Additional Provisions
Recommended Character Sets
A Document Instance SHOULD be authored using characters selected from the sets specified in .
Reference Fonts
When rendering codepoints matching one of the combinations of computed font family and codepoints listed in , 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.
This clause only applies to codepoints supported by the processor. See for codepoints that a processor is likely to encounter for various languages.
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.
Implementations can use fonts other than those specified in . Two fonts with equal metrics can have a different appearance, but flow identically.
Font Resources
A Presentation Processor SHALL support font resources of the following media types, as defined in [[!IANA-MEDIA-TYPES]]:
[[IANA-MEDIA-TYPES]] specifies that the media type
font/otf includes file extensions .ttf and .otf.
Document Encoding
A Document Instance SHALL be concretely encoded as a well-formed XML 1.0 [[!xml]] document using the UTF-8 character encoding as specified in [[!UNICODE]].
The resulting [[!xml]] document SHOULD NOT contain any of the following physical structures:
- entity declarations; and
- entity references other than to predefined entities.
These physical structures are intended to be prohibited in future versions of this specification.
The resulting [[xml]] document can contain character references, and entity references to predefined entities.
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.
Document Instances remain subject to the content conformance requirements specified at Section 3.1 of [[ttml2]]. In particular, a Document 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 [[ttml2]] prior to evaluating content conformance.
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 |
[[!ttml2]] |
| TT Parameter | ttp |
http://www.w3.org/ns/ttml#parameter |
[[!ttml2]] |
| TT Styling | tts |
http://www.w3.org/ns/ttml#styling |
[[!ttml2]] |
| TT Feature | none | http://www.w3.org/ns/ttml/feature/ |
[[!ttml2]] |
| EBU-TT Styling | ebutts |
urn:ebu:tt:style |
[[!EBU-TT-D]] |
| EBU-TT Metadata | ebuttm |
urn:ebu:tt:metadata |
[[!EBU-TT-D]] |
| IMSC Styling | itts |
http://www.w3.org/ns/ttml/profile/imsc1#styling |
This specification |
| IMSC Parameter | ittp |
http://www.w3.org/ns/ttml/profile/imsc1#parameter |
This specification |
| IMSC Metadata | ittm |
http://www.w3.org/ns/ttml/profile/imsc1#metadata |
This specification |
| IMSC Extension | none | http://www.w3.org/ns/ttml/profile/imsc1/extension/ |
This specification |
| IMSC 1.3 Text Profile Designator | none | See Section | |
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.
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.
Related Video Object
A Document Instance MAY be associated with a Related Video Object.
While this specification contains specific provisions when a Document Instance is associated with a Related Video Object, it does not prevent the use of a Document Instance with other kinds of Related Media Object, e.g. an audio object.
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.
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.
Mapping the Root Container Region to the Related Video Object
The Root Container Region of a Document Instance SHALL be mapped to a rectangular area within each image frame of the Related Video Object according to the following:
- the ratio of the width to the height of the rectangular area is equal to the display aspect ratio of the Root Container Region as determined in Appendix H of [[!ttml2]];
- the center of the rectangular area is collocated with the center of the image frame;
- the rectangular area is entirely within the image frame; and
- the rectangular area has a height or width equal to that of the image frame.
- 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:displayAspectRatio="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 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 the Root Container Region to the video frame, maintaining relative positioning between caption/subtitles and video elements but at the risk of clipping subtitles/captions.
- Subtitle/captions are authored using
ttp:displayAspectRatio="16 9"to match the 16:9 video, butittp:activeAreaindicates that the Active Area is smaller than the Root Container Region. The system, which uses a 4:3 display, elects to scale the video and the Root Container Region such that all of the subtitle/caption content within the Active Area is visible, with positional alignment maintained, at the cost of clipping some of the video.
Document complexity
A Text Profile document SHOULD conform to the Hypothetical Render Model specified at [[!imsc-hrm]].
This limits the complexity of the Text Profile document, so that playback systems can render the content synchronized with the author-specified display times.
Regions
Presented Region
A presented region is a temporally active region that satisfies the following conditions:
- the computed value of
tts:opacityis not equal to"0.0"; and - the computed value of
tts:displayis not"none"; and - the computed value of
tts:visibilityis not"hidden"; and - either (a) content is selected into the region or (b) the computed value of
tts:showBackgroundis equal to"always"and the computed value oftts:backgroundColorhas non-transparent alpha.
Dimensions and Position
All regions SHALL NOT extend beyond the Root Container Region, i.e. every coordinate in the set of coordinates of each region is also in the set of coordinates of the Root Container 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.
Maximum number
The number of presented regions in a given intermediate synchronic document SHALL NOT be greater than 4.
Extension Vocabulary
ittp:aspectRatio
The #aspectRatio feature is designated as permitted-deprecated in the profiles
defined by this specification. The #displayAspectRatio feature provides equivalent semantics.
The ittp:aspectRatio attributes allows authorial control of the mapping of the Root Container Region
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>+ // no linear white-space is implied or permitted
// between each <digit> token
|
An ittp:aspectRatio attribute is considered to be significant only when specified on the tt
element.
If specified on the tt element, the ittp:aspectRatio attribute specifies the display aspect
ratio of the Root Container Region. If not specified, then the display aspect ratio of the Root Container
Region is determined according to Appendix H of [[!ttml2]].
ittp:progressivelyDecodable
The #progressivelyDecodable feature is designated as permitted-deprecated in
the profiles defined by this specification.
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
headelement; - 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
pelement that lexically precedes anypelement 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.
<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>
[[ttml2]] specifies explicitly referencing of elements identified using xml:id in the following
circumstances:
- an element in
bodyreferencingregionelements. In this case, Requirement 4 above is always satisfied. - an element in
bodyreferencingstyleelements. In this case, Requirement 4 above is always satisfied. - a
regionelement referencingstyleelements. In this case, Requirement 4 above is always satisfied. - a
styleelement referencing otherstyleelements. In this case, Requirement 4 provides an optimization ofstyleelement ordering within theheadelement. - a
ttm:actorelement referencing attm:agentelement. In this case, Requirement 4 provides optimization of metadata elements ordering within the document. - a content element referencing
ttm:agentelements using thettm:agentattribute. In this case, Requirement 4 provides optimization of metadata elements ordering within the document.
itts:forcedDisplay
The itts:forcedDisplay attribute can be used to hide content when the processor has been configured to do so
via the application parameter displayForcedOnlyMode.
Annex 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 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 |
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, regardless
of the computed value of tts:visibility.
The itts:forcedDisplay attribute has no effect on content layout or composition, but merely determines
whether composed content is visible or not.
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.
As specified in [[ttml2]], 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.
It is expected that the functionality of itts:forcedDisplay will be mapped to a conditional
style construct in a future revision of this specification.
The presentation semantics associated with itts:forcedDisplay are intended to be compatible
with those associated with the forcedDisplayMode attribute defined in [[CFF]].
ittm:altText
The #altText feature is designated as permitted-deprecated in the profiles
defined by this specification. The altText named metadata item provides equivalent semantics.
ittm:altText allows an author to provide a text string equivalent for an element, which MAY be used to
support indexing of the content, or facilitate quality checking of the document. It can also be used by assistive
technologies.
The #altText feature is primarily useful in providing a text alternative to image-based
subtitles and captions, which are not supported by the Text Profile.
In contrast to the common use of alt attributes in [[HTML]], 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 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 namespace}>
Content: #PCDATA
</ittm:altText>
|
The ittm:altText element SHALL be a child of the metadata element.
ittp:activeArea
The Active Area of a Document Instance is the area within the Root Container Region that the author intends to be minimally visible to the viewer. This area typically fully contains all of the referenced regions within the Document Instance.
Under normal circumstances, the entirety of the Root Container Region is presented. However, under special
circumstances, such as when the related video object is cropped, a system can, for instance, use the
ittp:activeArea parameter to avoid cropping areas of the Root 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. The ittp:activeArea is
analogous to the Active Format Description (AFD) metadata commonly used in broadcast applications.
The Active Area is specified using the ittp:activeArea attribute.
If present, the ittp:activeArea attribute SHALL conform to the following syntax:
ittp:activeArea : leftOffset topOffset width height leftOffset | topOffset | width | height : <percentage> // where <percentage> is non-negative and not greater than 100%. |
The width percentage value is relative to the width of the Root Container Region.
The height percentage value is relative to the height of the Root Container Region.
The width and height percentage values are the width and height of the Active Area.
The leftOffset and topOffset percentage values specify an alignment point between the Root
Container Region and the Active Area.
The origin top left {x, y} percentage coordinates of the Active Area SHALL be calculated as follows:
x = leftOffset * (1 - width/100) y = topOffset * (1 - height/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.
The syntax of the ittp:activeArea parameter is such that the Active Area cannot extend outside the
Root Container Region in any dimension.
The ittp:activeArea attribute is considered to be significant only when specified on the tt
element.
If the ittp:activeArea attribute is not specified, the Active Area SHALL be the Root Container
Region.
itts:fillLineGap
The itts:fillLineGap attribute allows the author to control the application of background between
successive line areas.
If itts:fillLineGap="true" then the background of each inline area generated by descendant spans of the
p element SHALL extend to the before-edge and after-edge of its containing line area
(before-edge and after-edge are defined at Section 4.2.3 of [[XSL11]]).
The itts:fillLineGap attribute SHALL conform to the following:
| Values: | false | true |
| Initial: | false |
| Applies to: | p |
| Inherited: | yes |
| Percentages: | N/A |
| Animatable: | discrete |
In the following example, the p specifies itts:fillLineGap="true", and, as a result, no gap
exists between its lines.
itts:fillLineGap="true" removed (left) or
preserved (right). Blue lines have been added to show the before-edge and after-edge of each line
area, which are coincident for successive line areas.
Also, as illustrated in the following example, because the line areas of successive p elements are
contiguous, no gap exists between two successive p elements where itts:fillLineGap="true".
itts:fillLineGap="true" on the two
paragraphs of the top region, itts:fillLineGap="false" on the two paragraphs of the bottom region.
Profile Signaling
General
The ttp:contentProfiles attribute SHOULD be present on the tt element, with exactly one of its
values equal to the designator Text Profile.
The ttp:contentProfiles attribute is prohibited in some profiles, e.g. [[EBU-TT-D]] as
discussed in .
To minimize the risk of incompatibility, the use of the ttp:contentProfiles attribute and
other syntax introduced in [[ttml2]] is not recommended in documents intended to conform to profiles that are based on
[[TTML1]], e.g. [[SMPTE2052-1]], [[ttml-imsc1]] , [[ttml-imsc1.0.1]] , or [[ttml10-sdp-us]].
The ttp:contentProfiles attribute can include additional designators, beyond those specified
herein, including designators for other versions of this specification.
specifies the process by which the effective processor profile associated with the Document Instance is determined.
Signaling conformance using EBU-TT metadata
When using the ebuttm:conformsToStandard element specified in [[!EBU-TT-M]], the designators of the Text
Profile SHALL be used when indicating conformance to the corresponding profile.
See for a sample Document Instance that follows the recommendations of this section.
Style Resolution
The following style properties SHALL be subject to the Style Resolution procedures specified at Section 10.4 of [[!ttml2]]:
itts:fillLineGapitts:forcedDisplayebutts:linePaddingebutts:multiRowAlign
The style properties above can be specified as attributes of the initial element specified at
Section 10.1.1 of [[ttml2]].
Constraints on Features and Extensions
#color
The initial value of tts:color SHALL be "white".
This is consistent with [[SMPTE2052-1]].
The named color green defined in [[ttml2]] is equivalent to the RGB color triple
#008000 and is not full luminance. For full luminance green, an author can specify the RGB color triple
#00ff00 or the named color lime.
#extent-region
For each region element defined in the Document Instance, the specified value of the tts:extent style
property SHALL consist of two length expressions that use pixel (px), percentage (%), or root container relative units.
If no region element is defined in the Document Instance, then content flows into the default region;
conversely, if one or more region elements are defined in the Document Instance, the default region is not used.
The requirement above requires the author to explicitly specify the value of tts:extent in the Document Instance,
e.g. by specifying the tts:extent attribute for each region element.
#font and #source
A font element in the Document Instance SHALL not specify a src attribute.
All source elements that are children of a font element SHALL reference an external data resource.
A Processor MAY use the [[css-fonts-3]] §5 font matching algorithm for associating a font with a run of text.
#fontFamily-generic
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 .
If the computed value of tts:fontFamily is "default", then the used value of
tts:fontFamily SHALL be "monospaceSerif".
The term used value is defined in CSS 2.1, as normatively referenced by [[ttml2]].
#fontFamily
Linear white-space SHOULD NOT appear between components of the specified value of
tts:fontFamily.
#length-negative
Strictly negative length expressions SHALL NOT be used with attributes other than:
tts:disparity; andtts:textShadow.
#lineHeight
The specified value of the tts:lineHeight attribute SHOULD be such that specified style set of each
p element contains a tts:lineHeight property whose value is not normal.
Implementation of the normal value is not uniform at the time of this writing.
#origin
The value of the tts:origin attribute SHALL consist of two length expressions that use pixel (px) or percentage (%) units.
If the tts:position attribute is specified anywhere in a Document Instance, then the
tts:origin attribute SHALL NOT be specified anywhere in that Document Instance.
#position
All length components of the value of a tts:position attribute SHALL use pixel (px), percentage (%) or root
container relative units.
If the tts:origin attribute is specified anywhere in a Document Instance, then the
tts:position attribute SHALL NOT be specified anywhere in that Document Instance.
#resources
The resources element SHALL not contain any of the following children elements:
audio;data; andimage.
#rubyAlign
The specified value of the tts:rubyAlign attribute SHALL be such that its computed value is
center or spaceAround.
#textOutline-unblurred
The specified value of the tts:textOutline attribute SHALL be such that its computed value on any
span element is 10% or less than the computed value of tts:fontSize attribute on the same
element.
#textShadow
The specified value of the tts:textShadow attribute SHALL NOT have more than 4 <shadow>
values.
ebutts:linePadding
In addition to the style element, the attribute ebutts:linePadding MAY be specified on the
following elements:
regionbodydivpinitial
The processor:
- SHALL apply
ebutts:linePaddingtoponly; and - SHALL treat
ebutts:linePaddingas inheritable.
The ebutts:linePadding attribute only supports c length units.
In contrast to this specification, [[EBU-TT-D]] specifies that the attribute
ebutts:linePadding is allowed only on the style element.
ebutts:multiRowAlign
In addition to the style element, the attribute ebutts:multiRowAlign MAY be specified on the
following elements:
regionbodydivpinitial
The processor:
- SHALL apply
ebutts:multiRowAligntoponly; and - SHALL treat
ebutts:multiRowAlignas inheritable.
In contrast to this specification, [[EBU-TT-D]] specifies that the attribute
ebutts:multiRowAlign is allowed only on the style element.
Slanted Japanese-language text
Since (a) Japanese-language fonts typically do not include italic or oblique faces, and
(b) Japanese-language slanted vertical text consists of characters slanted in the vertical direction (and not
in the horizontal direction), it is recommended that tts:shear be specified to slant Japanese-language text
– instead of specifying tts:fontStyle="italic" or tts:fontStyle="oblique".
#fontVariant
The computed value of the tts:fontVariant attribute SHALL be one of the
following:
normal;sub; orsuper.
altText named metadata item
The altText named metadata item SHOULD NOT be present since
no image-based timed text information is used in a Text Profile Document Instance.
A altText named metadata item SHALL NOT be present in a Document Instance if any
ittm:altText element is also present.
#altText
The ittm:altText element SHOULD NOT be present since no image-based timed text information is used in a
Text Profile Document Instance.
A ittm:altText element SHALL NOT be present in a Document Instance if any altText named
metadata item element is also present.
#aspectRatio
The ittp:aspectRatio attribute SHOULD NOT be present in a Document Instance unless compatibility
with [[ttml-imsc1.0.1]] processors is desired.
The ittp:aspectRatio attribute SHALL not be present in a Document Instance if any
ttp:displayAspectRatio attribute is also present.
#displayAspectRatio
The ttp:displayAspectRatio attribute SHALL not be present in a Document Instance if any
ittp:aspectRatio attribute is also present.
#extent-root
If the Document Instance includes any length value that uses the px unit, tts:extent
SHALL be present on the tt element.
#frameRate
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.
#length-cell
c units SHALL NOT be present outside of the value of ebutts:linePadding.
#length-root-container-relative
When specified on a tts:extent or tts:position attribute:
rhunits SHALL NOT be used for horizontal length components; andrwunits SHALL NOT be used for vertical length components.
#tickRate
ttp:tickRate SHALL be present on the tt element if the document contains any time expression
that uses the t metric.
#timeBase-media
[[ttml2]] specifies that the default timebase is "media" if ttp:timeBase is not specified on
tt.
#time-clock-with-frames
As specified in [[ttml2]], a #time-clock-with-frames expression is translated to a media time M according
to M = 3600 · hours + 60 · minutes + seconds + (frames ÷ (ttp:frameRateMultiplier ·
ttp:frameRate)).
#timing
For any content element that contains br elements or text nodes, both the begin attribute and
one of either the end or dur attributes SHOULD be specified on the content element or at least one
of its ancestors.
usesForced named metadata item
The usesForced named metadata item does not apply since the condition attribute is
prohibited.
The itts:forcedDisplay attribute is used to specify forced content semantics.
#zIndex
This feature has no effect since, as specified at , presented regions do not overlap in a Document Instance.
