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# 5. The SMIL 2.0 Layout Modules

Editors
  Aaron Cohen (aaron.m.cohen@intel.com), Intel
  Dick Bulterman (Dick.Bulterman@oratrix.com), Oratrix
  Erik Hodge (ehodge@real.com), RealNetworks

## Table of contents

## 5.1 Introduction

This section is informative.

This section defines the SMIL 2.0 Layout Modules, which are composed of a BasicLayout module and three modules with additional functionality that build on top of the BasicLayout module: the AudioLayout, MultiWindowLayout, and HierarchicalLayout modules. The modules contain elements and attributes allowing for positioning of media elements on the visual rendering surface, and control of audio volume. Since these elements and attributes are defined in modules, designers of other markup languages can choose whether or not to include this functionality in their languages. Therefore, language designers incorporating other SMIL modules do not need to include the layout modules if sufficient layout functionality is already present.

## 5.2 Overview of the SMIL 2.0 BasicLayout Module

This section is informative.

The functionality in this module is essentially identical with the layout functionality in [SMIL10].

Like SMIL 1.0, SMIL 2.0 BasicLayout modules includes a layout model for organizing media elements into regions on the visual rendering surface. The layout element is used in the document head to declare a set of regions on which media elements are rendered. Media elements declare which region they are to be rendered into with the region attribute.

Each region has a set of CSS2 compatible properties such as top, left, height, width, and backgroundColor. These properties can be declared using a syntax defined by the type attribute of the layout element. In this way, media layout can be described using the either SMIL basic layout syntax or CSS2 syntax (note that these are not functionally identical). Other layout types are possible as well.

For example, to describe a region with the id "r" at location 15,20 that is 100 pixels wide by 50 pixels tall using the SMIL BasicLayout module:

        <layout>
        <region id="r" top="15px" left="20px" width="100px" height="50px"/>
        </layout>

To create the same region using CSS2 syntax:

        <layout type="text/css">
        [region="r"] { top: 15px; left: 20px; width: 100px; height:50px; }
        </layout>

To display a media element in the region declared above, specify the region's id as the region attribute of the media element:

        <ref region="r" src="http://..." />

Additionally, implementations may choose to allow using the CSS syntax to set the media layout directly. This can be done by using the selector syntax to set layout properties on the media elements. For example, to display all video and image elements in a rectangle at the same size and position as the examples above:

        <layout type="text/css">
        video, img { top:15px; left:20px; width:100px; height=50px; }
        </layout>

Note that multiple layout models could be specified within a control structure such as the SMIL switch element, each with a different type. The first layout with a type supported by the implementation will be the one used.

## 5.3 SMIL 2.0 BasicLayout Module Syntax and Semantics

This section is normative.

### 5.3.1 Elements and Attributes

This section defines the elements and attributes that make up the functionality in the SMIL BasicLayout module.

#### The layout element

The layout element determines how the elements in the document's body are positioned on an abstract rendering surface (either visual or acoustic).

The layout element must appear before any of the declared layout is used in the document. If present, the layout element must appear in the head section of the document. If a document contains no layout element, the positioning of the body elements is implementation-dependent.

It is recommended that profiles including the SMIL 2.0 BasicLayout also support the SMIL 2.0 BasicContentControl module. A document can then support multiple alternative layouts by enclosing several layout elements within the SMIL switch element. This could also be used to describe the document's layout using different layout languages. Support for the system test attributes in the SMIL BasicContentControl module also enables greater author flexibility as well as user accessibility.

Default layout values can be assigned to all renderable elements by selecting the empty layout element <layout></layout>. If the document does not include a layout element, then the positioning of media elements is implementation-dependent.

Element attributes

type
  This attribute specifies which layout language is used in the layout element. If the user agent does not understand this language, it must skip the element and all of its content up until the next </layout> tag. The default value of the type attribute is "text/smil-basic-layout". This identifier value supports SMIL 1.0 and SMIL 2.0 BasicLayout module layout semantics.

##### Element content

If the type attribute of the layout element has the value "text/smil-basic-layout", it may contain the following elements:

region
root-layout

Languages incorporating the BasicLayout module need to define what additional elements are allowed as children. If the type attribute of the layout element has another value, the element contains character data.

#### The region element

The region element controls the position, size and scaling of media object elements.

In the following example fragment, the position of a text element is set to a 5 pixel distance from the top border of the rendering window:

    <smil xmlns="http://www.w3.org/2001/SMIL20/WD/">
      <head>
        <layout>
            <root-layout width="320" height="480" />
                  <region id="a" top="5" />
        </layout>
      </head>
      <body>
        <text region="a" src="text.html" dur="10s" />
      </body>
    </smil>

The position of a region, as specified by its top, bottom, left, and right attributes, is always relative to the parent geometry, which is defined by the parent element. For the SMIL BasicLayout module, all region elements must have as their immediate parent a layout element, and the region position is defined relative to the root window declared in the sibling root-layout element. The intrinsic size of a region is equal to the size of the parent geometry.

When region sizes, as specified by width and height attributes are declared relative with the "%" notation, the size of a region is relative to the size of the parent geometry. Sizes declared as absolute pixel values maintain those absolute values.

Conflicts between the region size and position attributes width, height, bottom, left, right, and top are resolved according to the rules for placeholder elements as detailed below. The default values of region position and size attributes is specified as auto. This attribute value has the same meaning here that it does in [CSS2], when there is no distinction drawn between replaced and non-replaced element.

A placeholder element is one which has no intrinsic width or height, but does have a bounding-box which has a width and height. SMIL BasicLayout regions are placeholder elements. Placeholder elements are clipped to the bounding box.

The governing equation for the horizontal dimension is:

bbw (bounding-box-width) = left + width + right

Given that each of these three parameters can have either a value of "auto" or a defined value not "auto", then there are 8 possibilities:

Attribute values | Result before clipping to the bounding box
left | width | right | left | width | right
auto | auto | auto | 0 | bbw | 0
auto | auto | defined | 0 | bbw - right | right
auto | defined | auto | 0 | width | bbw - width
auto | defined | defined | bbw - right - width | width | right
defined | auto | auto | left | bbw - left | 0
defined | auto | defined | left | bbw - right - left | right
defined | defined | auto | left | width | bbw - left - width
defined | defined | defined | left | width | bbw - left - width

The vertical attributes height, bottom, and top are resolved similarly. The governing equation for the vertical dimension is:

bbh (bounding-box-height) = top + height + bottom

Given that each of these three parameters can have either a value of "auto" or a defined value not "auto", then there are 8 possibilities:

Attribute values | Result before clipping to the bounding box
top | height | bottom | top | height | bottom
auto | auto | auto | 0 | bbh | 0
auto | auto | defined | 0 | bbh - bottom | bottom
auto | defined | auto | 0 | height | bbh - height
auto | defined | defined | bbh - bottom - height | height | bottom
defined | auto | auto | top | bbh - top | 0
defined | auto | defined | top | bbh - bottom - top | bottom
defined | defined | auto | top | height | bbh - top - height
defined | defined | defined | top | height | bbh - top - height

Element attributes

The region element can have the following visual attributes:

backgroundColor
  The use and definition of this attribute are identical to the "background-color" property in the CSS2 specification. This attribute specifies the background color used to fill the area of a region displaying media that is not filled by the media. The display of the background color when the region is not in use by a media element is controlled by the showBackground attribute. Unlike SMIL 1.0, this module requires support for CSS2 system colors.
  The backgroundColor attribute may take on the CSS value inherit. This means that the background color will be that of the parent element. If the parent element does not have an applicable background color property, the default value of transparent is used instead.
background-color
  Deprecated. Equivalent to backgroundColor, which replaces this attribute. The language profile must define whether or not the background-color attribute is supported. If both the backgroundColor and background-color attributes are absent, then the background is transparent.
bottom
  The use and definition of this attribute are identical to the "bottom" property in the CSS2 specification. Attribute values can be non-negative "percentage" values, and a variation of the "length" values defined in CSS2. For "length" values, SMIL 2.0 BasicLayout only supports pixel units as defined in CSS2. It allows the author to leave out the "px" unit qualifier in pixel values (the "px" qualifier is required in CSS2). Conflicts between the region size attributes bottom, left, right, top, width, and height are resolved according to the rules for absolutely positioned, replaced elements in [CSS2]. The default value of this attribute is auto.
fit
  This attribute specifies the behavior if the intrinsic height and width of a visual media object differ from the values specified by the height and width attributes in the region element. This attribute does not have a one-to-one mapping onto a CSS2 property, but can be simulated in CSS2.
  This attribute can have the following values:
  fill
    Scale the object's height and width independently so that the content just touches all edges of the box.
  hidden
    Has the following effect:
    - If the intrinsic height (width) of the media object element is smaller than the height (width) defined in the region element, render the object starting from the top (left) edge and fill up the remaining height (width) with the background color.
    - If the intrinsic height (width) of the media object element is greater than the height (width) defined in the region element, render the object starting from the top (left) edge until the height (width) defined in the region element is reached, and clip the parts of the object below (right of) the height (width).
  meet
    Scale the visual media object while preserving its aspect ratio until its height or width is equal to the value specified by the height or width attributes, while none of the content is clipped. The object's left top corner is positioned at the top-left coordinates of the box, and empty space at the right or bottom is filled up with the background color.
  scroll
    A scrolling mechanism should be invoked when the element's rendered contents exceed its bounds.
  slice
    Scale the visual media object while preserving its aspect ratio so that its height or width are equal to the value specified by the height and width attributes while some of the content may get clipped. Depending on the exact situation, either a horizontal or a vertical slice of the visual media object is displayed. Overflow width is clipped from the right of the media object. Overflow height is clipped from the bottom of the media object.

The default value of fit is hidden.

Note that the fit attribute applies to visual media once it has an intrinsic two-dimensional size, such as images and video. It does not apply to visual media that is rendered and adapted to varying circumstances, such as the visual display of HTML, until its two-dimensional spatial dimensions have been determined, such as after an HTML page has been laid out to specific size.

height
  The use and definition of this attribute are identical to the "height" property in the CSS2 specification. Attribute values follow the same restrictions and rules as the values of the bottom attribute. The intrinsic height of a region is the same as that of the parent geometry. The default value of this attribute is auto.
left
  The use and definition of this attribute are identical to the "left" property in the CSS2 specification.
  Attribute values follow the same restrictions and rules as the values of the "bottom" attribute.
  The default value of this attribute is auto.
regionName
  This attribute assigns a name to this region element that can be referred to by the region attribute of media object elements. The regionName attribute is not a unique identifier; multiple region elements can share the same regionName attribute value.
right
  The use and definition of this attribute are identical to the "right" property in the CSS2 specification.
  Attribute values follow the same restrictions and rules as the values of the "bottom" attribute.
  The default value of this attribute is auto.
showBackground
  This attribute controls whether the backgroundColor of a region is shown when no media is being rendered to the region:
  - If the value of showBackground is always, then the background color will be shown in the region when no media object is rendering into that region. If the region is part of a hierarchical layout, then any ancestor regions must also either be active or have a showBackground value of always for the background color to be shown.
  - If the value of showBackground is whenActive, then the background color will be not be shown in the region when no media object is rendering into that region. If the region is part of a hierarchical layout, then the background color will also be shown when any descendent regions are active.
  The default value of showBackground is always.
top
  The use and definition of this attribute are identical to the "top" property in the CSS2 specification.
  Attribute values follow the same restrictions and rules as the values of the bottom attribute.
  The default value of this attribute is auto.
width
  The use and definition of this attribute are identical to the "width" property in the CSS2 specification.
  Attribute values follow the same restrictions and rules as the values of the bottom attribute. The intrinsic width of a region is the same as that of the parent geometry.
  The default value of this attribute is auto.
z-index
  The use and definition of this attribute are identical to the "z-index" property in the CSS2 specification, with the following exception:
  If two boxes generated by elements A and B have the same stack level, then:
  - If the display of an element A starts later than the display of an element B, the box of A is stacked on top of the box of B (temporal order).
  - Else, if the display of the elements starts at the same time, and an element A occurs later in the SMIL document text than an element B, the box of A is stacked on top of the box of B (document tree order as defined in CSS2).

A profile integrating the SMIL 2.0 BasicLayout module must provide a means of declaring an XML identifier on region elements.

#### The root-layout element

The root-layout element determines the value of the layout properties of the root element, which in turn determines the size of the window in which the SMIL presentation is rendered.

If more than one root-layout element is parsed within a single layout element, this is an error, and the document should not be displayed. This does not include root-layout elements skipped by the user agent (e.g. because the enclosing layout element was skipped due to an unrecognized type or a test attribute evaluated to false).

The semantics of the root-layout element are as in SMIL 1.0: the attributes of the root-layout element determine the size of the top level presentation window, and the declared sibling regions are arranged within this top level window. If either the height or width of the root-layout element is not specified, the value of the attribute is implementation-dependent.

Element attributes

The root-layout element can have the following attributes:

backgroundColor
  Defined in backgroundColor under the region element. Note that the default background color is transparent, which implies that, by default, the implementation-dependent window background will be shown.
background-color
  Defined in background-color under the region element.
height
  Sets the height of the root element. Only length values are allowed. For "length" values, SMIL 2.0 BasicLayout only supports pixel units as defined in CSS2. It allows the author to leave out the "px" unit qualifier in pixel values (the "px" qualifier is required in CSS2).
width
  Sets the width of the root element. Only length values are allowed. For "length" values, SMIL 2.0 BasicLayout only supports pixel units as defined in CSS2. It allows the author to leave out the "px" unit qualifier in pixel values (the "px" qualifier is required in CSS2).

Element content

The root-layout element is an empty element.

This element supports the SMIL 1.0 syntax where the root-layout element is an empty sibling of the top level region elements.

#### The region attribute

The region attribute is applied to an element in order to specify which rendering region is assigned to the element. The attribute refers to the abstract rendering region (either visual or acoustic) defined within the layout section of the document. The referenced abstract rendering region is determined by applying the following rules, in order:

1. Find all elements in the layout section with regionName attributes that are assigned the same value as that of the region attribute.
2. Remove the elements from this collection that are removed from the rendered presentation due to the processing of switch elements and test attributes.
3. If any elements remain, the media should be rendered in all of the referred to regions. If the application cannot render the media simultaneously in multiple regions, then the media should be rendered using the lexically first remaining element.
4. If no elements have a regionName attribute that is assigned the same value as that of the region attribute, then select the element in the layout section whose unique identifier is the value of the region attribute.

If this process selects no rendering surface defined in the layout section, the values of the formatting properties of this element are defined by the default layout values, which is described in the section on integration requirements for this module.

The language integrating this module must specify which elements have a region attribute and any inheritance of the attribute.

### 5.3.2 SMIL BasicLayout Module Details

SMIL 2.0 BasicLayout module is consistent with the visual rendering model defined in CSS2, it reuses the formatting properties defined by the CSS2 specification, and newly introduces the fit attribute [CSS2]. The reader is expected to be familiar with the concepts and terms defined in CSS2.

Integration Requirements

A profile integrating the SMIL 2.0 BasicLayout module must define the content models for the layout element if any elements beyond those specified here are to be allowed as children.

A profile integrating the SMIL 2.0 BasicLayout module must provide a means of declaring an XML identifier on region elements if the profile intends on referring to region elements by XML identifier. This value is used as the argument value to the region attribute. This is not required if the profile will only use the regionName method of referring to a region element.

A profile integrating the SMIL 2.0 BasicLayout module must specify which elements have a region attribute and any inheritance of the attribute.

If not otherwise defined by the profile, the default values of the layout attributes listed in the SMIL 2.0 layout modules will apply to presented elements not otherwise specifying layout semantics.

An element that does not refer to a valid region element will display in the default region. If not otherwise specified by the profile, the default region is defined as filling and aligned to the upper-left corner of the presentation window. This default region takes on default values for all other region attributes.

### 5.3.3 Document Type Definition (DTD) for the BasicLayout Module

See the full DTD for the SMIL Layout modules.

## 5.4 Overview of the SMIL 2.0 AudioLayout Module

This section is informative.

This section defines the functionality in the SMIL 2.0 AudioLayout module. This level contains an attribute providing audio rendering surface volume control.

### 5.4.1 Integration Requirements for the AudioLayout Module

This section is normative.

The functionality in this module builds on top of the functionality in the BasicLayout module, which is a required prerequisite for inclusion of the AudioLayout module.

### 5.4.2 Audio Volume Control

This section is normative.

SMIL 2.0 AudioLayout module supports control of aural media volumes via a new property on the region element, soundLevel. Multimedia assigned to a region with an explicit soundLevel attribute will have its audio rendered at the given relative sound intensity.

## 5.5 SMIL 2.0 AudioLayout Module Syntax and Semantics

This section is normative.

### 5.5.1 Elements and Attributes

This section defines the elements and attributes that make up the SMIL 2.0 AudioLayout module.

#### The region element

The region element defined in the BasicLayout module is extended with the addition of the soundLevel attribute.

Element attributes

The region element can have the following aural attributes:

soundLevel
  Specifies the relative volume of the audio portion of a media element assigned to play within the given region. This associates the region element with a sound reproduction unit. Cascaded region's will accumulate their respective sound level settings, as will be explained below. region's that are used for multiple sources apply their sound level setting to all of them. A sound source may be reproduced by different units, e.g., through application of the regionName attribute. In such a "multiple window" case a separate soundLevel may be applied to each instance of the sound source, one per region. Assigned level changes accumulate across nested regions by multiplying percentage values.

  Valid values are non-negative CSS2 percentage values. Percentage values are interpreted relative to the recorded volume of the media. The percentages are interpreted as a ratio of output to input signal level, and is defined in terms of dB:

  `dB change in signal level = 20 log10(percentage-value / 100)`

  A setting of '0%' plays the media silently. A value of '100%' will play the media at its recorded volume (0 dB). Similarly, a value of '200%' will play the media nearly twice as loud (6 dB) as it's recorded volume (subject to hardware limitations). The default value is '100%'. The absolute sound level of media perceived is further subject to system volume settings, which cannot be controlled with this attribute.

## 5.6 Overview of the SMIL 2.0 MultiWindowLayout Module

This section is informative.

This section defines the functionality in the SMIL 2.0 MultiWindowLayout module. This level contains elements and attributes providing for creation and control of multiple top level windows on the rendering device.

### 5.6.1 Integration Requirements for the MultiWindowLayout Module

This section is normative.

The functionality in this module builds on top of the functionality in the BasicLayout module, which is a required prerequisite for inclusion of the MultiWindowLayout module.

### 5.6.2 Multiple Top-Level Window Support

This section is normative.

In [[SMIL 1.0]], and the SMIL 2.0 BasicLayout module, each presentation is rendered into a single root window of a specific size/shape. The root window contains all of the regions used to manage the rendering of specific media objects.

This specification supports the concept of multiple top-level windows. Since there is no longer a single root window, we use the term top level instead. The assignment of the regions to individual top level windows allows independent placement and resizing of each top-level window, if supported by the including profile and implementation. The initial placement of the top level windows on the display device and any available means of relocating the top level windows is implementation-dependent.

A top level window is declared with the topLayout element in a manner similar to the SMIL 1.0 root-layout window, except that multiple instances of the topLayout element may occur within a single layout element:

    <layout>
      <topLayout id="WinV" title="Video" width="320" height="240"/>
        <region id="pictures" title="pictures" height="100%" fit="meet"/>
      </topLayout>
      <topLayout id="WinC" title="Captions" width="320" height="60">
        <region id="captions" title="caption text" top="90%" fit="meet"/>
      </topLayout>
    </layout>

In this example, two top-level windows are defined ("WinV" and "WinC"), and two regions are defined with one region ("pictures") assigned to WinV and the other ("captions") to WinC. The definitions of the top-level windows and the contained regions use a hierarchical syntax, unlike the older root-layout element.

The top-level windows function as rendering containers only, that is, they do not carry temporal significance. In other words, each window does not define a separate timeline or any other time-container properties. There is still a single master timeline for the SMIL presentation, no matter how many top-level windows have been created. This is important to allow synchronization between media displayed in separate top-level windows.

The display of top level windows can be controlled automatically by the player, or manually by the user of the application. If a window is closed (by the user) while any of the elements displayed in that window are active, there is no effect on the timeline (if any) of those elements. However, a player may choose not to decode content as a performance improvement. The means provided to a user to close top level windows is implementation-dependent.

For SMIL 1.0 compatibility, the root-layout element will continue to support SMIL 1.0 layout semantics. The new topLayout element will support the extension semantics and the improved, nested syntax.

Note also that any one region may belong to at most one top-level (or root-level) window. Regions not declared as children of a topLayout element belong to the root-layout window. If no root-layout element has been declared, the region is assigned to an additional window according to the semantics in the BasicLayout module.

## 5.7 SMIL 2.0 MultiWindowLayout Module Syntax and Semantics

This section is normative.

### 5.7.1 Elements and Attributes

This section defines the elements and attributes that make up the SMIL 2.0 MultiWindowLayout module.

#### The topLayout element

The topLayout element determines the size of the a window in which the SMIL presentation is rendered, as well as serving as a top level window in which to place child region elements.

Multiple topLayout elements may appear within a single layout element, each declaring an independent top-level window.

Each instance of a topLayout element determines the size of a separate top-level presentation window, and the descendant regions are arranged within this top-level window and relative to the coordinate system of this window.

This module also provides control over when topLayout windows open and close in a presentation. Note that the precise mapping of topLayout windows on to the host environment is implementation-dependent. It is expected that implementations will "pop up" independent desktop windows if they can, but other means of supporting multiple topLayouts, such as by using frames, are allowed. When automatically opening and closing windows, applications should try to comply with the WAI User Agent Guidelines [UAAG] and allow the user to choose whether to be warned that windows are being opened and closed, and given a method for disabling automatic opening and closing of windows.

Element attributes

The topLayout element can have the following attributes:

backgroundColor
  Defined in backgroundColor under the region element. Note that the default background color is transparent, which implies that, by default, the implementation-dependent window background will be shown.
close
  Specifies when the top level window should be closed. If the value of close is onRequest, then the top level window should not be closed automatically by the player and will only close if the user explicitly closes it via the user interface. If the value of close is whenNotActive, then the top level window should close automatically when no media is being displayed in any one of the window's regions. For timed media using the SMIL 2.0 timing and synchronization modules, this means when there is no media within its active duration or freeze period using any region of the topLayout. The default value of close is onRequest.
height
  Sets the height of the top-level window. Only length values are allowed.
open
  Specifies when the top level window should be opened. If the value of open is onStart, then the top level window should be opened when the presentation begins, and if closed, should not be reopened automatically during the presentation. If the value of open is whenActive, then, if not already open, the top level window should be opened when media is displayed in one of the window's regions. For timed media using the SMIL 2.0 timing and synchronization modules, this means when there is any media within its active duration or freeze period using any region of the topLayout. The default value of open is onStart.
width
  Sets the width of the top-level window. Only length values are allowed.

##### Element content

The topLayout element may contain any number of region elements, or be empty.

Allowing multiple topLayout elements within a single layout element supports multiple top level windows.

#### The layout element

##### Element content

The layout element defined in the SMIL BasicLayout module is extended with the addition that the topLayout element is added to the content model of the layout element if the type attribute of the layout element has the value "text/smil-basic-layout". In this case it can contain the following elements:

region
root-layout
topLayout

### 5.7.2 MultiWindowLayout Module Events

This module includes two events that may be included in the integrating language profile.

topLayoutOpenEvent
  Raised when a topLayout window opens. This event is delivered to the associated topLayout element. If a topLayout closes and then reopens when additional media becomes active in any of its regions, this event will be raised again, and will be raised every subsequent time it reopens.
topLayoutCloseEvent
  Raised when a topLayout closes for any reason. This event is delivered to the associated topLayout element. If a topLayout reopens when additional media becomes active in any of its regions, this event will be raised again if and when the topLayout closes again, and will be raised every subsequent time it closes.

#### Integration Requirements for the MultiWindowLayout Module Events

The language profile must specify the declarative names for binding these events, as well as the bubbling behavior of the events.

### 5.7.3 Document Type Definition (DTD) for the MultiWindowLayout Module

See the full DTD for the SMIL Layout modules.

## 5.8 Overview of the SMIL 2.0 HierarchicalLayout Module

This section is informative.

This section defines the functionality in the SMIL 2.0 HierarchicalLayout module. This module contains elements and attributes for advanced positioning of media elements on the visual rendering surface and builds upon the SMIL 2.0 BasicLayout module.

The SMIL 2.0 HierarchicalLayout module extends the basic layout model for organizing media elements into regions on the visual rendering surface providing much greater author control and flexibility. These extensions are important for certain classes of multimedia presentations in which author control of object placement is critical.

This module:

> - extends the definition of the region element to allow for the specification of hierarchical regions;
> - introduces a new layout element, regPoint, for controlling relative placement with an associated region (the registration element facility);
> - includes additional attributes, regPoint and regAlign, that specify how a media object's presentation can be aligned relative to the set of defined registration points (the registration alignment facility); and
> - provides additional attributes, top, bottom, left, and right, that specify exact positioning of objects within a region (the so-called sub-region positioning facility);
> - allows an object placed in a sub- region to set the sub- region's fit, z-index, and backgroundColor attributes.

### 5.8.1 Integration Requirements for the HierarchicalLayout Module

This section is normative.

The functionality in this module builds on top of the functionality in the BasicLayout module, which is a required prerequisite for inclusion of the HierarchicalLayout module.

## 5.9 New Features in the SMIL 2.0 HierarchicalLayout Module

This section is normative.

### 5.9.1 Hierarchical Region Layout

A new feature in this module is support for hierarchical layout. This allows for the declaration of regions nested inside other regions, much like regions are laid out inside the top level window declared by the topLayout element. For example, the following declares a top level window of 640 by 480 pixels, regions "left" and "right" which covers the left and right sides of the window respectively, and a hierarchical region "inset" that is centered within "right".

    <layout>
            <topLayout width="640px" height="480px" />
                    <region id="left" top="0%" left="0%"
                                 width="50%" height="100%" />
                    <region id="right" top="0%" left="50%"
                                   width="50%" height="100%">
                            <region id="inset" top="25%" left="25%"
                                             width="50%" height="50%" />
                    </region>
            </topLayout>
    </layout>

The resulting layout looks like this:

By default, each hierarchical region shares the z-index (depth) value of its parent. Hierarchical regions may also introduce their own local z-index value. In this case, all hierarchical regions with a common direct parent define local z-indexes within the z-index value of their parent. For example, if a parent region has a z-index value of "4" and two hierarchical children of that parent define z-indexes of "1" and "2", respectively, then each of these are treated as further sub-divisions of the parent's z-index of "4".

If two hierarchical regions with the same z-index attribute value overlap, the existing rules for z-index processing (from the BasicLayout module) are applied. Specifically, the rule concerning time priority is maintained, meaning that in the case of a z-index conflict, the media visible in the overlap will be determined by the region that is rendering the media that has most recently begun in time. If the conflicting media began at the same time, then the rule using the textual order of the media elements in the SMIL document is applied.

For example:

    <layout>
       <root-layout width="640px" height="480px" />
       <region id="whole" top="0px" left="0px" width="640px"
                                    height="480px" z-index="5"/>
       <region id="right" top="0px" left="320px" width="320px"
                                    height="480px" z-index="4">
           <region id="inset" top="140px" left="80" width="160px"
                                    height="200px" z-index="6"/>
           <region id="inset2" top="140px" left="80" width="160px"
                                    height="200px" z-index="6"/>
           <region id="inset3" top="140px" left="80" width="160px"
                                    height="200px" z-index="7"/>
       </region>
    </layout>
    ...
    <par>
            <img id="A" region="whole" src="imageA.jpg" dur="10s"/>
            <img id="B" region="inset" src="imageB.jpg" dur="10s"/>
    </par>
    <par>
            <img id="D" region="inset2" src="imageD.jpg" begin="1s" dur="10s"/>
            <img id="C" region="inset" src="imageC.jpg" begin="0s" dur="10s"/>
    </par>
    <par>
            <img id="E" region="inset2" src="imageE.jpg" dur="10s"/>
            <img id="F" region="inset3" src="imageF.jpg" dur="10s"/>
    </par>

1. In the first "par", image "A" and image "B" begin at the same time. Image A will obscure image "B", even though the z-index of "inset is greater than that of "whole". This is because the z-index of "right", which is the region containing "inset" is less than that of "whole".
2. In the second "par", images "C" and "D" are rendered into regions occupying the same area of the rendering surface. Image "C" will be shown for one second and then obscured by Image "D", since "D" begins after image "C". Note that lexical order is irrelevant here.
3. In the third "par", the z-index of region "inset" is considered when computing stacking between siblings, and therefore image "F" will be shown, but image "E" will be obscured for the entire 10 seconds that they are both active.

### 5.9.2 Sub-Region Layout

Where hierarchical layout provides a facility for defining a set of regions with a common parent, it does not provide any facility for finer control within a region where a given media object will be placed. The SMIL 2.0 HierarchicalLayout module solves this problem by defining a set of attributes which, when placed on an object using the region attribute, allow that media item to be declared with an explicitly positioned sub-region of a given region. These attributes are collectively referred to as sub-region positioning attributes. A sub-region is a child of the region declared in the region attribute on the media element. The sub-region positioning argument values follow the conventions of placeholder elements described in the section on the region element .

For example, suppose a region "d" is defined:

      <layout>
         ...
        <region id="d" ... />
         ...
      </layout>

The following code describes the placement of an object in a sub-region at a particular offset within a region, using the SMIL 2.0 HierarchicalLayout syntax:

         <ref id="a" ... region="d" top="5%" left="3" />

Each placement attribute defines a new, temporary child region for the referenced media object. In this case, the top-left point of the media object is displayed 5% from the top and 3 pixels from the left of region "d", and extends to the right and bottom edges of the region "d".

All other placement operations, such as the fit attribute, now operate on the sub-region. For example, the following document fragment describes a region and a media object reference that make use of sub-region positioning:

      <layout>
         ...
        <region id="d" ... fit ="fill" />
         ...
      </layout>

      <body>     ...
         <ref src="..." ... region="d" fit="hidden"
                    top="5%" left="3" bottom="10%" right="15%" />
         ...
      </body>

In this example, the effective boundaries of the sub-region for the placement of this object are defined by declaring the top, bottom, left and right edges of the region to the values shown, and then filling the resulting sub-region with the specified image as directed by the fit attribute. If the size of the media object being displayed is smaller than that of the resulting sub-region, the display will be similar to:

The use of sub-region placement is intended as a light-weight alternative to defining a large number of single-use regions. Often, the dimensions used for the sub-region will match the dimensions of the media object being placed, but in all cases the values of the fit attribute will govern rendering of the object in the sub-region. The other attributes on the media element that would have been applied to a referenced region are applied to the sub-region instead. Note that the default values for the sub-region attributes are all 'auto', resulting that, by default, a sub-region is created having the same size and position as the parent region.

Rules for handling clipping of objects within regions based on the sub-region attributes are provided below.

### 5.9.3 Media Object fit, z-index, and backgroundColor

The SMIL 2.0 HierarchicalLayout module includes the ability to use the fit, z-index, and backgroundColor attributes on objects displayed in a sub-region in order to declare different behavior from that on the region element.

### 5.9.4 Registration Points

A registration element is an element defined within this module that is used to define a point within a region and a default object alignment algorithm about that point. The element can be used in a media object element, where it is associated with a region and an optional override alignment algorithm. The placement values within registration elements can be either percentages or pixels.

The use of registration points allows for consistent relative placement across regions. As such, registration points are defined outside of any single region.

For example, the following code describes two registration points (with id values "midPoint" and "topMargin"), one of which is defined as a relative location and one at a fixed pixel location, using the SMIL 2.0 HierarchicalLayout syntax:

        <layout>
          <regPoint id="midPoint" top="50%" left="50%" regAlign="center" />
          <regPoint id="topMargin" top="10" left="15" regAlign="topLeft" />
          <region id="a" ... />
          <region id="b" ... />
        </layout>

In this example, the registration point with the id value "midPoint" has a default alignment algorithm that centers the media object about the defined point, while the registration point with the id value "topMargin" has a default alignment algorithm that places the top-left point of the media object at the registration point.

Various media elements could be displayed in the regions using the alignment points as follows:

        <ref region="a" src="rtsp://..." dur="2s" regPoint="midPoint" />
        <ref region="b" src="http://..." dur="2s"
                             regPoint="midPoint" regAlign="bottomRight"/>
        <ref region="a" src="http://..." dur="2s" regPoint="topMargin" />
        <ref region="b" src="http://..." dur="2s"
                             regPoint="topMargin" regAlign="center"/>

In the first example, a media object is centered in the middle of region a. In the second example, a media object has its bottom right corner centered in the middle of region b. Similarly, in the third example, a media object has its top left corner placed at a point 10,15 within region a, and in the fourth example, an object is centered around the point 10,15 in region b.

Registration points can be used to coordinate the placement of a set of media objects that do not share the same sizes. (For example, a set of images can be aligned to the center of a region.) They can also be used to coordinate the display of images about a particular point in a region, as in:

       <layout>
          <regPoint id="middle" top="50%" left="50%" regAlign="center" />
          <region id="a" ... />
       </layout>
       ...
       <seq>
        <ref region="a" src="rtsp://..." dur="2s" regPoint="middle"
                                            regAlign="bottomRight"/>
        <ref region="a" src="http://..." dur="2s" regPoint="middle"
                                            regAlign="bottomLeft"/>
        <ref region="a" src="http://..." dur="2s" regPoint="middle"
                                            regAlign="topLeft"/>
        <ref region="a" src="http://..." dur="2s" regPoint="middle"
                                            regAlign="topRight"/>
      </seq>

In this example, four objects are aligned over time to the middle of region. If any media element extends outside the bounds of a region, it will be clipped to the region.

Note that registration points are global within the context of a layout, and are not tied to a particular region, but can be reused across regions. As such, pixel-based offsets should be used with care.

For authoring convenience, SMIL HierarchicalLayout module provides several pre-defined region registration points including topLeft, topMid, topRight, midLeft, center, midRight, bottomLeft, bottomMid, and bottomRight.

For example, media objects can be centered in any region like this:

        <ref ..." regPoint="center" regAlign="center" />

The default value of regAlign for a region is topLeft. If the regAlign attribute is used without a regPoint attribute, the alignment operation is relative to the upper left point of the region containing this object, that is, the behavior is the same as if the regPoint were to be specified as topLeft.

Rules for handling clipping of objects within regions based on the regPoint and regAlign attributes are defined below.

## 5.10 SMIL 2.0 HierarchicalLayout Syntax and Semantics

This section is normative.

### 5.10.1 Elements and attributes

This section defines the elements and attributes that make up the SMIL 2.0 HierarchicalLayout module.

#### The layout element

This element is defined as in the BasicLayout module with extensions presented here.

In order to support the registration point functionality described above, one new element is added to the content model of the layout element.

Element attributes

SMIL HierarchicalLayout module does not provide any new attributes to the layout element.

Element content

If the type attribute of the layout element has the value "text/smil-basic-layout", it can contain the following elements:

region
root-layout
topLayout
regPoint

All element content except regPoint are defined above in the BasicLayout and MultiWindowLayout modules. The regPoint element is described below.

#### The region element

The region element controls the position, size and scaling of media object elements. This module extends the definition of the region element to include the definition of hierarchical regions.

The position of a region, as specified by its top and left attributes, is always relative to the parent geometry, which is defined by the parent element. For the SMIL 2.0 HierarchicalLayout module, all hierarchical region elements must have as their immediate parent a region or topLayout element. The position of the hierarchical region is defined relative to that parent element. The intrinsic size of a region is equal to the size of the parent geometry.

When region sizes, as specified by width and height attributes are declared relative with the "%" notation, the size of the hierarchical region is relative to the size of the parent region. Sizes declared as absolute pixel values are absolute values even when used with a child region.

Note that a (hierarchical) region may be defined in such a way as to extend beyond the limits of its parent. In this case the child region must be clipped to the parent boundaries.

If a z-index attribute is defined on the hierarchical region, it is evaluated as a local index within that of the parent.

Element attributes

In the HierarchicalLayout module, the region element has no additional attributes beyond that provided in the other included layout modules. However, the semantics of the z-index attribute are extended to support hierarchical regions.

z-index
  This attribute is defined as in the BasicLayout module with extensions presented here.
  The z-index attribute defines the level of the region within the parent region stacking context. Elements assigned to higher level regions are rendered in front of lower level regions within the same parent region. Child regions are always placed in front of their parent region. This results in a two stage sorting of region visibility: first by parent-child containment, and then by z-index among siblings.

  Just as with simple non-hierarchical regions, the stacking order of hierarchical regions may be affected by temporal activation. A region becomes active either when media begins rendering into it, or when one of its child regions becomes active. If two sibling regions have the same z-index, the region most recently made active is in front of the other region.

Element content

SMIL HierarchicalLayout module extends the region element content model to optionally include other region elements.

#### The regPoint element

The regPoint element determines the (x, y) coordinates of a point relative to a region upper-left corner for use in aligning elements in the document's body on regions within a visual rendering surface. A regPoint may be defined using absolute (pixel) or relative (percentage) based values. The regPoint functionality is not defined and may not be used for media without intrinsic size.

For the purposes of regPoint functionality, media and regions are defined to be rectangular, with perpendicular sides, with the sides ordered clockwise top, right, bottom, and left. The top side is the edge closest to the point or edge of the display device considered "up".

The regPoint element may only appear as an immediate child of a layout element.

If the registration point functionality is used on a media object that also uses sub-region positioning, the registration point applies to the subregion.

If the registration point or alignment functionality is used on a media object, the interaction between the regPoint attribute value, the regAlign attribute value, and the fit attribute value of the region in which the media object is displayed is as follows:

fill fit value:
  This depends on the value of the regAlign attribute. (Note: in all cases, the media must maintain proper alignment over the regPoint):
  topLeft regAlign value:
    Scale the object's height and width independently so that the content just touches the bottom and right edges of the box.
  topMid regAlign value:
    Scale the object's height and width independently so that the content just touches the bottom edge of the box, and the nearest (to the regPoint) of the left or right edges of the box.
  topRight regAlign value:
    Scale the object's height and width independently so that the content just touches the bottom and left edges of the box.
  midLeft regAlign value:
    Scale the object's height and width independently so that the content just touches the nearest (to the regPoint) of the top or bottom edges of the box, and touches the right edge of the box.
  center regAlign value:
    Scale the object's height and width independently so that the content just touches the nearest (to the regPoint) of the top or bottom edges of the box, and touches the nearest (to the regPoint) of the left or right edges of the box.
  midRight regAlign value:
    Scale the object's height and width independently so that the content just touches the nearest (to the regPoint) of the top or bottom edges of the box, and touches the left edge of the box.
  bottomLeft regAlign value:
    Scale the object's height and width independently so that the content just touches the top and right edges of the box.
  bottomMid regAlign value:
    Scale the object's height and width independently so that the content just touches the top edge of the box, and the nearest (to the regPoint) of the left or right edges of the box.
  bottomRight regAlign value:
    Scale the object's height and width independently so that the content just touches the top and left edges of the box.
hidden fit value:
  Align the media over the given regPoint per the regAlign attribute. If the media so positioned extends past the bounds of the region, clip the excess media. If the media so positioned doesn't meet the bounds of the region, fill the remaining space with the background color of the region.
meet fit value:
  This depends on the value of the regAlign attribute. Any part of the region that is not covered by the media is then filled with the region's background color. (Note: in all cases, the media must maintain proper alignment over the regPoint):
  topLeft regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the right or bottom edge of the box while none of the content is clipped.
  topMid regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches at least one of the left, right, or bottom edges while none of the content is clipped.
  topRight regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the left or bottom edge of the box while none of the content is clipped.
  midLeft regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches at least one of the top, right, or bottom edges while none of the content is clipped.
  center regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches at least one of the top, left, right, or bottom edges while none of the content is clipped.
  midRight regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches at least one of the top, left, or bottom edges while none of the content is clipped.
  bottomLeft regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top or right edge of the box while none of the content is clipped.
  bottomMid regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches at least one of the top, left, or right edges while none of the content is clipped.
  bottomRight regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top or left edge of the box while none of the content is clipped.
scroll fit value:
  Align the media over the given regPoint per the regAlign attribute. If any part of the media extends beyond the bounds of the region, a scrolling mechanism should be invoked that allows viewing of the media that is clipped beyond the bounds of the region.
slice fit value:
  This depends on the value of the regAlign attribute. (Note: in all cases, the media must maintain proper alignment over the regPoint):
  topLeft regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the right or bottom edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped.
  topMid regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the left, right, or bottom edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped. Clipping will occur on up to two sides of the region.
  topRight regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the left or bottom edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped.
  midLeft regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top, right, or bottom edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped. Clipping will occur on up to two sides of the region.
  center regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top, left, right, or bottom edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped. Clipping will occur on up to three sides of the region.
  midRight regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top, left, or bottom edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped. Clipping will occur on up to two sides of the region.
  bottomLeft regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top or right edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped.
  bottomMid regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top, left, or right edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped. Clipping will occur on up to two sides of the region.
  bottomRight regAlign value:
    Scale the visual media object's height and width while maintaining its aspect ratio so that the content just touches the top or left edge of the box, whichever requires the largest scale factor. Any content that extends beyond the bounds of the region is clipped.

For example, a wide-screen video can be made to play in "letterbox" mode in a region, whose width-to-height ratio is smaller, by using regPoint ="center" and regAlign="center" and setting the region's fit value to "meet". The result is the video will touch the left and right edges of the region and will be centered vertically with the gaps above and below filled in with the region's background color.

Element attributes

top
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the registration point relative to the top of the region. The default value of top is auto.
bottom
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the registration point relative to the bottom of the region. The default value of bottom is auto.
left
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the registration point relative to the left location of the region. The default value of left is auto.
right
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the registration point relative to the right location of the region. The default value of right is auto.
regAlign
  This attribute specifies the default alignment algorithm which is associated with a regPoint relative to the media object. If no value is specified the value of topLeft is the default for the regPoint. The following values are allowed:
  topLeft
    Align the top-left corner of the object with the registration point.
  topMid
    Align the top-middle point of the object with the registration point. The top-middle is the point offset width/2 to the right of the object top-left corner.
  topRight
    Align the top-right corner of the object with the registration point.
  midLeft
    Align the middle-left point of the object with the registration point. The middle-left is the point offset height/2 down from the object top-left corner.
  center
    Align the center of the object with the registration point.
  midRight
    Align the middle-right point of the object with the registration point. The middle-right is the point offset height/2 down from the object top-right corner.
  bottomLeft
    Align the bottom-left corner of the object with the registration point.
  bottomMid
    Align the bottom-middle point of the object with the registration point. The bottom-middle is the point offset width/2 right from the object bottom-left corner.
  bottomRight
    Align the bottom-right corner of the object with the registration point.

Element content

None.

### 5.10.2 SMIL HierarchicalLayout Positioning and Presentation Attributes

While the SMIL 2.0 BasicLayout module provides only the region attribute on elements to place them on the rendering surface, the HierarchicalLayout module includes attributes to refine the position of media content within a region, and to refine the visual presentation of the media within the region.

This module provides fine control over the background color surrounding media elements by allowing the media element to declare the background color of a region in which the media is being shown.

One set of attributes (the sub-region positioning attributes) allows a sub-region to be defined that is a child of a declared region and is wholly contained within the enclosing layout region for that media object; the other set of attributes can be used to define a registration point to be used with that object and an optional layout algorithm that will override the default algorithm associated with the registration point.

If the fit attribute and alignment attributes regPoint and regAlign are relevant to the placement of a particular media object, the interaction is the same as described in the definition of regPoint. If sub-region positioning attributes are used on a media object along with fit or the alignment attributes regPoint and regAlign, these attributes apply to the sub-region. In this case the fit setting on the referenced region element does not apply to the sub-region.

For both sub-region positioning and registration point use, the value of the z-index attribute on the associated region is used. If media objects overlap spatially, existing rules for resolving z-index conflicts are applied.

Note that placement within the region may be defined in such a way as to extend the media object beyond the limits of the region. In this case the media object must be clipped to the region boundaries.

#### Media specific background color

backgroundColor
  This attribute allows specifying the background color that will be used when a media element is being shown in a sub-region. The range of legal values are the same as the backgroundColor attribute on the region element. The default value for this attribute is transparent.

#### Sub-region Positioning Attributes

top
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the top edge of the sub-region relative to the top of the region. The default value of top is auto.
bottom
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the bottom edge of the sub-region relative to the bottom of the region. The default value of bottom is auto.
height
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the bottom edge of the sub-region relative to the top side of the region. The default value of height is auto.
left
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the left edge of the sub-region relative to the left of the region. The default value of left is auto.
right
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the right edge of the sub-region relative to the right side of the region. The default value of right is auto.

width
  This value uniquely identifies the position or the distance (using CSS2 pixel or non-negative percentage values) of the right edge of the sub-region relative to the left side of the region. The default value of width is auto.

Conflicts between the region size attributes bottom, height, left, right, top, and width are resolved according to the rules for placeholder elements described in the section on the region element .

The sub-region positioning attributes will be ignored if they are used on an element without a region attribute that specifies the identifier of region element in the layout section.

#### Registration point attributes

The regPoint attribute is used in conjunction with the regPoint element. If a regPoint attribute is missing or refers to a non-existent regPoint element the value of the regAlign attributes are applied to the top-left point of the region containing the media object.

regPoint
  This value uniquely identifies the registration point to be used for the placement of the object. The value should be an XML identifier of a regPoint element.
  The following values are predefined registration points that do not have to be declared as regPoint elements before they are used:
  topLeft
    Same as using top="0%" left="0%" on the element without the regPoint attribute.
  topMid
    Same as using top="0%" left="50%" on the element without the regPoint attribute.
  topRight
    Same as using top="0%" left="100%" on the element without the regPoint attribute.
  midLeft
    Same as using top="50%" left="0%" on the element without the regPoint attribute.
  center
    Same as using top="50%" left="50%" on the element without the regPoint attribute.
  midRight
    Same as using top="50%" left="100%" on the element without the regPoint attribute.
  bottomLeft
    Same as using top="100%" left="0%" on the element without the regPoint attribute.
  bottomMid
    Same as using top="100%" left="50%" on the element without the regPoint attribute.
  bottomRight
    Same as using top="100%" left="100%" on the element without the regPoint attribute.
  Note that the predefined registration points have the same meaning relative to the region as the regAlign attribute values have relative to the media. If no value is given, the default registration point of topLeft (top="0%", left="0%") is assumed, that is, the media is aligned with the top-left of the region.
regAlign
  This value uniquely identifies the registration algorithm to be used for the regPoint defined for the object, and overrides any regAlign attribute on the referenced regPoint element. Permissible values are those defined under the regAlign attribute for the regPoint element. If used without an explicit regPoint attribute, the value is relative to the top left point of the region used by the associated media object.

#### Region fit override

The fit attribute is used on an element in conjunction with the region attribute to control the display of the element on the rendering surface. A value of the fit attribute given on the element will override the value of fit declared in the referenced region element.

fit
  This attribute specifies the behavior if the intrinsic height and width of a visual media object differ from the values specified by the height and width values of the region. It takes the same values as the fit attribute on the region element, and has the same semantics with the exception that the declared fit only applies to the sub-region declared by the media element using it, and not to other elements which may use the same parent region.

#### Region z-index override

The z-index attribute is used on an element in conjunction with the sub-region positioning attributes to control the display of the element on the rendering surface. A value of the z-index attribute given on the element will set the z-index for the sub-region within the context of the parent region stacking order.

z-index
  This attribute specifies the stacking order behavior for the sub-region. It takes the same values as the z-index attribute on the region element, and has the same semantics with the exception that the declared z-index only applies to the sub-region declared by the media element, and not to other elements which may use the same parent region.

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