TTML Profiles for Internet Media Subtitles and Captions 1.0 (IMSC1)

Abstract

This document specifies two profiles of [TTML1]: a text-only profile and an image-only profile. These profiles are intended to be used across subtitle and caption delivery applications worldwide, thereby simplifying interoperability, consistent rendering and conversion to other subtitling and captioning formats.

It is feasible to create documents that simultaneously conform to both [ttml10-sdp-us] and the text-only profile.

The document defines extensions to [TTML1], as well as incorporates extensions specified in [ST2052-1] and [EBU-TT-D].

Both profiles are based on [SUBM].

6. Common Constraints

6.1 Document Encoding

A Document Instance SHALL use UTF-8 character encoding as specified in [UNICODE].

6.2 Foreign Element and Attributes

A Document Instance MAY contain elements and attributes that are neither specifically permitted nor forbidden by a profile.

A transformation processor SHOULD preserve such elements or attributes whenever possible.

Note

Document Instances remain subject to the structural requirements of [TTML1].

6.3 Namespaces

The following namespaces (see [xml-names]) are used in this specification:

Name	Prefix	Value	Defining Specification
XML	`xml`	`http://www.w3.org/XML/1998/namespace`	[xml-names]
TT	`tt`	`http://www.w3.org/ns/ttml`	[TTML1]
TT Parameter	`ttp`	`http://www.w3.org/ns/ttml#parameter`	[TTML1]
TT Styling	`tts`	`http://www.w3.org/ns/ttml#styling`	[TTML1]
TT Feature	none	`http://www.w3.org/ns/ttml/feature/`	[TTML1]
SMPTE-TT Extension	`smpte`	`http://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt`	[ST2052-1]
EBU-TT Styling	`ebutts`	`urn:ebu:tt:style`	[EBU-TT-D]
EBU-TT Metadata	`ebuttm`	`urn:ebu:tt:metadata`	[EBU-TT-D]
IMSC 1.0 Styling	`itts`	`http://www.w3.org/ns/ttml/profile/imsc1#styling`	This specification
IMSC 1.0 Parameter	`ittp`	`http://www.w3.org/ns/ttml/profile/imsc1#parameter`	This specification
IMSC 1.0 Metadata	`ittm`	`http://www.w3.org/ns/ttml/profile/imsc1#metadata`	This specification
IMSC 1.0 Extension	none	`http://www.w3.org/ns/ttml/profile/imsc1/extension/`	This specification
IMSC 1.0 Text Profile Designator	none	`http://www.w3.org/ns/ttml/profile/imsc1/text`	This specification
IMSC 1.0 Image Profile Designator	none	`http://www.w3.org/ns/ttml/profile/imsc1/image`	This specification

The namespace prefix values defined above are for convenience and Document Instances MAY use any prefix value that conforms to [xml-names].

The namespaces defined by this specification are mutable [namespaceState]; all undefined names in these namespaces are reserved for future standardization by the W3C.

6.4 Overflow

A Document Instance SHOULD be authored assuming strict clipping of content that falls out of region areas, regardless of the computed value of tts:overflow for the region.

Note

As specified in [TTML1], tts:overflow has no effect on the extent of the region, and hence the total normalized drawing area S(E_n) at 9.3 Paint Regions.

6.6 Synchronization

Each intermediate synchronic document of the Document Instance is intended to be displayed on a specific frame and removed on a specific frame of the Related Video Object.

When mapping a media time expression M to a frame F of a Related Video Object, e.g. for the purpose of rendering a Document Instance onto the Related Video Object, the presentation processor SHALL map M to the frame F with the presentation time that is the closest to, but not less, than M.

Note

In typical scenario, the same video program (the Related Video Object) will be used for Document Instance authoring, delivery and user playback. The mapping from media time expression to Related Video Object above allows the author to precisely associate subtitle video content with video frames, e.g. around scene transitions. In circumstances where the video program is downsampled during delivery, the application can specify that, at playback, the relative video object be considered the delivered video program upsampled to is original rate, thereby allowing subtitle content to be rendered at the same temporal locations it was authored.

6.7 Extensions

6.7.1 ittp:aspectRatio

The ittp:aspectRatio attributes allows authorial control of the mapping of the root container of a Document Instance to each image frame of the Related Video Object.

If present, the ittp:aspectRatio attribute SHALL conform to the following syntax:

ittp:aspectRatio
  : numerator denominator          // with int(numerator) != 0 and int(denominator) != 0
                                   // where int(s) parses string s as a decimal integer.

numerator | denominator
  : <digit>+

The root container of a Document Instance SHALL be mapped to each image frame of the Related Video Object according to the following:

If ittp:aspectRatio is present, the root container SHALL be mapped to a rectangular area within the image frame such that:
1. the ratio of the width to the height of the rectangular area is equal to ittp:aspectRatio,
2. the center of the rectangular area is collocated with the center of the image frame,
3. the rectangular area is entirely within the image frame, and
4. the rectangular area has a height or width equal to that of the image frame.
Otherwise, the root container of a Document Instance SHALL be mapped to the image frame in its entirety.

An ittp:aspectRatio attribute is considered to be significant only when specified on the tt element.

Example 2

Region with xml:id="A" in the following document would be positioned 20% from the left edge of an image frame with an aspect ratio of 16:9, or 10% from the left edge of an image frame with an aspect ratio of 4:3.

<tt
  xmlns="http://www.w3.org/ns/ttml"
  xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
  xmlns:tts="http://www.w3.org/ns/ttml#styling"
  xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
  xmlns:ittp="http://www.w3.org/ns/ttml/profile/imsc1#parameter"
  ittp:aspectRatio="4 3"
  tts:extent="400px 300px"
  ttp:profile="..."
 >
 ...
 <region xml:id="A" tts:origin="40px 10px" tts:extent="320px 10px">
 ...
</tt>

In other words, tts:extent, when present on the tt element, determines the size of "px" units relative to the root container. ttp:aspectRatio is independently used to map the root container to each image frame of the Related Video Object.

Note

The ittp:aspectRatio parameter effectively defines the intended display aspect ratio (DAR) of the root container, while the tts:extent style property on the root element effectively defines the intended storage aspect ratio (SAR) of the root container.

Note

The mapping algorithm above allows the author to precisely control caption/subtitle position relative to elements within each frame of the video program, e.g. to match the position of actors. This mapping algorithm does not however specify the presentation of either the video frame or root container on the ultimate display device. This presentation depends on many factors, including user input, and can involve displaying only parts of the content. Authors are therefore encouraged to follow best practices for the intended target applications. Below are selected examples:

A 16:9 video program is authored to ensure adequate presentation on 4:3 display devices using a center-cut. Accordingly subtitle/captions are authored using ttp:aspectRatio="4 3", allowing the combination to be displayed on both 4:3 and 16:9 display devices while preserving both caption/subtitles content and the relative position of caption/subtitles with video elements.
A playback system zooms the content of example (a) to fill a 21:9 display, perhaps as instructed by the user. The system elects to scale the root container to fit vertically within the display (maintaining its aspect ratio as authored), at the cost of losing relative positioning between caption/subtitles and video elements.
The system described in (b) instead elects to map the root container to the video frame, maintaining relative positioning between caption/subtitles and video elements but at the risk of clipping subtitles/captions.

6.7.2 ittp:progressivelyDecodable

A progressively decodable Document Instance is structured to facilitate presentation before the document is received in its entirety, and can be identified using ittp:progressivelyDecodable attribute.

A progressively decodable Document Instance is a Document Instance that conforms to the following:

no attribute or element of the TTML timing vocabulary is present within the head element;
given two intermediate synchronic documents A and B of the Document Instance, with start times TA and TB, respectively, TA is not greater than TB if A includes a p element that lexically precedes any p element that B includes;
no attribute of the TTML timing vocabulary is present on a descendant element of p; and
no element E1 explicitly references another element E2 where the opening tag of E2 is lexically subsequent to the opening tag of E1.

If present, the ittp:progressivelyDecodable attribute SHALL conform to the following syntax:

ittp:progressivelyDecodable
  : "true"
  | "false"

An ittp:progressivelyDecodable attribute is considered to be significant only when specified on the tt element.

If not specified, the value of ittp:progressivelyDecodable SHALL be considered to be equal to "false".

A Document Instance for which the computed value of ittp:progressivelyDecodable is "true" SHALL be a progressively decodable Document Instance.

A Document Instance for which the computed value of ittp:progressivelyDecodable is "false" is neither asserted to be a progressively decodable Document Instance nor asserted not to be a progressively decodable Document Instance.

Example 3

<tt
  xmlns="http://www.w3.org/ns/ttml"
  xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
  xmlns:tts="http://www.w3.org/ns/ttml#styling"
  xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
  xmlns:ittp="http://www.w3.org/ns/ttml/profile/imsc1#parameter"
  ittp:progressivelyDecodable="true"
  ttp:profile="..."
 >
 ...
</tt>

Note

[TTML1] specifies explicitly referencing of elements identified using xml:id in the following circumstances:

an element in body referencing region elements. In this case, Requirement 4 above is always satisfied.
an element in body referencing style elements. In this case, Requirement 4 above is always satisfied.
a region element referencing style elements. In this case, Requirement 4 above is always satisfied.
a style element referencing other style elements. In this case, Requirement 4 provides an optimization of style element ordering within the head element.
a ttm:actor element referencing a ttm:agent element. In this case, Requirement 4 provides optimization of metadata elements ordering within the document.
a content element referencing ttm:agent elements using the ttm:agent attribute. In this case, Requirement 4 provides optimization of metadata elements ordering within the document.

6.7.3 itts:forcedDisplay

itts:forcedDisplay can be used to hide content whose computed value of tts:visibility is "visible" when the processor has been configured to do so via the application parameter displayForcedOnlyMode.

If and only if the value of displayForcedOnlyMode is "true", a content element with a itts:forcedDisplay computed value of "false" SHALL NOT produce any visible rendering, but still affect layout, regardless of the computed value of tts:visibility.

The itts:forcedDisplay attribute SHALL conform to the following:

Values:	`false \| true`
Initial:	`false`
Applies to:	`body`, `div`, `p`, `region`, `span`
Inherited:	yes
Percentages:	N/A
Animatable:	discrete

Annex C. Forced content (non-normative) illustrates the use of itts:forcedDisplay in an application in which a single document contains both hard of hearing captions and translated foreign language subtitles, using itts:forcedDisplay to display translation subtitles always, independently of whether the hard of hearing captions are displayed or hidden.

The presentation processor SHALL accept an optional boolean parameter called displayForcedOnlyMode, whose value MAY be set by a context external to the presentation processor. If not set, the value of displayForcedOnlyMode SHALL be assumed to be equal to "false".

The algorithm for setting the displayForcedOnlyMode parameter based on the circumstances under which the Document Instance is presented is left to the application.

Example 4

...
<head>
	...
	<region xml:id="r1" tts:origin="10% 2%" tts:extent="80% 10%" tts:color="white" itts:forcedDisplay="true" tts:backgroundColor="black"/>
	<region xml:id="r2" tts:origin="10% 80%" tts:extent="80% 88%" tts:color="white" tts:backgroundColor="black"/>
	...
</head>
...
<div>
	 <p region="r1" begin="1s" end="6s">Lycée</p>

	 <!-- the following will not appear if displayForcedOnlyMode='true' -->
	 <p region="r2" begin="4s" end="6s">Nous étions inscrits au même lycée.</p>
</div>
...

Note

As specified in [TTML1], the background of a region can be visible even if the computed value of tts:visibility equals "hidden" for all active content within. The background of a region for which itts:forcedDisplay equals "true" can therefore remain visible even if itts:forcedDisplay equals "false" for all active content elements within the region and displayForcedOnlyMode equals "true". Authors can avoid this situation, for instance, by ensuring that content elements and the regions that they are flowed into always have the same value of itts:forcedDisplay.

Note

Although itts:forcedDisplay, like all the TTML style attributes, has no defined semantics on a br content element, itts:forcedDisplay will apply to a br content element if it is either defined on an ancestor content element of the br content element or it is applied to a region element corresponding to a region that the br content element is being flowed into.

Note

It is expected that the functionality of itts:forcedDisplay will be mapped to a conditional style construct in a future revision of this specification.

Note

The presentation semantics associated with itts:forcedDisplay are intended to be compatible with those associated with the forcedDisplayMode attribute defined in [CFF].

6.7.4 ittm:altText

ittm:altText allows an author to provide a text string equivalent for an element, typically an image. This text equivalent MAY be used to support indexing of the content and also facilitate quality checking of the document during authoring.

The ittm:altText element SHALL conform to the following syntax:

<ittm:altText
  xml:id = ID
  xml:lang = string
  xml:space = (default|preserve)
  {any attribute not in the default namespace, any TT namespace or any IMSC 1.0 namespace}>
  Content: #PCDATA
</ittm:altText>

The ittm:altText element SHALL be a child of the metadata element.

8. Image Profile Constraints specifies the use of the ittm:altText element with images.

Example 5

...
<div region="r1" begin="1s" end="6s" smpte:backgroundImage="1.png">
  <metadata>
  <ittm:altText>Nous étions inscrits au même lycée.</ttm:title>
  </metadata>
</div>
...

Note

In contrast to the common use of alt attributes in [HTML5], the ittm:altText attribute content is not intended to be displayed in place of the element if the element is not loaded. The ittm:altText attribute content can however be read and used by assistive technologies.

6.8 Region

6.8.1 Presented Region

A presented region is a temporally active region that satisfies the following conditions:

the computed value of tts:opacity is not equal to "0.0"; and
the computed value of tts:display is not "none"; and
the computed value of tts:visibility is not "hidden"; and
either (a) content is selected into the region or (b) the computed value of tts:showBackground is equal to "always" and the computed value of tts:backgroundColor has non-transparent alpha.

6.8.2 Dimensions and Position

All regions SHALL NOT extend beyond the root container, i.e. the intersection of the sets of coordinates belonging to a region and the sets of coordinates belonging to the root container is the set of coordinates belonging to the region.

No two presented regions in a given intermediate synchronic document SHALL overlap, i.e. the intersection of the sets of coordinates within each presented region is empty.

6.8.3 Maximum number

The number of presented regions in a given intermediate synchronic document SHALL NOT be greater than 4.

6.9 Profile Signaling

The ttp:profile attribute SHOULD be present on the tt element and equal to the designator of the IMSC1 profile to which the Document Instance conforms, and the ttp:profile element SHOULD NOT be present, unless:

the Document Instance also conforms to [EBU-TT-D], in which case the ttp:profile attribute and the ttp:profile element SHOULD NOT be present, and instead the designator of the IMSC1 profile to which the Document Instance conforms and the URI "urn:ebu:tt:distribution:2014-01" SHOULD each be carried in an ebuttm:conformsToStandard element as specified in [EBU-TT-D]; or
the Document Instance also conforms to [ttml10-sdp-us], in which case the ttp:profile attribute SHOULD NOT be present. [ttml10-sdp-us] requires that the ttp:profile element be present and that its use attribute be set to a specified value.

The ttp:profile and ebuttm:conformsToStandard elements SHALL NOT signal conformance to both Image Profile and Text Profile in a given Document Instance.

6.10 Hypothetical Render Model

It SHALL be possible to apply the Hypothetical Render Model specified in Section 9. Hypothetical Render Model to any sequence of consecutive intermediate synchronic documents without error as defined in Section 9.2 General.

6.11 Features and Extensions

See 4. Conformance for a definition of permitted and prohibited.

Feature	Disposition	Additional provision
Relative to the TT Feature namespace
`#animation`	permitted
`#backgroundColor-block`	permitted
`#backgroundColor-region`	permitted
`#cellResolution`	permitted	If the Document Instance includes any length value that uses the `c` expression, `ttp:cellResolution` SHOULD be present on the `tt` element.
`#clockMode`	prohibited
`#clockMode-gps`	prohibited
`#clockMode-local`	prohibited
`#clockMode-utc`	prohibited
`#core`	permitted
`#display-block`	permitted
`#display-inline`	permitted
`#display-region`	permitted
`#display`	permitted
`#dropMode`	prohibited
`#dropMode-dropNTSC`	prohibited
`#dropMode-dropPAL`	prohibited
`#dropMode-nonDrop`	prohibited
`#extent-root`	permitted	If the Document Instance includes any length value that uses the `px` expression, `tts:extent` SHALL be present on the `tt` element.
`#extent`	permitted
`#frameRate`	permitted	If the Document Instance includes any clock time expression that uses the `frames` term or any offset time expression that uses the `f` metric, the `ttp:frameRate` attribute SHALL be present on the `tt` element.
`#frameRateMultiplier`	permitted
`#layout`	permitted
`#length-cell`	permitted	`c` units SHALL NOT be present outside of the value of `ebutts:linePadding`.
`#length-integer`	permitted
`#length-negative`	prohibited
`#length-percentage`	permitted
`#length-pixel`	permitted
`#length-positive`	permitted
`#length-real`	permitted
`#length`	permitted
`#markerMode`	prohibited
`#markerMode-continuous`	prohibited
`#markerMode-discontinuous`	prohibited
`#metadata`	permitted
`#opacity`	permitted
`#origin`	permitted
`#overflow`	permitted
`#overflow-visible`	permitted
`#pixelAspectRatio`	prohibited
`#presentation`	permitted	See constraints applied to `#profile.`
`#profile`	permitted	See 6.9 Profile Signaling.
`#showBackground`	permitted
`#structure`	permitted
`#styling-chained`	permitted
`#styling-inheritance-content`	permitted
`#styling-inheritance-region`	permitted
`#styling-inline`	permitted
`#styling-nested`	permitted
`#styling-referential`	permitted
`#styling`	permitted
`#subFrameRate`	prohibited
`#tickRate`	permitted	`ttp:tickRate` SHALL be present on the `tt` element if the document contains any time expression that uses the `t` metric.
`#timeBase-clock`	prohibited
`#timeBase-media`	permitted	NOTE: [TTML1] specifies that the default timebase is `"media"` if `ttp:timeBase` is not specified on `tt`.
`#timeBase-smpte`	prohibited
`#time-clock-with-frames`	permitted
`#time-clock`	permitted
`#time-offset-with-frames`	permitted
`#time-offset-with-ticks`	permitted
`#time-offset`	permitted
`#timeContainer`	permitted
`#timing`	permitted	All time expressions within a Document Instance SHOULD use the same syntax, either `clock-time` or `offset-time`. For any content element that contains `br` elements or text nodes or a `smpte:backgroundImage` attribute, the `begin` and `end` attributes SHOULD be specified on the content element or at least one of its ancestors.
`#transformation`	permitted	See constraints at #profile.
`#visibility-block`	permitted
`#visibility-region`	permitted
`#writingMode-horizontal-lr`	permitted
`#writingMode-horizontal-rl`	permitted
`#writingMode-horizontal`	permitted
`#zIndex`	permitted
Extension	Disposition	Provisions
Relative to the IMSC 1.0 Extension namespace
`#aspectRatio`	permitted
`#forcedDisplay`	permitted
`#progressivelyDecodable`	permitted
`#altText`	permitted

Note

As specified in [TTML1], a #time-offset-with-frames expression is translated to a media time M according to M = 3600 · hours + 60 · minutes + seconds + (frames ÷ (ttp:frameRateMultiplier · ttp:frameRate)).

7. Text Profile Constraints

7.1 Profile Designator

This profile is associated with the following profile designator:

Profile Name	Profile Designator
IMSC 1.0 Text	`http://www.w3.org/ns/ttml/profile/imsc1/text`

Note

As specified in 6.11 Features and Extensions, the presence of the ttp:profile attribute is not required by this profile. The profile designator specified above is intended to be generally used to signal conformance of a Document Instance to the profile. The details of such signaling depends on the application, and can, for instance, use metadata structures out-of-band of the Document Instance.

7.2 Recommended Character Sets

A Document Instance SHOULD be authored using characters selected from the sets specified in B. Recommended Character Sets.

7.3 Reference Fonts

The flow of text within a region depends the dimensions and spacing (kerning) between individual glyphs. The following allows, for instance, region extents to be set such that text flows without clipping.

When rendering codepoints matching one of the combinations of computed font family and codepoints listed in A. Reference Fonts, a processor SHALL use a font that generates a glyph sequence whose dimension is substantially identical to the glyph sequence that would have been generated by one of the specified reference fonts.

Note

Implementations can use fonts other than those specified in A. Reference Fonts. Two fonts with equal metrics can have a different appearance, but flow identically.

7.4 Features and Extensions

See 4. Conformance for a definition of permitted and prohibited.

Feature	Disposition	Additional provisions
Relative to the TT Feature namespace
`#backgroundColor-inline`	permitted
`#backgroundColor`	permitted
`#bidi`	permitted
`#content`	permitted
`#color`	permitted	The initial value of `tts:color` SHALL be `"white"`. NOTE: This is consistent with [ST2052-1].
`#direction`	permitted
`#displayAlign`	permitted
`#extent-region`	permitted	The `tts:extent` attribute SHALL be present on all `region` elements, where it SHALL use either `px` units or "percentage" syntax.
`#fontFamily-generic`	permitted	In absence of specific instructions on the choice of font families, and in order to enhance reproducibility of line fitting, authors are encouraged to use the `monospaceSerif` or `proportionalSansSerif` generic font families, for which reference font metrics are defined at A. Reference Fonts. If the computed value of `tts:fontFamily` is `"default"`, then the used value of `tts:fontFamily` SHALL be `"monospaceSerif"`. NOTE: The term used value is defined in CSS 2.1, as normatively referenced by [TTML1].
`#fontFamily-non-generic`	permitted
`#fontFamily`	permitted
`#fontSize-anamorphic`	prohibited
`#fontSize-isomorphic`	permitted
`#fontSize`		See individual disposition of `#fontSize-anamorphic` and `#fontSize-isomorphic`.
`#fontStyle-italic`	permitted
`#fontStyle-oblique`	permitted
`#fontStyle`	permitted
`#fontWeight-bold`	permitted
`#fontWeight`	permitted
`#length-em`	permitted
`#lineBreak-uax14`		The processor SHALL implement the `#lineBreak-uax14` feature defined in the TT Feature namespace.
`#lineHeight`	permitted	As implementation of the `"normal"` value is not uniform at the time of this writing, `tts:lineHeight` SHOULD NOT be set to `"normal"` and SHOULD be explicitly specified such that the specified style set of each `p` element contains a `tts:lineHeight` property whose value is not assigned by initial value fallback.
`#nested-div`	permitted
`#nested-span`	permitted
`#origin`	permitted	The `tts:origin` attribute SHALL use `px` units or "percentage" representation, and SHALL NOT use `em` units.
`#padding-1`	permitted
`#padding-2`	permitted
`#padding-3`	permitted
`#padding-4`	permitted
`#padding`	permitted
`#textAlign-absolute`	permitted
`#textAlign-relative`	permitted
`#textAlign`	permitted
`#textDecoration-over`	permitted
`#textDecoration-through`	permitted
`#textDecoration-under`	permitted
`#textDecoration`	permitted
`#textOutline-blurred`	prohibited
`#textOutline-unblurred`	permitted
`#textOutline`	permitted	The computed value of `tts:textOutline` on a `span` element SHALL be 10% or less than the computed value of `tts:fontSize` on the same element.
`#unicodeBidi`	permitted
`#visibility`	permitted
`#visibility-inline`	permitted
`#wrapOption`	permitted
`#writingMode`	permitted
`#writingMode-vertical`	permitted
Extension	Disposition	Provisions
Relative to the SMPTE-TT Extension Namespace
`#image`	prohibited
Relative to the IMSC 1.0 Extension namespace
`#linePadding`	permitted	If used, the attribute `ebutts:linePadding` MAY be specified on elements `region`, `body`, `div` and `p` in addition to `style`. The processor: SHALL apply `ebutts:linePadding` to `p` only; and SHALL treat `ebutts:linePadding` as inheritable. NOTE: The `ebutts:linePadding` attribute only supports `c` length units.
`#multiRowAlign`	permitted	If used, the attribute `ebutts:multiRowAlign` MAY be specified on elements `region`, `body`, `div` and `p` in addition to `style` The processor: SHALL apply `ebutts:multiRowAlign` to `p` only; and SHALL treat `ebutts:multiRowAlign` as inheritable.

Note

In contrast to this specification, [EBU-TT-D] specifies that the attributes ebutts:linePadding and ebutts:multiRowAlign are allowed only on the style element.

8. Image Profile Constraints

8.1 Profile Designator

This profile is associated with the following profile designator:

Profile Name	Profile Designator
IMSC 1.0 Image	`http://www.w3.org/ns/ttml/profile/imsc1/image`

Note

8.2 Presented Image

8.2.1 Definition

A presented image is a div element with a smpte:backgroundImage attribute that flows into a presented region.

8.2.2 Constraints

In a given intermediate synchronic document, each presented region SHALL contain at most one div element, which SHALL be a presented image.

8.2.3 Intermediate Synchronic Document Construction

For the purposes of constructing an intermediate synchronic document, a div element with a smpte:backgroundImage attribute SHALL NOT be considered empty.

8.3 smpte:backgroundImage Constraints

If a smpte:backgroundImage attribute is applied to a div element:

the width and height (in pixels) of the image source referenced by smpte:backgroundImage SHALL be equal to the width and height (as specified by the tts:extent attribute using px units) of the region in which the div element is presented;
the div element SHOULD contain a metadata element containing an ittm:altText element that is a Text Alternative of the image referenced by the smpte:backgroundImage attribute; and
The smpte:backgroundImage attribute SHALL reference a PNG datastream as specified in [PNG]. If a pHYs chunk is present, it SHALL indicate square pixels. Note that if no pixel aspect ratio is carried, the default of square pixels is assumed.

Note

In [TTML1], tts:extent and tts:origin do not apply to div elements. In order to individually position multiple div elements, each div can be associated with a distinct region with the desired tts:extent and tts:origin.

8.4 Features and Extensions

See 4. Conformance for a definition of permitted and prohibited.

Feature	Disposition	Additional provisions
Relative to the TT Feature namespace
`#backgroundColor-inline`	prohibited
`#backgroundColor`		See individual disposition of `#backgroundColor-inline`, `#backgroundColor-region` and `#backgroundColor-block`.
`#bidi`		See individual disposition of `#direction`, `#unicodeBidi` and `#writingMode-horizontal`.
`#color`	prohibited
`#content`	permitted	The `p`, `span` and `br` elements SHALL NOT be present. See Section 8.2.2 Constraints for constraints on `div` elements.
`#direction`	prohibited
`#displayAlign`	prohibited
`#extent-region`	permitted	The `tts:extent` attribute SHALL be present on all `region` elements, where it SHALL use `px` units.
`#fontFamily`	prohibited
`#fontFamily-generic`	prohibited
`#fontFamily-non-generic`	prohibited
`#fontSize`	prohibited
`#fontSize-anamorphic`	prohibited
`#fontSize-isomorphic`	prohibited
`#fontStyle`	prohibited
`#fontStyle-italic`	prohibited
`#fontStyle-oblique`	prohibited
`#fontWeight`	prohibited
`#fontWeight-bold`	prohibited
`#length-em`	prohibited
`#lineBreak-uax14`		No processor requirement is specified.
`#lineHeight`	prohibited
`#nested-div`	prohibited
`#nested-span`	prohibited	NOTE: The prohibition of `span` elements by this profile implies the prohibition of this feature.
`#padding`	prohibited
`#padding-1`	prohibited
`#padding-2`	prohibited
`#padding-3`	prohibited
`#padding-4`	prohibited
`#textAlign`	prohibited
`#textAlign-absolute`	prohibited
`#textAlign-relative`	prohibited
`#textDecoration`	prohibited
`#textDecoration-over`	prohibited
`#textDecoration-through`	prohibited
`#textDecoration-under`	prohibited
`#textOutline`	prohibited
`#textOutline-blurred`	prohibited
`#textOutline-unblurred`	prohibited
`#unicodeBidi`	prohibited
`#visibility`		See individual disposition of `#visibility-inline`, `#visibility-region` and `#visibility-block`.
`#visibility-inline`	prohibited
`#wrapOption`	prohibited
`#writingMode`		See individual disposition of `#writingMode-vertical` and `#writingMode-horizontal`.
`#writingMode-vertical`	prohibited
Extension	Disposition	Provisions
Relative to the SMPTE-TT Extension namespace
`#image`	permitted	`smpte:backgroundImage` MAY be used according to 8.3 smpte:backgroundImage Constraints with the semantics of the attribute defined by Section 5.5.2 of [ST2052-1]. `smpte:backgroundImageHorizontal` and `smpte:backgroundImageVertical` SHALL NOT be used. `smpte:image` SHALL NOT be used.

9. Hypothetical Render Model

9.1 Overview (non-normative)

This Section specifies the Hypothetical Render Model illustrated in Fig. 1 Hypothetical Render Model .

The purpose of the model is to limit Document Instance complexity. It is not intended as a specification of the processing requirements for implementations. For instance, while the model defines a glyph buffer for the purpose of limiting the number of glyphs displayed at any given point in time, it neither requires the implementation of such a buffer, nor models the sub-pixel character positioning and anti-aliased glyph rendering that can be used to produce text output.

The model operates on successive intermediate synchronic documents obtained from an input Document Instance, and uses a simple double buffering model: while an intermediate synchronic document E_n is being painted into Presentation Buffer P_n (the "front buffer" of the model), the previous intermediate synchronic document E_n-1 is available for display in Presentation Buffer P_n-1 (the "back buffer" of the model).

The model specifies an (hypothetical) time required for completely painting an intermediate synchronic document as a proxy for complexity. Painting includes drawing region backgrounds, rendering and copying glyphs, and decoding and copying images. Complexity is then limited by requiring that painting of intermediate synchronic document E_n completes before the end of intermediate synchronic document E_n-1.

Whenever applicable, constraints are specified relative to root container dimensions, allowing subtitle sequences to be authored independently of Related Video Object resolution.

To enable scenarios where the same glyphs are used in multiple successive intermediate synchronic documents, e.g. to convey a CEA-608/708-style roll-up (see [CEA-608] and [CEA-708]), the Glyph Buffers G_n and G_n-1 store rendered glyphs across intermediate synchronic documents, allowing glyphs to be copied into the Presentation Buffer instead of rendered, a more costly operation.

Similarly, Decoded Image Buffers D_n and D_n-1 store decoded images across intermediate synchronic documents, allowing images to be copied into the Presentation Buffer instead of decoded.

9.2 General

The Presentation Compositor SHALL render in Presentation Buffer P_n each successive intermediate synchronic document E_n using the following steps in order:

clear the pixels, except for the first intermediate synchronic document E₀ for the which the pixels of P₀ SHALL be assumed to have been cleared;
paint, according to stacking order, all background pixels for each region;
paint all pixels for background colors associated with text or image subtitle content; and
paint the text or image subtitle content.

The Presentation Compositor SHALL start rendering E_n:

at the presentation time of E₀ minus Initial Painting Delay (IPD), if n = 0; or
at the presentation time of E_n-1, if n > 0.

The duration DUR(E_n) for painting an intermediate synchronic document E_n in the Presentation Buffer P_n SHALL be:

DUR(E_n) = S(E_n) / BDraw + DUR_T(E_n) + DUR_I(E_n)

where

S(E_n) is the total normalized drawing area for intermediate synchronic document E_n, as specified in 9.3 Paint Regions;
BDraw is the normalized background drawing performance factor;
DUR_T(E_n) is the duration, in seconds, for painting the text subtitle content for intermediate synchronic document E_n, as specified in Section 9.5 Paint Text; and
DUR_I(E_n) is the duration, in seconds, for painting the image subtitle content for intermediate synchronic document E_n, as specified in Section .

The contents of the Presentation Buffer P_n SHALL be transferred instantaneously to Presentation Buffer P_n-1 at the presentation time of intermediate synchronic document E_n, making the latter available for display.

Note

It is possible for the contents of Presentation Buffer P_n-1 to never be displayed. This can happen if Presentation Buffer P_n is copied twice to Presentation Buffer P_n-1 between two consecutive video frame boundaries of the Related Video Object.

It SHALL be an error for the Presentation Compositor to fail to complete painting pixels for E_n before the presentation time of E_n.

Unless specified otherwise, the following table SHALL specify values for IPD and BDraw.

Parameter	Initial value
Initial Painting Delay (IPD)	1 s
Normalized background drawing performance factor (BDraw)	12 s^-1

Note

BDraw effectively sets a limit on fillings regions - for example, assuming that the root container is ultimately rendered at 1920×1080 resolution, a BDraw of 12 s^-1 would correspond to a fill rate of 1920×1080×12/s=23.7×2²⁰pixels s^-1.

Note

IPD effectively sets a limit on the complexity of any given intermediate synchronic document.

9.3 Paint Regions

The total normalized drawing area S(E_n) for intermediate synchronic document E_n SHALL be

S(E_n) = CLEAR(E_n) + PAINT(E_n )

where CLEAR(E₀) = 0 and CLEAR(E_{n | n > 0}) = 1, i.e. the root container in its entirety.

Note

To ensure consistency of the Presentation Buffer, a new intermediate synchronic document requires clearing of the root container.

PAINT(E_n) SHALL be the normalized area to be painted for all regions that are used in intermediate synchronic document E_n according to:

PAINT(E_n) = ∑_{R_i∈R_p} NSIZE(R_i) ∙ NBG(R_i)

where R_p SHALL be the set of presented regions in the intermediate synchronic document E_n.

NSIZE(R_i) SHALL be given by:

NSIZE(R_i) = (width of R_i ∙ height of R_i ) ÷ (root container height ∙ root container width)

Example 6

For a region R_i in with tts:extent="250px 50px" within a root container with tts:extent="1920px 1080px", NSIZE(R_i) = 0.603.

NBG(R_i) SHALL be the total number of tts:backgroundColor attributes associated with the given region R_i in the intermediate synchronic document. A tts:backgroundColor attribute is associated with a region when it is explicitly specified (either as an attribute in the element, or by reference to a declared style) in the following circumstances:

it is specified on the region layout element that defines the region; or
it is specified on a div, p, span or br content element that is to be flowed into the region for presentation in the intermediate synchronic document (see [TTML1] for more details on when a content element is followed into a region); or
it is specified on a set animation element that is to be applied to content elements that are to be flowed into the region for presentation in the intermediate synchronic document (see [TTML1] for more details on when a set animation element is applied to content elements).

Even if a specified tts:backgroundColor is the same as specified on the nearest ancestor content element or animation element, specifying any tts:backgroundColor SHALL require an additional fill operation for all region pixels.

9.4 Paint Images

The Presentation Compositor SHALL paint into the Presentation Buffer P_n all visible pixels of presented images of intermediate synchronic document E_n.

For each presented image, the Presentation Compositor SHALL either:

if an identical image is present in Decoded Image Buffer D_n, copy the image from Decoded Image Buffer D_n to the Presentation Buffer P_n using the Image Copier; or
if an identical image is present in Decoded Image Buffer D_n-1, i.e. an identical image was present in intermediate synchronic document E_n-1, copy using the Image Copier the image from Decoded Image Buffer D_n-1 to both the Decoded Image Buffer D_n and the Presentation Buffer P_n; or
otherwise, decode the image using the Image Decoder the image into the Presentation Buffer P_n and Decoded Image Buffer D_n.

Two images SHALL be identical if and only if they reference the same encoded image source.

The duration DUR_I(E_n) for painting images of an intermediate synchronic document E_n in the Presentation Buffer SHALL be as follows:

DUR_I(E_n) = ∑_{I_i ∈ I_c} NRGA(I_i) / ICpy + ∑_{I_j ∈ I_d} NSIZ(I_j) / IDec

where

I_c is the set of images copied when painting intermediate synchronic document E_n;
I_d is the set of images decoded when painting intermediate synchronic document E_n;
IDec is the image decoding rate; and
ICpy is the normalized image copy performance factor.

NRGA(I_i) is the Normalized Image Area of presented image I_i and SHALL be equal to:

NRGA(I_i)= (width of I_i ∙ height of I_i ) ÷ ( root container height ∙ root container width )

NSIZ(I_i) SHALL be the number of pixels of presented image I_i.

The contents of the Decoded Image Buffer D_n SHALL be transferred instantaneously to Decoded Image Buffer D_n-1 at the presentation time of intermediate synchronic document E_n.

The total size occupied by images stored in Decoded Image Buffers D_n or D_n-1 SHALL be the sum of their Normalized Image Area.

The size of Decoded Image Buffers D_n or D_n-1 SHALL be the Normalized Decoded Image Buffer Size (NDIBS).

Unless specified otherwise, the following table SHALL specify ICpy, IDec, and NDBIS.

Parameter	Initial value
Normalized image copy performance factor (ICpy)	6
Image Decoding rate (IDec)	1 × 2²⁰ pixels s^-1
Normalized Decoded Image Buffer Size (NDIBS)	0.9885

9.5 Paint Text

In the context of this section, a glyph is a tuple consisting of (i) one character and (ii) the computed values of the following style properties:

tts:color
tts:fontFamily
tts:fontSize
tts:fontStyle
tts:fontWeight
tts:textDecoration
tts:textOutline

Note

While one-to-one mapping between characters and typographical glyphs is generally the rule in some scripts, e.g. latin script, it is the exception in others. For instance, in arabic script, a character can yield multiple glyphs depending on its position in a word. The Hypothetical Render Model always assumes a one-to-one mapping, but reduces the performance of the glyph buffer for scripts where one-to-one mapping is not the general rule (see GCpy below).

For each glyph associated with a character in a presented region of intermediate synchronic document E_n, the Presentation Compositor SHALL:

if an identical glyph is present in Glyph Buffer G_n, copy the glyph from Glyph Buffer G_n to the Presentation Buffer P_n using the Glyph Copier; or
if an identical glyph is present in Glyph Buffer G_n-1, i.e. an identical glyph was present in intermediate synchronic document E_n-1, copy using the Glyph Copier the glyph from Glyph Buffer G_n-1 to both the Glyph Buffer G_n and the Presentation Buffer P_n; or
otherwise render using the Glyph Renderer the glyph into the Presentation Buffer P_n and Glyph Buffer G_n.

Fig. 2 Example of Presentation Compositor Behavior for Text Rendering

The duration DUR_T(E_n) for rendering the text of an intermediate synchronic document E_n in the Presentation Buffer is as follows:

DUR_T(E_n) = ∑_{g_i ∈ Γ_r} NRGA(g_i) / Ren(g_i) + ∑_{g_j ∈ Γ_c} NRGA(g_j) / GCpy

where

Γ_r is the set of glyphs rendered into the Presentation Buffer P_n using the Glyph Renderer in intermediate synchronic document E_n;
Γ_c is the set of glyphs copied to the Presentation Buffer P_n using the Glyph Copier in intermediate synchronic document E_n;
Ren(g_i) is the text rendering performance factor for glyph g_i; and
GCpy is the normalized glyph copy performance factor.

The Normalized Rendered Glyph Area NRGA(g_i) of a glyph g_i SHALL be equal to:

NRGA(g_i) = (fontSize of g_i as percentage of root container height)²

Note

NRGA(G_i) does not take into account decorations (e.g. underline), effects (e.g. outline) or actual typographical glyph aspect ratio. An implementation can determine an actual buffer size needs based on worst-case glyph size complexity.

The contents of the Glyph Buffer G_n SHALL be copied instantaneously to Glyph Buffer G_n-1 at the presentation time of intermediate synchronic document E_n.

It SHALL be an error for the sum of NRGA(g_i) over all glyphs Glyph Buffer G_n to be larger than the Normalized Glyph Buffer Size (NGBS).

Unless specified otherwise, the following table specifies values of GCpy, Ren and NGBS.

Normalized glyph copy performance factor (GCpy)
Script property (see Standard Annex #24 at [UNICODE]) for the character of g_i	GCpy
latin, greek, cyrillic, hebrew or common	12
any other value	3
Text rendering performance factor Ren(G_i)
Block property (see [UNICODE]) for the character of g_i	Ren(G_i)
CJK Unified Ideograph	0.6
any other value	1.2
Normalized Glyph Buffer Size (NGBS)
1

Note

The choice of font by the presentation processor can increase rendering complexity. For instance, a cursive font can generally result in a given character yielding different typographical glyphs depending on context, even if latin script is used.

Computed Font Family	Code Points	Reference Font
`monospaceSerif`	All code points specified in B. Recommended Character Sets	Courier New or Liberation Mono
`proportionalSansSerif`	All code points specified in B. Recommended Character Sets, excluding the code points defined for Hebrew and Arabic scripts.	Arial or Helvetica or Liberation Sans

Table 1. Common Character Set.
(Basic Latin)
U+0020 - U+007E
(Latin-1 Supplement)
U+00A0 - U+00FF
(Latin Extended-A)
U+0152 : LATIN CAPITAL LIGATURE OE
U+0153 : LATIN SMALL LIGATURE OE
U+0160 : LATIN CAPITAL LETTER S WITH CARON
U+0161 : LATIN SMALL LETTER S WITH CARON
U+0178 : LATIN CAPITAL LETTER Y WITH DIAERESIS
U+017D : LATIN CAPITAL LETTER Z WITH CARON
U+017E : LATIN SMALL LETTER Z WITH CARON
(Latin Extended-B)
U+0192 : LATIN SMALL LETTER F WITH HOOK
(Spacing Modifier Letters)
U+02DC : SMALL TILDE
(Combining Diacritical Marks)
U+0301 : COMBINING ACUTE ACCENT
(General Punctuation)
U+2010 - U+2015 : Dashes
U+2016 - U+2027 : General punctuation
U+2030 - U+203A : General punctuation
(Currency symbols)
U+20AC : EURO SIGN
(Letterlike Symbols)
U+2103 : DEGREES CELSIUS
U+2109 : DEGREES FAHRENHEIT
U+2120 : SERVICE MARK SIGN
U+2122 : TRADE MARK SIGN
(Number Forms)
U+2153 - U+215F : Fractions
(Mathematical Operators)
U+2212 : MINUS SIGN
U+221E : INFINITY
(Box Drawing)
U+2500 : BOX DRAWINGS LIGHT HORIZONTAL
U+2502 : BOX DRAWINGS LIGHT VERTICAL
U+250C : BOX DRAWINGS LIGHT DOWN AND RIGHT
U+2510 : BOX DRAWINGS LIGHT DOWN AND LEFT
U+2514 : BOX DRAWINGS LIGHT UP AND RIGHT
U+2518 : BOX DRAWINGS LIGHT UP AND LEFT
(Block Elements)
U+2588 : FULL BLOCK
(Geometric Shapes)
U+25A1 : WHITE SQUARE
(Musical Symbols)
U+2669 : QUARTER NOTE
U+266A : EIGHTH NOTE
U+266B : BEAMED EIGHTH NOTES

Table 2. Supplementary Character Sets.
Primary language subtag	Characters
sq, fi, da, nl, en, de, is, no, sv, ca, fr, it	no supplementary characters
lv, lt, et, tr, hr, cs, pl, sl, sk	(Latin Extended-A) U+0100 - U+017F
ro	(Latin Extended-A) U+0100 - U+017F (Latin Extended-B) U+0218 - U+0219 U+021A - U+021B
el	(Combining Diacritical Marks) U+0308 (Greek and Coptic) U+0386 - U+038A U+038C U+038E - U+03A1 U+03A3 - U+03CE
pt, es	(Currency symbols) U+20A1 - U+20A2 U+20B3
ar	(Arabic) U+0609 U+060C - U+060D U+061B U+061E - U+061F U+0621 - U+063A U+0640 - U+0652 U+0660 - U+066D U+0670
he	(Hebrew) U+05B0 - U+05C3 U+05D0 - U+05EA U+05F3 - U+05F4
bs, bg, mk, ru, sr, uk	(Latin Extended-A) U+0100 - U+017F (Spacing Modifier Letters) U+02BC (Cyrillic) U+0400 - U+045F U+048A - U+04F9 (Letterlike Symbols) U+2116
kk	(Latin Extended-A) U+0100 - U+017F (Cyrillic) U+0400 - U+045F U+048A - U+04F9
hu	(Latin Extended-A) U+0100 - U+017F (General Punctuation) U+2052 (Miscellaneous Mathematical Symbols-A) U+27E8–U+27E9

I. Compatibility with other TTML-based specifications (non-normative)

I.1 Overview

This specification is designed to be compatible with [ST2052-1], [EBU-TT-D] and [ttml10-sdp-us]. Specifically, it is possible to create a document that:

conforms to one of [ST2052-1], [EBU-TT-D] or [ttml10-sdp-us], and also conforms to Text Profile; or
conforms to [ST2052-1], and also conforms to Image Profile.

This specification is also intended to allow straightforward conversion of a document that conforms to the text or image profiles of [CFF] to the Text Profile or Image Profile, respectively.

I.2 EBU-TT-D

The Text Profile is a strict syntactic superset of [EBU-TT-D].

A document that conforms to [EBU-TT-D] therefore generally also conforms to the Text Profile, with a few exceptions, including:

[EBU-TT-D] does not constrain document complexity using an HRM;
[EBU-TT-D] recommends use of UTF-8 encoding but allows alternate encodings; and
[EBU-TT-D] does not constrain the number of presented regions in a given intermediate synchronic document.

Note that the ttp:profile attribute is not allowed by [EBU-TT-D], and the ebuttm:conformsToStandard element is used instead to signal Text Profile, as specified in 6.9 Profile Signaling.

It is not possible for a document that conforms to [EBU-TT-D] to also conform to Image Profile, and vice-versa, notwithstanding the special case where the document also conforms to Text Profile as noted at 5. Profiles.

The following is an example of a document that conforms to both Text Profile and [EBU-TT-D]. Note the presence of two ebuttm:conformsToStandard elements, one of which equals the Text Profile designator:

Example 12

<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://www.w3.org/ns/ttml" xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
    xmlns:tts="http://www.w3.org/ns/ttml#styling" xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
    xmlns:ebutts="urn:ebu:tt:style" xml:lang="en" ttp:timeBase="media" xmlns:ebuttm="urn:ebu:tt:metadata" >
    <head>
        <metadata>
            <ebuttm:documentMetadata>
                <ebuttm:conformsToStandard>urn:ebu:tt:distribution:2014-01</ebuttm:conformsToStandard>
                <ebuttm:conformsToStandard>http://www.w3.org/ns/ttml/profile/imsc1/text</ebuttm:conformsToStandard>
            </ebuttm:documentMetadata>
        </metadata>
        <styling>
            <style xml:id="baseStyle" tts:color="#FFFFFF" tts:lineHeight="100%"/>
            <style xml:id="blackBackground" tts:backgroundColor="#000000"/>
            <style xml:id="greenBackground" tts:backgroundColor="#00FF00"/>
            <style xml:id="startEnd" tts:textAlign="start" ebutts:multiRowAlign="end"/>
            <style xml:id="centerStart" tts:textAlign="center" ebutts:multiRowAlign="start"/>
        </styling>
        <layout>
            <region xml:id="area1" tts:origin="15% 10%" tts:extent="70% 20%" style="greenBackground" tts:displayAlign="center"/>
            <region xml:id="area2" tts:origin="15% 70%" tts:extent="70% 20%" style="blackBackground" tts:displayAlign="center"/>
        </layout>
    </head>
    <body>
        <div style="baseStyle">
            <p xml:id="s1" region="area1" style="startEnd" begin="00:00:01" end="00:00:09">
                multiRowAlign="end"<br/>textAlign="start"
            </p>
            <p xml:id="s2" region="area2" style="centerStart" begin="00:00:01" end="00:00:09">
                multiRowAlign="start"<br/>textAlign="center"
            </p>
        </div>
    </body>
</tt>

I.3 SDP-US

The Text Profile is a strict syntactic superset of [ttml10-sdp-us].

A document that conforms to [ttml10-sdp-us] therefore also generally conforms to the Text Profile, with a few exceptions, including:

[ttml10-sdp-us] does not constrain document complexity using an HRM.

[ttml10-sdp-us] requires a specific value of the use attribute of the ttp:profile. As a result, Text Profile is not signaled using the ttp:profile attribute. Instead, as specified in 5.4 Profile Resolution Semantics, the Text Profile can be signaled by the Document Interchange Context and/or the Document Processing Context. Alternatively, a processor can choose to process a document as a Text Profile document if the ttp:profile element signals [ttml10-sdp-us], since [ttml10-sdp-us] is feasibly interoperable with Text Profile.

It is not possible for a document that conforms to [ttml10-sdp-us] to also conform to Image Profile, and vice-versa, notwithstanding the special case where the document also conforms to Text Profile as noted at 5. Profiles.

As an illustration, Example 3 at [ttml10-sdp-us] conforms to both Text Profile and [ttml10-sdp-us].

I.4 SMPTE-TT (SMPTE ST 2052-1)

[ST2052-1] specifies the use of the DFXP Full Profile (see Appendix F.3 at [TTML1]) supplemented by a number of extensions, including http://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt#image.

This specification defines practical constraints on [ST2052-1], supplemented by a few extensions defined at F. Extensions. These constraints and extensions are intended to reflect industry practice.

As a result, particular care is required when creating a document intended to be processed according to both [ST2052-1] and Text Profile or Image Profile. In particular:

in contrast to Text Profile and Image Profile, [ST2052-1] allows documents to contain both smpte:backgroundImage attributes and any of p, span, or br elements;
Image Profile allows only a subset of the http://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt#image extension;
[ST2052-1] does not support the #aspectRatio, #forcedDisplay, #linePadding and #multiRowAlign extensions that impact presentation; and
when the designator "http://www.smpte-ra.org/schemas/2052-1/2010/profiles/smpte-tt-full" is used as a value for ttp:profile element or attribute (see Section 5.8 at [ST2052-1]), Text Profile or Image Profile is signaled by the Document Interchange Context and/or the Document Processing Context.

The following is an example of a document that conforms to both Text Profile and [ST2052-1]:

Example 13

<?xml version="1.0" encoding="UTF-8"?>
<tt xml:lang="en" xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
    xmlns:ttp="http://www.w3.org/ns/ttml#parameter" ttp:profile="http://www.smpte-ra.org/schemas/2052-1/2010/profiles/smpte-tt-full"
    xmlns:tts="http://www.w3.org/ns/ttml#styling" ttp:frameRate="24">
    <head>
        <layout>
            <region xml:id="area1" tts:origin="10% 70%" tts:extent="80% 20%" tts:showBackground="whenActive" tts:backgroundColor="red" tts:displayAlign="center" tts:color="white"/>
        </layout>
    </head>
    <body tts:lineHeight="100%">
        <div>
            <p region="area1" begin="00:00:01.01" end="00:00:03">This should appear on frame 25.</p>
            <p region="area1" begin="00:00:04" end="00:00:06">This should appear on frame 96.</p>
            <p region="area1" begin="00:00:07.33" end="00:00:09">This should appear on frame 176.</p>
        </div>
    </body>
</tt>

I.5 CFF-TT

This specification was derived from the text and image profiles specified in Section 6 at [CFF], and is intended to be a superset in terms of capabilities. Additional processing is however generally necessary to convert a document from [CFF] to this specification. In particular:

the namespace of the progressivelyDecodable attribute is different;
the forcedDisplayMode attribute in [CFF] is renamed to forcedDisplay in this specification;
the [CFF] HRM does not specifies GCpy as a function of script;
in [CFF], the attribute ttp:frameRate is not subject to the requirements specified at 6.11 Features and Extensions; and
[CFF] requires the use of the ttp:profile element, whereas this specification recommends the use of the ttp:profile attribute.

Abstract

Status of This Document

1. Scope

2. Documentation Conventions

3. Terms and Definitions

4. Conformance

5. Profiles

5.1 General

5.2 Text Profile

5.3 Image Profile

5.4 Profile Resolution Semantics

6. Common Constraints

6.1 Document Encoding

6.2 Foreign Element and Attributes

6.3 Namespaces

6.4 Overflow

6.5 Related Video Object

6.6 Synchronization

6.7 Extensions

6.7.1 ittp:aspectRatio

6.7.2 ittp:progressivelyDecodable

6.7.3 itts:forcedDisplay

6.7.4 ittm:altText

6.8 Region

6.8.1 Presented Region

6.8.2 Dimensions and Position

6.8.3 Maximum number

6.9 Profile Signaling

6.10 Hypothetical Render Model

6.11 Features and Extensions

7. Text Profile Constraints

7.1 Profile Designator

7.2 Recommended Character Sets

7.3 Reference Fonts

7.4 Features and Extensions

8. Image Profile Constraints

8.1 Profile Designator

8.2 Presented Image

8.2.1 Definition

8.2.2 Constraints

8.2.3 Intermediate Synchronic Document Construction

8.3 smpte:backgroundImage Constraints

8.4 Features and Extensions

9. Hypothetical Render Model

9.1 Overview (non-normative)

9.2 General

9.3 Paint Regions

9.4 Paint Images

9.5 Paint Text

A. Reference Fonts

B. Recommended Character Sets

C. Forced content (non-normative)

D. WCAG Considerations

E. Sample Document Instance (non-normative)

F. Extensions

F.1 General

F.2 #progressivelyDecodable

F.3 #aspectRatio

F.4 #forcedDisplay

F.5 #altText

F.6 #linePadding

F.7 #multiRowAlign

G. XML Schema Definitions (non-normative)

H. Extensibility Objectives (non-normative)

I. Compatibility with other TTML-based specifications (non-normative)

I.1 Overview

I.2 EBU-TT-D

I.3 SDP-US

I.4 SMPTE-TT (SMPTE ST 2052-1)

I.5 CFF-TT

J. References

J.1 Normative references

J.2 Informative references