TTML Profiles for Internet Media Subtitles and Captions 1.0.1 (IMSC1)

5.1 General

Notwithstanding special cases, e.g. a Document Instance that contains no p, span, br element and no smpte:backgroundImage attribute, it is generally not possible to construct a Document Instance that conforms to the Text Profile and Image Profile simultaneously, and it is not possible to construct a Document 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 distinct Document Instances, one conforming to the Text Profile and the other conforming to the Image Profile, SHOULD be offered. In addition, the Text Profile Document Instance SHOULD be associated with the Image Profile Document 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.

Annex D. WCAG Considerations specifically discusses this specification in the context of the [WCAG20] guidelines.

5.2 Text Profile

The Text Profile consists of Sections 6. Common Constraints and 7. Text Profile Constraints.

5.3 Image Profile

The Image Profile consists of Sections 6. Common Constraints and 8. Image Profile Constraints.

5.4 Profile Resolution Semantics

For the purpose of content processing, the determination of the resolved profile SHOULD take into account both the signaled profile, as defined in 6.9 Profile Signaling, and profile metadata, as designated by either (or both) the Document Interchange Context or (and) the Document Processing Context, which MAY entail inspecting document content.

If the resolved profile is not a profile supported by the Processor but is feasibly interoperable with the Text Profile, then the resolved profile is the Text Profile; otherwise, if the resolved profile is not a profile supported by the Processor but is feasibly interoperable with the Image Profile, then the resolved profile is the Image Profile.

If the resolved profile is a profile supported by the Processor, then the Processor SHOULD process the Document Instance according to the resolved profile. If the resolved profile is neither Text Profile nor Image Profile, processing is outside the scope of this specification.

If the resolved profile is undetermined or not supported by the Processor, then the Processor SHOULD nevertheless process the Document Instance using one of its supported profiles, with a preference for the Text Profile over the Image Profile; otherwise, processing MAY be aborted.

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.

6.3 Namespaces

The following namespaces (see [xml-names]) are used in 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.

6.5 Related Video Object

A Document Instance MAY be associated with a Related Video Object.

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.

6.7 Extensions

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

ittp:progressivelyDecodable
  : "true"
  | "false"

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

<?xml version="1.0" encoding="UTF-8"?>
<tt xml:lang="en"
    xmlns="https://www.w3.org/ns/ttml"
    xmlns:ttm="https://www.w3.org/ns/ttml#metadata"
    xmlns:tts="https://www.w3.org/ns/ttml#styling"
    xmlns:ttp="https://www.w3.org/ns/ttml#parameter"
    xmlns:ittp="https://www.w3.org/ns/ttml/profile/imsc1#parameter"
    xmlns:itts="https://www.w3.org/ns/ttml/profile/imsc1#styling"
    ittp:aspectRatio="16 9"
    ttp:profile="https://www.w3.org/ns/ttml/profile/imsc1/text">
    <head>
        <layout>
            <region xml:id="r1" tts:showBackground="whenActive" tts:origin="10% 2%" tts:extent="80% 10%" tts:color="white" itts:forcedDisplay="true" tts:backgroundColor="black"/>
            <region xml:id="r2" tts:showBackground="whenActive" tts:origin="10% 80%" tts:extent="80% 10%" tts:color="white" tts:backgroundColor="black"/>
        </layout>
    </head>
    <body>
        <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>
    </body>
</tt>

<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>

<?xml version="1.0" encoding="UTF-8"?>
<tt xml:lang="fr"
xmlns="https://www.w3.org/ns/ttml"
xmlns:ttp="https://www.w3.org/ns/ttml#parameter"
ttp:profile="https://www.w3.org/ns/ttml/profile/imsc1/image"
xmlns:tts="https://www.w3.org/ns/ttml#styling"
xmlns:smpte="https://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt"
xmlns:ittm="https://www.w3.org/ns/ttml/profile/imsc1#metadata" tts:extent="320px 240px">
    <head>
        <layout>
            <region xml:id="area1" tts:origin="20px 215px" tts:extent="180px 20px"/>
        </layout>
    </head>
    <body>
        <div begin="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>

ittp:activeArea
  : leftOffset topOffset width height
leftOffset | topOffset | width | height
  : <percentage>                // where <percentage> is non-negative and not greater than 100%.

x = leftOffset * (1 - width/100)
y = topOffset * (1 - height/100)

<?xml version="1.0" encoding="UTF-8"?>
<tt xml:lang="en"
    xmlns="https://www.w3.org/ns/ttml"
    xmlns:ttm="https://www.w3.org/ns/ttml#metadata"
    xmlns:tts="https://www.w3.org/ns/ttml#styling"
    xmlns:ttp="https://www.w3.org/ns/ttml#parameter"
    xmlns:ittp="https://www.w3.org/ns/ttml/profile/imsc1#parameter"
    ittp:activeArea="50% 50% 80% 80%"
    tts:extent="640px 480px"
    ttp:profile="https://www.w3.org/ns/ttml/profile/imsc1/text">
    <head>
        <layout>
            <region xml: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"/>
            <region xml: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"/>
            <region xml: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>
            <p region="area1" begin="0s" end="6s">This region is within the editorial area.</p>
            <p region="area2" begin="0s" end="6s">This region is within the editorial area.</p>
            <p region="area3" begin="0s" end="6s">This region is not.</p>
        </div>
    </body>
</tt>

6.7.6 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.

Example 7

<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="https://www.w3.org/ns/ttml"
    xmlns:ttp="https://www.w3.org/ns/ttml#parameter"
    xmlns:tts="https://www.w3.org/ns/ttml#styling"
    xmlns:itts="https://www.w3.org/ns/ttml/profile/imsc1#styling"
    ttp:timeBase="media"
    xml:lang="en"
    ttp:cellResolution="50 30"
    ttp:profile="https://www.w3.org/ns/ttml/profile/imsc1/text">
  <head>
    <styling>
      <style xml:id="spanStyle" tts:color="#ffffff" tts:backgroundColor="#000000" />
      <style xml:id="spanStyleSmall" tts:color="#000000" tts:backgroundColor="#dfbb02" tts:fontSize="50%"/>
      <style xml:id="spanStyleBig" tts:color="#ffffff" tts:backgroundColor="#b75800" tts:fontSize="150%"/>
      <style xml:id="paragraphStyle" tts:fontFamily="monospaceSerif" tts:textAlign="center"
          tts:fontSize="200%" tts:lineHeight="165%"  itts:fillLineGap="true"/>
    </styling>
    <layout>
      <region xml:id="bottom" tts:origin="10% 10%" tts:extent="80% 80%" tts:displayAlign="after" />
    </layout>
  </head>
  <body>
    <div>
      <p xml:id="subtitle1" region="bottom" begin="00:00:00.000" end="00:00:30.000" style="paragraphStyle">
        <span style="spanStyle">##Line gaps##</span><br/>
        <span style="spanStyle">The quick </span><span style="spanStyleBig">brown</span><span style="spanStyle"> fox</span><br/>
        <span style="spanStyle">jumps over the </span><span style="spanStyleSmall">lazy</span><span style="spanStyle"> dog</span><br/>
        <span style="spanStyle">##Line gaps##</span>
      </p>
    </div>
  </body>
</tt>

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".

Example 8

<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="https://www.w3.org/ns/ttml"
    xmlns:ttp="https://www.w3.org/ns/ttml#parameter"
    xmlns:tts="https://www.w3.org/ns/ttml#styling"
    xmlns:itts="https://www.w3.org/ns/ttml/profile/imsc1#styling"
    ttp:timeBase="media"
    xml:lang="en"
    ttp:cellResolution="50 30"
    ttp:profile="https://www.w3.org/ns/ttml/profile/imsc1/text">
  <head>
    <styling>
      <style xml:id="spanStyle" tts:color="#ffffff" tts:backgroundColor="#000000" />
      <style xml:id="paragraphStyleNoGap" tts:fontFamily="monospaceSerif" tts:textAlign="center" tts:fontSize="200%"
          tts:lineHeight="165%" itts:fillLineGap="true"/>
      <style xml:id="paragraphStyle" tts:fontFamily="monospaceSerif" tts:textAlign="center" tts:fontSize="200%"
          tts:lineHeight="165%" itts:fillLineGap="false"/>
    </styling>
    <layout>
      <region xml:id="bottom" tts:origin="10% 10%" tts:extent="80% 80%" tts:displayAlign="after" />
      <region xml:id="top" tts:origin="10% 10%" tts:extent="80% 80%" tts:displayAlign="before" />
    </layout>
  </head>
  <body>
    <div region="bottom" begin="00:00:00.000" end="00:00:30.000">
      <p xml:id="subtitle1" style="paragraphStyle">
        <span style="spanStyle">Paragraph 1</span>
      </p>
      <p xml:id="subtitle1" style="paragraphStyle">
        <span style="spanStyle">Paragraph 2</span>
      </p>
    </div>
    <div region="top" begin="00:00:00.000" end="00:00:30.000">
      <p xml:id="subtitle1" style="paragraphStyleNoGap">
        <span style="spanStyle">Paragraph 1</span>
      </p>
      <p xml:id="subtitle1" style="paragraphStyleNoGap">
        <span style="spanStyle">Paragraph 2</span>
      </p>
    </div>
  </body>
</tt>

6.8 Region

6.9 Profile Signaling

The ttp:profile attribute SHOULD be present on the tt element and equal to the designator of the IMSC 1.0 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 neither the ttp:profile attribute nor the ttp:profile element are present, and, instead, the designator of the IMSC 1.0 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] and illustrated in I.2 EBU-TT-D; or

  Note

  Multiple other ebuttm: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.

• the Document Instance also conforms to [ttml10-sdp-us], in which case the ttp:profile attribute is not present and the ttp:profile element is present, as illustrated in I.3 SDP-US.

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, prohibited and optional.

6.12 Style Resolution

The following style properties are subject to the Style Resolution procedures specified at Section 8.4 of [TTML1]:

• itts:fillLineGap
• itts:forcedDisplay
• ebutts:linePadding
• ebutts:multiRowAlign

7.1 Profile Designator

This profile is associated with the following profile designator:

7.2 Recommended Character Sets

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

#PCDATA content within p and span elements of a Document Instance SHOULD NOT include the TAB (U+0009) character.

7.3 Reference Fonts

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.

7.4 Features and Extensions

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

8.1 Profile Designator

This profile is associated with the following profile designator:

8.2 Presented Image

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.

8.4 Features and Extensions

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

9.1 Overview (non-normative)

This Section specifies the Hypothetical Render Model illustrated in Figure 3.

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 the dimensions of the Root Container Region, 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:

1. 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;
2. paint, according to stacking order, all background pixels for each region;
3. paint all pixels for background colors associated with text or image subtitle content; and
4. 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.

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.

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 Region in its entirety.

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) = ∑_Ri∈Rp 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 Region height ∙ Root Container Region width)

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) = ∑_{Ii ∈ Ic} NRGA(I_i) / ICpy + ∑_{Ij ∈ Id} 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 Region height ∙ Root Container Region 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.

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

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.

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) = ∑_{gi ∈ Γr} NRGA(g_i) / Ren(g_i) + ∑_{gj ∈ Γc} 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 Region height)²

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.