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].
Status Update (9 April 2020): In the document header, the "Latest IMSC1 Recommendation" link (pointing to the specific "TTML Profiles for Internet Media Subtitles and Captions 1.0.1" Recommendation) was replaced by a "Latest IMSC Recommendation" link pointing to the most recently published Recommendation in the "TTML Profiles for Internet Media Subtitles and Captions" family of Recommendations.
This document was published by theTimed Text Working Group as a Recommendation. Comments regarding this document are welcome. Please file them in thegithub repository.
This specification defines two profiles of [TTML1]: a text-only profile and an image-only profile. These profiles are intended for subtitle and caption delivery worldwide, including dialog language translation, content description, captions for deaf and hard of hearing, etc.
The text profile is a syntactic superset of [ttml10-sdp-us], and a document can 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].
This specification uses the same conventions as [TTML1] for the specification of parameter attributes, styling attributes and metadata elements. In particular:
Section 2.3 of [TTML1] specifies conventions used in the XML representation of elements; and
Sections 6.2 and 8.2 of [TTML1] specify conventions used when specifying the syntax of attribute values, such as in6.7Extensions.
All content of this specification that is not explicitly marked as non-normative is considered to be normative. If a section or appendix header contains the expression "non-normative", then the entirety of the section or appendix is considered non-normative.
This specification usesFeature andExtension designations as defined in Appendices D.1 and E.1 at [TTML1]:
when making reference to content conformance, these designations refer to the syntactic expression or the semantic capability associated with each designatedFeature orExtension; and
when making reference to processor conformance, these designations refer to processing requirements associated with each designatedFeature orExtension.
If the name of an element referenced in this specification is not namespace qualified, then the TT namespace applies (see6.3Namespaces.)
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key wordsMAY,SHALL,SHALL NOT,SHOULD, andSHOULD NOT are to be interpreted as described in [RFC2119].
SHALL satisfy all normative provisions specified by the profile;
MAY include any vocabulary, syntax or attribute value associated with aFeature orExtension whose disposition ispermitted oroptional in the profile;
SHALL NOT include any vocabulary, syntax or attribute value associated with aFeature orExtension whose disposition isprohibited in the profile.
Note
ADocument Instance, by definition, satisfies the requirements of Section 3.1 at [TTML1], and hence aDocument Instance that conforms to a profile defined herein is also a conforming TTML1 Document Instance.
SHALL satisfy the Generic Processor Conformance requirements at Section 3.2.1 of [TTML1];
SHALL satisfy all normative provisions specified by the profile; and
SHALL implement presentation semantic support for everyFeature andExtension designated aspermitted by the profile, subject to any additional constraints on eachFeature andExtension as specified by the profile.
MAY implement presentation semantic support for everyFeature andExtension designated asoptional by the profile, subject to any additional constraints on eachFeature andExtension as specified by the profile.
SHALL satisfy the Generic Processor Conformance requirements at Section 3.2.1 of [TTML1];
SHALL satisfy all normative provisions specified by the profile; and
SHALL implement transformation semantic support for everyFeature andExtension designated aspermitted by the profile, subject to any additional constraints on eachFeature andExtension as specified by the profile.
MAY implement transformation semantic support for everyFeature andExtension designated asoptional by the profile, subject to any additional constraints on eachFeature andExtension as specified by the profile.
Note
The use of the termpresentation processor (transformation processor) within this specification does not imply conformance to the DFXP Presentation Profile (DFXP Transformation Profile) specified in [TTML1]. In other words, it is not considered an error for apresentation processor (transformation processor) to conform to a profile defined in this specification without also conforming to the DFXP Presentation Profile (DFXP Transformation Profile).
Thepermitted andprohibited dispositions do not refer to the specification of attp:feature orttp:extension element as being permitted or prohibited within attp:profile element.
5.Profiles
5.1General
Notwithstanding special cases, e.g. aDocument Instance that contains nop,span,br element and nosmpte:backgroundImage attribute, it is generally not possible to construct aDocument Instance that conforms to theText Profile andImage Profile simultaneously, and it is not possible to construct aDocument Instance that results in the presentation of both text data and image data.
In applications that require subtitle/caption content in image form to be simultaneously available in text form, two distinctDocument Instances, one conforming to theText Profile and the other conforming to theImage Profile,SHOULD be offered. In addition, theText ProfileDocument InstanceSHOULD be associated with theImage ProfileDocument Instance such that, when image content is encountered, assistive technologies have access to its corresponding text form. The method by which this association is made is left to each application.
Note
Theittm:altText element specified6.7.4ittm:altText also allows text equivalent string to be associated with an image, e.g. to support indexation of the content and also facilitate quality checking of the document during authoring.
AnnexD.WCAG Considerations specifically discusses this specification in the context of the [WCAG20] guidelines.
For the purpose of content processing, the determination of the resolved profileSHOULD take into account both the signaled profile, as defined in6.9Profile Signaling, and profile metadata, as designated by either (or both) theDocument Interchange Context or (and) theDocument Processing Context, whichMAY entail inspecting document content.
If the resolved profile is not a profile supported by theProcessor but is feasibly interoperable with theText Profile, then the resolved profile is theText Profile; otherwise, if the resolved profile is not a profile supported by theProcessor but is feasibly interoperable with theImage Profile, then the resolved profile is theImage Profile.
If the resolved profile is a profile supported by theProcessor, then theProcessorSHOULD process theDocument Instance according to the resolved profile. If the resolved profile is neitherText Profile norImage Profile, processing is outside the scope of this specification.
If the resolved profile is undetermined or not supported by theProcessor, then theProcessorSHOULD nevertheless process theDocument Instance using one of its supported profiles, with a preference for theText Profile over theImage Profile; otherwise, processingMAY be aborted.
Document Instances remain subject to the content conformance requirements specified at Section 3.1 of [TTML1]. In particular, aDocument Instance can contain elements and attributes not in any TT namespace, i.e. in foreign namespaces, since such elements and attributes are pruned by the algorithm at Section 4 of [TTML1] prior to evaluating content conformance.
Note
For validation purposes it is good practice to define and use a content specification for all foreign namespace elements and attributes used within aDocument Instance.
6.3Namespaces
The following namespaces (see [xml-names]) are used in this specification:
The namespace prefix values defined above are for convenience andDocument InstancesMAY 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 theW3C.
6.4Overflow
ADocument InstanceSHOULD be authored assuming strict clipping of content that falls out of region areas, regardless of the computed value oftts: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(En) at9.3Paint Regions.
In typical scenario, the same video program (theRelated Video Object) will be used forDocument Instance authoring, delivery and user playback. The mapping from media time expression toRelated 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.
If present, theittp:aspectRatio attributeSHALL 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>+ // no linear white-space is implied or permitted // between each <digit> token
Theittp:aspectRatio parameter effectively defines the display aspect ratio (DAR) of the root container, while thetts:extent style property on the root element effectively defines the storage aspect ratio (SAR) of theRoot Container Region. As a result, when bothtts:extent andittp:aspectRatio are specified on thett element, the effective pixel aspect ratio (PAR) of theRoot Container Region is equal to the ratio of the DAR to the SAR.
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 orRoot Container Region 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 usingittp: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 theRoot Container Region 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 theRoot Container Region to the video frame, maintaining relative positioning between caption/subtitles and video elements but at the risk of clipping subtitles/captions.
6.7.2ittp:progressivelyDecodable
Aprogressively decodable Document Instance is structured to facilitate presentation before the document is received in its entirety, and can be identified usingittp:progressivelyDecodable attribute.
Aprogressively decodable Document Instance is aDocument Instance that conforms to the following:
no attribute or element of the TTML timing vocabulary is present within thehead element;
given twointermediate synchronic documentsA andB of theDocument Instance, with start timesTA andTB, respectively,TA is not greater thanTB ifA includes ap element that lexically precedes anyp element thatB includes;
no attribute of the TTML timing vocabulary is present on a descendant element ofp; and
no elementE1 explicitly references another elementE2 where the opening tag ofE2 is lexically subsequent to the opening tag ofE1.
If present, theittp:progressivelyDecodable attributeSHALL conform to the following syntax:
ittp:progressivelyDecodable :"true" |"false"
Anittp:progressivelyDecodable attribute is considered to be significant only when specified on thett element.
If not specified, the value ofittp:progressivelyDecodableSHALL be considered to be equal to"false".
[TTML1] specifies explicitly referencing of elements identified usingxml:id in the following circumstances:
an element inbody referencingregion elements. In this case, Requirement 4 above is always satisfied.
an element inbody referencingstyle elements. In this case, Requirement 4 above is always satisfied.
aregion element referencingstyle elements. In this case, Requirement 4 above is always satisfied.
astyle element referencing otherstyle elements. In this case, Requirement 4 provides an optimization ofstyle element ordering within thehead element.
attm:actor element referencing attm:agent element. In this case, Requirement 4 provides optimization of metadata elements ordering within the document.
a content element referencingttm:agent elements using thettm:agent attribute. In this case, Requirement 4 provides optimization of metadata elements ordering within the document.
6.7.3itts:forcedDisplay
itts:forcedDisplay can be used to hide content whose computed value oftts:visibility is"visible" when the processor has been configured to do so via the application parameterdisplayForcedOnlyMode.
If and only if the value ofdisplayForcedOnlyMode is"true", a content element with aitts:forcedDisplay computed value of"false"SHALL NOT produce any visible rendering, regardless of the computed value oftts:visibility.
Theitts:forcedDisplay attribute has no effect on content layout or composition, but merely determines whether composed content is visible or not.
Theitts:forcedDisplay attributeSHALL conform to the following:
Values:
false | true
Initial:
false
Applies to:
body,div,p,region,span
Inherited:
yes
Percentages:
N/A
Animatable:
discrete
AnnexC.Forced content (non-normative) illustrates the use ofitts:forcedDisplay in an application in which a single document contains both hard of hearing captions and translated foreign language subtitles, usingitts:forcedDisplay to display translation subtitles always, independently of whether the hard of hearing captions are displayed or hidden.
Thepresentation processorSHALL accept an optional boolean parameter calleddisplayForcedOnlyMode, whose valueMAY be set by a context external to thepresentation processor. If not set, the value ofdisplayForcedOnlyModeSHALL be assumed to be equal to"false".
The algorithm for setting thedisplayForcedOnlyMode parameter based on the circumstances under which theDocument Instance is presented is left to the application.
Example 4
<?xml version="1.0" encoding="UTF-8"?><ttxml:lang="en"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"xmlns:itts="http://www.w3.org/ns/ttml/profile/imsc1#styling"ittp:aspectRatio="16 9"ttp:profile="http://www.w3.org/ns/ttml/profile/imsc1/text"><head><layout><regionxml:id="r1"tts:showBackground="whenActive"tts:origin="10% 2%"tts:extent="80% 10%"tts:color="white"itts:forcedDisplay="true"tts:backgroundColor="black"/><regionxml:id="r2"tts:showBackground="whenActive"tts:origin="10% 80%"tts:extent="80% 10%"tts:color="white"tts:backgroundColor="black"/></layout></head><body><div><pregion="r1"begin="1s"end="6s">Lycée</p><!-- the following will not appear if displayForcedOnlyMode='true' --><pregion="r2"begin="4s"end="6s">Nous étions inscrits au même lycée.</p></div></body></tt>
Note
As specified in [TTML1], the background of a region can be visible even if the computed value oftts:visibility equals"hidden" for all active content within. The background of a region for whichitts:forcedDisplay equals"true" can therefore remain visible even ifitts:forcedDisplay equals"false" for all active content elements within the region anddisplayForcedOnlyMode 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 ofitts:forcedDisplay.
Note
Althoughitts:forcedDisplay, like all the TTML style attributes, has no defined semantics on abr content element,itts:forcedDisplay will apply to abr content element if it is either defined on an ancestor content element of thebr content element or it is applied to a region element corresponding to a region that thebr content element is being flowed into.
Note
It is expected that the functionality ofitts:forcedDisplay will be mapped to a conditional style construct in a future revision of this specification.
Note
The presentation semantics associated withitts:forcedDisplay are intended to be compatible with those associated with theforcedDisplayMode attribute defined in [CFF].
6.7.4ittm:altText
ittm:altText allows an author to provide a text string equivalent for an element, typically an image. This text equivalentMAY be used to support indexing of the content and also facilitate quality checking of the document during authoring.
Theittm:altText elementSHALL conform to the following syntax:
<?xml version="1.0" encoding="UTF-8"?><ttxml:lang="fr"xmlns="http://www.w3.org/ns/ttml"xmlns:ttp="http://www.w3.org/ns/ttml#parameter"ttp:profile="http://www.w3.org/ns/ttml/profile/imsc1/image"xmlns:tts="http://www.w3.org/ns/ttml#styling"xmlns:smpte="http://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt"xmlns:ittm="http://www.w3.org/ns/ttml/profile/imsc1#metadata"tts:extent="320px 240px"><head><layout><regionxml:id="area1"tts:origin="20px 215px"tts:extent="180px 20px"/></layout></head><body><divbegin="1s"end="9s"region="area1"smpte:backgroundImage="altText1-img.png"><metadata><ittm:altText>Nous étions inscrits au même lycée.</ittm:altText></metadata></div></body></tt>
Note
In contrast to the common use ofalt attributes in [HTML5], theittm:altText attribute content is not intended to be displayed in place of the element if the element is not loaded. Theittm:altText attribute content can however be read and used by assistive technologies.
6.7.5ittp:activeArea
TheActive Area of aDocument Instance is the area within theRoot Container Region that the author intends to be minimally visible to the viewer. This area typically fully contains all of the referenced regions within theDocument Instance.
Note
Under normal circumstances, the entirety of theRoot Container Region is presented. However, under special circumstances, such as when therelated video object is cropped, a system can, for instance, use theittp:activeArea parameter to avoid cropping areas of theRoot Container Region that are intended to be visible to the viewer. The specific behavior of the system is however left undefined intentionally: the system can select a presentation mode appropriate to the display shape, user preferences, etc. Theittp:activeArea is analogous to the Active Format Description (AFD) metadata commonly used in broadcast applications.
TheActive Area is specified using theittp:activeArea attribute.
If present, theittp:activeArea attributeSHALL conform to the following syntax:
ittp:activeArea : leftOffset topOffset width heightleftOffset | topOffset | width | height :<percentage> // where<percentage> is non-negative and not greater than 100%.
The use of left and top offset positions is co-incident with the [css3-background]background-position property where a two percentage value position is used.
Note
The syntax of theittp:activeArea parameter is such that theActive Area cannot extend outside theRoot Container Region in any dimension.
Theittp:activeArea attribute is considered to be significant only when specified on thett element.
<?xml version="1.0" encoding="UTF-8"?><ttxml:lang="en"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:activeArea="50% 50% 80% 80%"tts:extent="640px 480px"ttp:profile="http://www.w3.org/ns/ttml/profile/imsc1/text"><head><layout><regionxml:id="area1"tts:origin="10% 10%"tts:extent="80% 10%"tts:backgroundColor="blue"tts:displayAlign="center"tts:textAlign="center"tts:color="white"tts:fontSize="24px"/><regionxml:id="area2"tts:origin="10% 80%"tts:extent="80% 10%"tts:backgroundColor="blue"tts:displayAlign="center"tts:textAlign="center"tts:color="white"tts:fontSize="24px"/><regionxml:id="area3"tts:origin="10% 92%"tts:extent="80% 6%"tts:backgroundColor="red"tts:displayAlign="center"tts:textAlign="center"tts:color="yellow"tts:fontSize="24px"/></layout></head><body><div><pregion="area1"begin="0s"end="6s">This region is within the editorial area.</p><pregion="area2"begin="0s"end="6s">This region is within the editorial area.</p><pregion="area3"begin="0s"end="6s">This region is not.</p></div></body></tt>
6.7.6itts:fillLineGap
Theitts:fillLineGap attribute allows the author to control the application of background between successive line areas.
Ifitts:fillLineGap="true" then the background of each inline area generated by descendant spans of thep elementSHALL extend to thebefore-edge andafter-edge of its containing line area (before-edge andafter-edge are defined at Section 4.2.3 of [XSL11]).
Theitts:fillLineGap attributeSHALL conform to the following:
Values:
false | true
Initial:
false
Applies to:
p
Inherited:
yes
Percentages:
N/A
Animatable:
discrete
In the following example, thep specifiesitts:fillLineGap="true", and, as a result, no gap exists between its lines.
Figure1 Illustrative rendition of the example immediately above withitts:fillLineGap="true" removed (left) or preserved (right). Blue lines have been added to show thebefore-edge andafter-edge of each line area, which are coincident for successive line areas.
Also, as illustrated in the following example, because the line areas of successivep elements are contiguous, no gap exists between two successivep elements whereitts:fillLineGap="true".
Figure2 Illustrative rendition of the example immediately above, whereitts:fillLineGap="true" on the two paragraphs of the top region,itts:fillLineGap="false" on the two paragraphs of the bottom region.
6.8Region
6.8.1Presented Region
Apresented region is a temporally active region that satisfies the following conditions:
the computed value oftts:opacity is not equal to"0.0"; and
the computed value oftts:display is not"none"; and
the computed value oftts:visibility is not"hidden"; and
either (a) content is selected into the region or (b) the computed value oftts:showBackground is equal to"always" and the computed value oftts:backgroundColor has non-transparent alpha.
6.8.2Dimensions and Position
All regionsSHALL NOT extend beyond theRoot Container Region, i.e. every coordinate in the set of coordinates of each region is also in the set of coordinates of theRoot Container Region.
Thettp:profile attributeSHOULD be present on thett element and equal to the designator of the IMSC 1.0 profile to which theDocument Instance conforms, and thettp:profile elementSHOULD NOT be present, unless:
theDocument Instance also conforms to [EBU-TT-D], in which case neither thettp:profile attribute nor thettp:profile element are present, and, instead, the designator of the IMSC 1.0 profile to which theDocument Instance conforms and the URI"urn:ebu:tt:distribution:2014-01"SHOULD each be carried in anebuttm:conformsToStandard element as specified in [EBU-TT-D] and illustrated inI.2EBU-TT-D; or
Note
Multiple otherebuttm:conformsToStandard elements can be present in addition to the two recommended above, indicating simultaneous conformance to profiles defined in other specifications, including future versions of this specification.
theDocument Instance also conforms to [ttml10-sdp-us], in which case thettp:profile attribute is not present and thettp:profile element is present, as illustrated inI.3SDP-US.
See4.Conformance for a definition ofpermitted,prohibited andoptional.
Feature
Disposition
Additional provision
Relative to the TT Feature namespace
#animation
permitted
#backgroundColor-block
permitted
#backgroundColor-region
permitted
#cellResolution
permitted
If theDocument Instance includes any length value that uses thec expression,ttp:cellResolutionSHOULD be present on thett 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 theDocument Instance includes any length value that uses thepx expression,tts:extentSHALL be present on thett element.
#extent
permitted
#frameRate
permitted
If theDocument Instance includes any clock time expression that uses theframes term or any offset time expression that uses thef metric, thettp:frameRate attributeSHALL be present on thett element.
#frameRateMultiplier
permitted
#layout
permitted
#length-cell
permitted
c unitsSHALL NOT be present outside of the value ofebutts:linePadding.
ttp:tickRateSHALL be present on thett element if the document contains any time expression that uses thet metric.
#timeBase-clock
prohibited
#timeBase-media
permitted
NOTE: [TTML1] specifies that the default timebase is"media" ifttp:timeBase is not specified ontt.
#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 aDocument InstanceSHOULD use the same syntax, eitherclock-time oroffset-time.
For any content element that containsbr elements or text nodes or asmpte:backgroundImage attribute, both thebegin attribute and one of either theend ordur attributesSHOULD be specified on the content element or at least one of its ancestors.
NOTE: This feature is optional such that aprocessor that conforms to the earlier version of this specification also conforms to this version.
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)).
6.12Style Resolution
The following style properties are subject to theStyle Resolution procedures specified at Section 8.4 of [TTML1]:
itts:fillLineGap
itts:forcedDisplay
ebutts:linePadding
ebutts:multiRowAlign
7.Text Profile Constraints
7.1Profile 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 in6.11Features and Extensions, the presence of thettp:profile attribute is not required by this profile. The profile designator specified above is intended to be generally used to signal conformance of aDocument Instance to the profile. The details of such signaling depends on the application, and can, for instance, use metadata structures out-of-band of theDocument Instance.
#PCDATA content withinp andspan elements of aDocument InstanceSHOULD NOT include the TAB (U+0009) character.
Note
No presentation semantics are specified for the TAB (U+0009) character.
7.3Reference Fonts
When rendering codepoints matching one of the combinations of computed font family and codepoints listed inA.Reference Fonts, a processorSHALL 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
This clause only applies to codepoints supported by the processor. See7.2Recommended Character Sets for codepoints that a processor is likely to encounter for various languages.
Note
When a content author sets a bounding box for a subtitle, they want to maximize the likelihood that the text will fit within it when displayed by the processor. If the processor doesn't use the specific font the content author had in mind, the font actually used might cause the text to grow in size so that it no longer fits in the bounding box. This is further compounded by differences in the way text wraps when a font has bigger glyphs, which might increase the number of lines used, and increased line spacing, which might also push some of the text outside the bounding box. To help ensure that things such as text size, line breaking, and line height behave as expected relative to the size of the bounding box set by the content author, the author can use one of the reference fonts defined by this specification. This specification requires processors to support one or more fonts with similar font metrics as reference fonts. Note that, however, the reference fonts as currently defined only cover characters used for a few writing systems – in particular, a subset of those based on Latin, Greek, Cyrillic, Hebrew, and Arabic scripts.
Note
Implementations can use fonts other than those specified inA.Reference Fonts. Two fonts with equal metrics can have a different appearance, but flow identically.
7.4Features and Extensions
See4.Conformance for a definition ofpermitted,prohibited andoptional.
NOTE 2: The named color"green" defined in [TTML1] is equivalent to the RGB triplet#008000 and is not full luminance. For full luminance green, an author can specify the RGB triplet#00ff00ff or the named color"lime".
#direction
permitted
#displayAlign
permitted
#extent-region
permitted
Thetts:extent attributeSHALL be present on allregion elements, where itSHALL usepx 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 themonospaceSerif orproportionalSansSerif generic font families, for which reference font metrics are defined atA.Reference Fonts.
If the computed value oftts:fontFamily is"default", then the used value oftts:fontFamilySHALL be"monospaceSerif".
NOTE: The termused value is defined in CSS 2.1, as normatively referenced by [TTML1].
#fontFamily-non-generic
permitted
#fontFamily
permitted
Linear white-spaceSHOULD NOT appear between components of the specified value oftts:fontFamily.
TheprocessorSHALL 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:lineHeightSHOULD NOT be set to"normal" andSHOULD be explicitly specified such that the specified style set of eachp element contains atts:lineHeight property whose value is not assigned by initial value fallback.
#nested-div
permitted
#nested-span
permitted
#origin
permitted
Thetts:origin attributeSHALL usepx units or "percentage" representation, andSHALL NOT useem 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 oftts:textOutline on aspan elementSHALL be 10% or less than the computed value oftts: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 Extension namespace
#linePadding
permitted
If used, the attributeebutts:linePaddingMAY be specified on elementsregion,body,div andp in addition tostyle.
NOTE: This feature is optional such that aprocessor that conforms to the earlier version of this specification also conforms to this version.
Note
In contrast to this specification, [EBU-TT-D] specifies that the attributesebutts:linePadding andebutts:multiRowAlign are allowed only on thestyle element.
8.Image Profile Constraints
8.1Profile 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
As specified in6.11Features and Extensions, the presence of thettp:profile attribute is not required by this profile. The profile designator specified above is intended to be generally used to signal conformance of aDocument Instance to the profile. The details of such signaling depends on the application, and can, for instance, use metadata structures out-of-band of theDocument Instance.
8.2Presented Image
8.2.1Definition
Apresented image is adiv element with asmpte:backgroundImage attribute that flows into apresented region.
8.2.3Intermediate Synchronic Document Construction
For the purposes of constructing anintermediate synchronic document, adiv element with asmpte:backgroundImage attributeSHALL NOT be considered empty.
8.3smpte:backgroundImage Constraints
If asmpte:backgroundImage attribute is applied to adiv element:
the width and height (in pixels) of the image source referenced bysmpte:backgroundImageSHALL be equal to the width and height (as specified by thetts:extent attribute usingpx units) of the region in which thediv element is presented;
thediv elementSHOULD contain ametadata element containing anittm:altText element that is aText Alternative of the image referenced by thesmpte:backgroundImage attribute; and
Thesmpte:backgroundImage attributeSHALL reference a PNG datastream as specified in [PNG]. If a pHYs chunk is present, itSHALL indicate square pixels. Note that if no pixel aspect ratio is carried, the default of square pixels is assumed.
Note
In [TTML1],tts:extent andtts:origin do not apply todiv elements. In order to individually position multiplediv elements, eachdiv can be associated with a distinctregion with the desiredtts:extent andtts:origin.
8.4Features and Extensions
See4.Conformance for a definition ofpermitted,prohibited andoptional.
smpte:backgroundImageHorizontal andsmpte:backgroundImageVerticalSHALL NOT be used.
smpte:imageSHALL NOT be used.
Relative to the IMSC Extension namespace
#fillLineGap
prohibited
Note
The rendering semantics ofsmpte:backgroundImage are not identical to those ofbackground-image specified at Section 7.8.3 of [XSL11]. In particular, Section 5.5.6 at [ST2052-1] amends the semantics ofbackground-image by specifying values for itsmin-height andmin-width properties.
9.Hypothetical Render Model
9.1Overview (non-normative)
This Section specifies the Hypothetical Render Model illustrated inFigure3.
The purpose of the model is to limitDocument 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 specifies an (hypothetical) time required for completely painting anintermediate 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 ofintermediate synchronic document En completes before the end ofintermediate synchronic document En-1.
Whenever applicable, constraints are specified relative to the dimensions of theRoot Container Region, allowing subtitle sequences to be authored independently ofRelated Video Object resolution.
To enable scenarios where the same glyphs are used in multiple successiveintermediate synchronic documents, e.g. to convey a CEA-608/708-style roll-up (see [CEA-608] and [CEA-708]), the Glyph Buffers Gn and Gn-1 store rendered glyphs acrossintermediate synchronic documents, allowing glyphs to be copied into the Presentation Buffer instead of rendered, a more costly operation.
Similarly, Decoded Image Buffers Dn and Dn-1 store decoded images acrossintermediate synchronic documents, allowing images to be copied into the Presentation Buffer instead of decoded.
9.2General
The Presentation CompositorSHALL render in Presentation Buffer Pn each successiveintermediate synchronic document En using the following steps in order:
clear the pixels, except for the firstintermediate synchronic document E0 for the which the pixels of P0SHALL 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 CompositorSHALL start rendering En:
at the presentation time of E0 minus Initial Painting Delay (IPD), if n = 0; or
DURI(En) is the duration, in seconds, for painting the image subtitle content forintermediate synchronic document En, as specified in Section.
The contents of the Presentation Buffer PnSHALL be transferred instantaneously to Presentation Buffer Pn-1 at the presentation time ofintermediate synchronic document En, making the latter available for display.
Note
It is possible for the contents of Presentation Buffer Pn-1 to never be displayed. This can happen if Presentation Buffer Pn is copied twice to Presentation Buffer Pn-1 between two consecutive video frame boundaries of theRelated Video Object.
ItSHALL be an error for the Presentation Compositor to fail to complete painting pixels for En before the presentation time of En.
Unless specified otherwise, the following tableSHALL specify values for IPD and BDraw.
BDraw effectively sets a limit on fillings regions - for example, assuming that theRoot Container Region 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.
NBG(Ri)SHALL be the total number oftts:backgroundColor attributes associated with the given region Ri in theintermediate synchronic document. Atts: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 theregion layout element that defines the region; or
it is specified on adiv,p,span orbr content element that is to be flowed into the region for presentation in theintermediate synchronic document (see [TTML1] for more details on when a content element is followed into a region); or
it is specified on aset animation element that is to be applied to content elements that are to be flowed into the region for presentation in theintermediate synchronic document (see [TTML1] for more details on when aset animation element is applied to content elements).
Even if a specifiedtts:backgroundColor is the same as specified on the nearest ancestor content element or animation element, specifying anytts:backgroundColorSHALL require an additional fill operation for all region pixels.
9.4Paint Images
The Presentation CompositorSHALL paint into the Presentation Buffer Pn all visible pixels of presented images ofintermediate synchronic document En.
For eachpresented image, the Presentation CompositorSHALL either:
if an identical image is present in Decoded Image Buffer Dn, copy the image from Decoded Image Buffer Dn to the Presentation Buffer Pn using the Image Copier; or
if an identical image is present in Decoded Image Buffer Dn-1, i.e. an identical image was present inintermediate synchronic document En-1, copy using the Image Copier the image from Decoded Image Buffer Dn-1 to both the Decoded Image Buffer Dn and the Presentation Buffer Pn; or
otherwise, decode the image using the Image Decoder the image into the Presentation Buffer Pn and Decoded Image Buffer Dn.
Two imagesSHALL be identical if and only if they reference the same encoded image source.
The duration DURI(En) for painting images of anintermediate synchronic document En in the Presentation BufferSHALL be as follows:
DURI(En) = ∑Ii ∈ Ic NRGA(Ii) / ICpy + ∑Ij ∈ Id NSIZ(Ij) / IDec
The contents of the Decoded Image Buffer DnSHALL be transferred instantaneously to Decoded Image Buffer Dn-1 at the presentation time ofintermediate synchronic document En.
The total size occupied by images stored in Decoded Image Buffers Dn or Dn-1SHALL be the sum of their Normalized Image Area.
The size of Decoded Image Buffers Dn or Dn-1SHALL be the Normalized Decoded Image Buffer Size (NDIBS).
Unless specified otherwise, the following tableSHALL 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
9.5Paint Text
In the context of this section, aglyph 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).
if an identicalglyph is present in Glyph Buffer Gn, copy theglyph from Glyph Buffer Gn to the Presentation Buffer Pn using the Glyph Copier; or
if an identicalglyph is present in Glyph Buffer Gn-1, i.e. an identicalglyph was present inintermediate synchronic document En-1, copy using the Glyph Copier theglyph from Glyph Buffer Gn-1 to both the Glyph Buffer Gn and the Presentation Buffer Pn; or
otherwise render using the Glyph Renderer theglyph into the Presentation Buffer Pn and Glyph Buffer Gn.
Figure4 Example of Presentation Compositor Behavior for Text Rendering
NRGA(Gi) 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 GnSHALL be copied instantaneously to Glyph Buffer Gn-1 at the presentation time ofintermediate synchronic document En.
ItSHALL be an error for the sum of NRGA(gi) over allglyphs Glyph Buffer Gn 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 gi
GCpy
latin, greek, cyrillic, hebrew or common
12
any other value
3
Text rendering performance factor Ren(Gi)
Block property (see [UNICODE]) for the character of gi
Ren(Gi)
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.
When authoring textual content, authors are encouraged to select from sets of characters based on the language indicated usingxml:lang. The idea is to increase the confidence that the text will be presented correctly by implementations targeting specific locales.
Specifically, for a given language, an authorSHOULD choose characters from the set resulting from the union of the following sets:
the main exemplar character set specified for the language in [CLDR] as well as any corresponding uppercase characters as specified in [UNICODE]; and
the punctuation exemplar character set specified for the language in [CLDR];
the digits and symbols included in all numberSystems specified for the language in [CLDR];
the common character set specified in Table 1; and
supplementary characters specified for the language in Table 2, if any.
Some of these sets overlap.
Table 1 captures the set of characters intended to be available to authors across all languages. The terms used in the table are defined in [UNICODE].
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 specifies supplementary character set that have proven useful in captioning and subtitling applications for a number of selected languages. Table 2 is non-exhaustive, and will be extended as needs arise.
Figure5 below illustrates the use of forced content, i.e.itts:forcedDisplay anddisplayForcedOnlyMode. The content withitts:forcedDisplay="true" is the French translation of the "High School" sign. The content withitts:forcedDisplay="false" are French subtitles capturing a voiceover.
Figure5 Illustration of the use of itts:forcedDisplay
When the user selects French as the playback language but does not select French subtitles,displayForcedOnlyMode is set to"true", causing the display of the sign translation, which is useful to any French speaker, but hiding the voiceover subtitles as the voiceover is heard in French.
If the user selects French as the playback language and also selects French subtitles, e.g. if the user is hard-of-hearing,displayForcedOnlyMode is set to"false", causing the display of both the sign translation and the voiceover subtitles.
The algorithm for setting thedisplayForcedOnlyMode parameter and selecting the appropriate combination of subtitle and audio tracks depends on the application.
D.WCAG Considerations
In order to meet the guidelines in [WCAG20], the following considerations apply.
Guideline 1.1 of [WCAG20] recommends that an implementation provideText Alternatives for all non-text content. In the context of this specification, thisText Alternative is intended primarily to support users of the subtitles who cannot see images. Since the images of anImage ProfileDocument Instance usually represent subtitle or caption text, the guidelines for authoring text equivalent strings given atImages of text of [HTML5] are appropriate.
Thus, for each subtitle in anImage ProfileDocument Instance, a text equivalent content in aText ProfileDocument InstanceSHOULD be written so that it conveys all essential content and fulfills the same function as the corresponding subtitle image. In the context of subtitling and captioning, this content will be (as a minimum) the verbatim equivalent of the image without précis or summarization. However, the authorMAY include extra information to the text equivalent string in cases where styling is applied to the text image with a deliberate connotation, as afunctional replacement for the applied style.
For instance, in subtitling and captioning, italics can be used to indicate an off screen speaker context (for example a voice from a radio). An author can choose to include this functional information in the text equivalent; for example, by including the word "Radio: " before the image equivalent text. Note that images in anImage ProfileDocument Instance that are intended for use ascaptions, i.e. intended for a hard of hearing audience, might already include this functional information in the rendered text.
Guideline 1.1 of [WCAG20] also recommends that accessibleText Alternatives must be "programmatically determinable." This means that the text must be able to be read and used by the assistive technologies (and the accessibility features in browsers) that people with disabilities use. It also means that the user must be able to use their assistive technology to find the alternative text (that they can use) when they land on the non-text content (that they can't use).
E.Sample Document Instance (non-normative)
The following sampleDocument Instances conforms to theText Profile andImage Profile, respectively. These samples are for illustration only, and are neither intended to capture current or future practice, nor exercise all normative prose contained in this specification.
Example 13
<?xml version="1.0" encoding="UTF-8"?><ttxml:lang="en"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"ttp:profile="http://www.w3.org/ns/ttml/profile/imsc1/text"><head><layout><regionxml:id="area1"tts:origin="10% 10%"tts:extent="80% 10%"tts:backgroundColor="black"tts:displayAlign="center"tts:color="red"/></layout></head><body><div><pregion="area1"begin="0s"end="6s">Lorem ipsum dolor.</p></div></body></tt>
The following sections define extension designations, expressed as relative URIs (fragment identifiers) relative to the IMSC Extension Namespace base URI.
Apresentation processor supports the#altText feature if it implements presentation semantic support for values of theittm:altText element.
F.6#linePadding
Atransformation processor supports the#linePadding feature if it recognizes and is capable of transforming values of theebutts:linePadding attribute specified in [EBU-TT-D].
Apresentation processor supports the#linePadding feature if it implements presentation semantic support for values of theebutts:linePadding attribute specified in [EBU-TT-D].
F.7#multiRowAlign
Atransformation processor supports the#multiRowAlign feature if it recognizes and is capable of transforming values of theebutts:multiRowAlign attribute specified in [EBU-TT-D].
Apresentation processor supports the#multiRowAlign feature if it implements presentation semantic support for values of theebutts:multiRowAlign attribute specified in [EBU-TT-D].
XML Schema definitions (see [xmlschema-1]) for extension vocabulary defined by this specification are providedhere for convenience.
These definitions are non-normative and are not sufficient to validate conformance of aDocument Instance.
In any case where a definition specified by this appendix diverge from the prose of the specification, then the latter takes precedence.
H.Extensibility Objectives (non-normative)
This section documents extensibility objectives for this specification.
This specification is intended to allow:
other profiles of TTML and future revisions of this specification to specify support for documents and/or processors conforming to Text Profile or Image Profile, in addition to specifying additional extensions;
subject to the structural requirements of [TTML1], content from external namespaces to be present in a document that conforms to Text Profile or Image Profile (a) without affecting transformation or presentation, and (b) to be carried through by atransformation processor (see6.2Foreign Element and Attributes);
a document that conforms to Text Profile or Image Profile to be embedded in other XML documents.
I.Compatibility with other TTML-based specifications (non-normative)
I.1Overview
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 [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.2EBU-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
Thettp:profile attribute and element are not allowed by [EBU-TT-D]. Theebuttm:conformsToStandard element is used instead, as discussed at6.9Profile 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 at5.Profiles.
The following is an example of a document that conforms to both Text Profile and [EBU-TT-D]. Note the presence of twoebuttm:conformsToStandard elements, one of which equals the Text Profile designator:
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 theuse attribute of thettp:profile. As a result, Text Profile is not signaled using thettp:profile attribute. Instead, as specified in5.4Profile 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 thettp: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 at5.Profiles.
[ST2052-1] specifies the use of the DFXP Full Profile (see Appendix F.3 at [TTML1]) supplemented by a number of extensions, includinghttp://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 atF.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 bothsmpte:backgroundImage attributes and any ofp,span, orbr elements;
Image Profile allows only a subset of thehttp://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 forttp: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 16
<?xml version="1.0" encoding="UTF-8"?><ttxml: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><regionxml:id="area1"tts:origin="10% 70%"tts:extent="80% 20%"tts:showBackground="whenActive"tts:backgroundColor="red"tts:displayAlign="center"tts:color="white"/></layout></head><bodytts:lineHeight="100%"><div><pregion="area1"begin="00:00:01.01"end="00:00:03">This should appear on frame 25.</p><pregion="area1"begin="00:00:04"end="00:00:06">This should appear on frame 96.</p><pregion="area1"begin="00:00:07.33"end="00:00:09">This should appear on frame 176.</p></div></body></tt>
I.5CFF-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 theprogressivelyDecodable attribute is different;
theforcedDisplayMode attribute in [CFF] is renamed toforcedDisplay in this specification;
the [CFF] HRM does not specifies GCpy as a function of script;
The editor wishes to especially acknowledge the following contributions by members: Glenn Adams, Skynav; John Birch, Invited expert; Mike Dolan, Invited expert; Nigel Megitt, British Broadcasting Corporation; Thierry Michel,W3C; Andreas Tai, Institut für Rundfunktechnik.
The editor also wishes to acknowledge Digital Entertainment Content Ecosystem (DECE) for contributing to the initial document for the Member Submission.
K.Privacy and Security Considerations (non-normative)
TTML security
The security and privacy considerations of [rfc3023] and [TTML1] apply, particularly in relation to document parsing. XML Entities are excluded from the Reduced XML Infoset of TTML and are therefore not considered part ofDocument Instances; nevertheless implementations are encouraged to provide protection against recursive entity expansion or prevent entity expansion altogether in processors.
Privacy of preference
A user agent that selects, and causes to download or interpret aDocument Instance, might indicate to the origin server that the user has a need for captions or subtitles, and also the language preference of the user for captions or subtitles. That is a small piece of information about the user. However, the offering of aDocument Instance, and the choice whether to retrieve and consume it, are characteristics of the application that makes the offer (e.g. a web application based on [HTML]), rather than of theDocument Instance itself.
Security and Privacy related to external images
TheImage Profile includes a mechanism for referencing external images. A user agent that downloads external images during media playback indicates to the origin server of the images the progress of the user's media consumption. In many cases such media progress information is available to the origin server of the media via other mechanisms, for example by scripting or by monitoring streaming media requests.
User agents that do not enforce cross origin policies when downloading external images expose such media progress information and potentially other user tracking information to other origins without the consent of the web site serving the media and without the consent of the user. This specification defines no APIs and makes no statement on how implementations are expected to obtain referenced images.
