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Adds support for BT 2100 PQ HDR pixels in PNG #284

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Adds support for BT 2100 PQ HDR pixels in PNG #284

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a4662e7
Add tts:hdrAbsoluteLuminanceGain (issue #233)
palemieux Mar 23, 2017
6510c8a
Add support for HDR PNG
palemieux Mar 30, 2017
2c34bdb
Add generated HTML
palemieux Mar 30, 2017
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Expand Up @@ -6802,6 +6802,13 @@ based on the actual character content of that document.</p>
<div3 id="embedded-content-vocabulary-image">
<head>image</head>
<p>The <el>image</el> element is used to define an author supplied <loc href="#terms-image-resource">image resource</loc>.</p>

<note role="clarification">
<p>
<specref ref="png-hdr-signaling"/> specifies a means of using ITU BT.2100 PQ full-range images with the PNG Format.
</p>
</note>

<p>An <el>image</el> element may appear in two contexts: (1) as a child of a <loc href="#embedded-content-vocabulary-resources"><el>resources</el></loc> element and
(2) as a child of an element in the <loc href="#element-vocab-group-block">Block.class</loc> element group, namely, as a child of a
<loc href="#content-vocabulary-div"><el>div</el></loc> or <loc href="#content-vocabulary-p"><el>p</el></loc> element, or
Expand Down Expand Up @@ -10005,6 +10012,144 @@ represented by this attribute are based upon that defined by <bibref ref="xsl11"
&sect; 7.9.9.</p>
</note>
</div3>
<div3 id="style-attribute-hdrAbsoluteLuminanceGain">
<head>tts:hdrAbsoluteLuminanceGain</head>
<p>The <att>tts:hdrAbsoluteLuminanceGain</att> allows the author to specify the absolute luminance of a region when it is composited onto images with greater dynamic range
than sRGB images.</p>
<note role="clarification">
<p>The sRGB colorspace specifies an absolute reference white point luminance of 80 cd∙m<sup>-2</sup>.
As a result, sRGB elements, i.e. elements whose color is specified in the sRGB space, cannot be
reliably composited with high-dynamic range (HDR) images intended for display with significantly
greater peak luminance, e.g. images that use the PQ EOTF specified in <bibref ref="bt2100_1"/>. Specifically:
<ulist>
<item><p>sRGB elements can appear too dim if they remains unscaled and the average luminance of the scene
exceeds 80 cd∙m<sup>-2</sup>; and
</p></item>
<item><p>conversely, sRGB elements can appear too bright if they are uniformly scaled to an arbitrary large
luminance, say 10,000 cd∙m<sup>-2</sup>.
</p>
</item>
</ulist>
</p>
</note>
<p>This attribute may be specified by any
element type
that permits use of attributes in the TT Style Namespace; however, this attribute applies
as a style property only to those element types indicated in the following table.</p>
<table id="style-property-details-hdrAbsoluteLuminanceGain" role="common">
<col css="width: 25%;"/>
<col/>
<tbody>
<tr>
<td><emph>Values:</emph></td>
<td>
<code><loc href="#style-value-non-negative-integer">&lt;non-negative-integer&gt;</loc></code>
</td>
</tr>
<tr>
<td><emph>Initial:</emph></td>
<td><code>1</code></td>
</tr>
<tr>
<td><emph>Applies to:</emph></td>
<td>
<loc href="#content-vocabulary-region"><el>region</el></loc>
</td>
</tr>
<tr>
<td><emph>Inherited:</emph></td>
<td>yes</td>
</tr>
<tr>
<td><emph>Percentages:</emph></td>
<td>N/A</td>
</tr>
<tr>
<td><emph>Animatable:</emph></td>
<td>discrete</td>
</tr>
</tbody>
</table>

<p>When applying <att>tts:hdrAbsoluteLuminanceGain</att>, the linear optical output value
(C<sub>r</sub>, C<sub>g</sub>, C<sub>b</sub>)
of the components of a pixel of a region is given by:
</p>
<p>
(C<sub>r</sub>, C<sub>g</sub>, C<sub>b</sub>) = 80 cd∙m<sup>-2</sup> ∙ <att>tts:hdrAbsoluteLuminanceGain</att> ∙ (r, g, b)
</p>
<p>
where (r, g, b) are the normalized linear sRGB components of the pixel.
</p>

<table id="hdr-computation-example" role="example">
<caption>Example &ndash; Absolute linear optical output calculation using tts:hdrAbsoluteLuminanceGain</caption>
<tbody>
<tr>
<td>
<slist>
<sitem>
Given <code>tts:hdrAbsoluteLuminanceGain="1.5"</code>, the absolute linear optical output of the components of a rendered pixel with color <code>rgb(218,165,32)</code> is:
</sitem>
<sitem>
(109.83, 56.28, 1.1) cd∙m<sup>-2</sup> ≈ 80 cd∙m<sup>-2</sup> ∙ 1.5 ∙((218/255)<sup>2.4</sup>, (165/255)<sup>2.4</sup>, (32/255)<sup>2.4</sup>)
</sitem>
</slist>
</td>
</tr>
</tbody>
</table>

<p>
<specref ref="absolute-hdr-compositing"/> provides a detailed example of compositing sRGB pixels onto HDR pixels using <att>tts:hdrAbsoluteLuminanceGain</att>.
</p>

<note role="clarification">
<p>
As specified in <specref ref="style-value-color"/>, blending of a <loc href="#terms-document-instance">document instance</loc> onto a target image is typically performed using pixel components encoded using
the inverse EOTF of the target image, as opposed to pixel components expressed in
linear light. As a result, the use of semi-transparent regions can yield
perceptually different results depending on the inverse EOTF used by the target
image, especially when there is a large luminance difference between the pixels of the <loc href="#terms-document-instance">document instance</loc> and the target image, e.g. video. Authors are therefore encouraged to consider visual results involving semi-transparent elements.
</p>
</note>

<p>If a computed value of the property associated with this attribute is not supported,
then a <loc href="#terms-presentation-processor">presentation processor</loc> must use the closest value.</p>

<p>The style is illustrated by the following example.</p>
<table id="style-attribute-hdrAbsoluteLuminanceGain-example-1" role="example">
<caption>Example Fragment &ndash; tts:hdrAbsoluteLuminanceGain</caption>
<tbody>
<tr>
<td>
<eg xml:space="preserve">
&lt;region xml:id="r1"&gt;
&lt;style tts:extent="60% 20%"/&gt;
&lt;style tts:origin="10% 70%"/&gt;
&lt;style tts:backgroundColor="black"/&gt;
&lt;style tts:color="white"/&gt;
&lt;style tts:hdrAbsoluteLuminanceGain="3.8"/&gt;
&lt;/region&gt;
&lt;region xml:id="r2"&gt;
&lt;style tts:extent="60% 20%"/&gt;
&lt;style tts:origin="10% 10%"/&gt;
&lt;style tts:backgroundColor="black"/&gt;
&lt;style tts:color="white"/&gt;
&lt;/region&gt;
...
&lt;p region="r1"&gt;
The absolute luminance of this text is specified higher 3.8 times brighter than the other text.
&lt;/p&gt;
&lt;p region="r2"&gt;
The other text.
&lt;/p&gt;
</eg>
</td>
</tr>
</tbody>
</table>
</div3>
<div3 id="style-attribute-ipd">
<head>tts:ipd</head>
<p>The <att>tts:ipd</att> attribute is used to specify the
Expand Down Expand Up @@ -23195,13 +23340,22 @@ href="http://www.w3.org/2001/tag/doc/namespaceState-2006-01-09.html">The Disposi
of Names in an XML Namespace</titleref>, TAG Finding, 9 January 2006. (See
<xspecref href="http://www.w3.org/2001/tag/doc/namespaceState-2006-01-09.html">http://www.w3.org/2001/tag/doc/namespaceState-2006-01-09</xspecref>.)
</bibl>

<bibl id="png" key="PNG">David Duce, <titleref
href="http://www.w3.org/TR/2003/REC-PNG-20031110">Portable Network Graphics (PNG) Specification (Second Edition)</titleref>.
</bibl>

<bibl id="qaf-sg" key="QAF SG">Karl Dubost, et al.,
<titleref
href="http://www.w3.org/TR/2005/REC-qaframe-spec-20050817/">QA
Framework: Specifications Guidelines</titleref>, W3C Recommendation, 17
August 2005. (See
<xspecref href="http://www.w3.org/TR/qaframe-spec/">http://www.w3.org/TR/qaframe-spec/</xspecref>.)
</bibl>
<bibl id="bt2100_1" key="ITU BT.2100-1">Recommendation ITU-R BT.2100-1,
<titleref
href="https://www.itu.int/rec/R-REC-BT.2100">Image parameter values for high dynamic range television for use in production and international programme exchange</titleref>
</bibl>
<bibl id="smiththreed" key="SMITH">Smith, Michael D.; Malia, Jason, <titleref
href="http://ieeexplore.ieee.org/document/7269419/">Controlling Miniaturization in Stereoscopic 3D Imagery</titleref>, in Motion Imaging Journal, SMPTE , vol.124, no.3, pp.50-58, April 2015 doi: 10.5594/m001513.
</bibl>
Expand Down Expand Up @@ -25170,6 +25324,182 @@ where <emph>chunking</emph> refers to the subdivision of resource content into c

</inform-div1>

<inform-div1 id="absolute-hdr-compositing">
<head>HDR Compositing Example</head>
<p>
The following illustrates the use of <att>tts:hdrAbsoluteLuminanceGain</att> to composite sRGB pixels onto HDR pixels.
</p>
<olist>
<item>
<p>
Let (r, g, b) be a full-range 8-bit sRGB pixel with opacity a between 0 and 1.
</p>
</item>
<item>
<p>
Let (R, G, B) and (R<sub>c</sub>, G<sub>c</sub>, B<sub>c</sub>) be full-range 10-bit pixels in the system colorimetry specified in <bibref ref="bt2100_1"/> using PQ EOTF and full-range quantization, with opacity A and A<sub>c</sub> between 0 and 1.
</p>
</item>
<item>
<p>
Inverse the 8-bit full-range quantization:
</p><p>
(r, g, b) / 255 → (r, g, b)
</p>
</item>
<item>
<p>
Linearize using the sRGB EOTF:
</p><p>
(r<sup>2.4</sup>, g<sup>2.4</sup>, b<sup>2.4</sup>) → (r, g, b)
</p>
</item>
<item>
<p>
Compute HDR absolute luminance using the <att>tts:hdrAbsoluteLuminanceGain</att> attribute and the sRGB illuminant:
</p><p>
80 cd∙m<sup>-2</sup> ∙ <att>tts:hdrAbsoluteLuminanceGain</att> ∙ (r, g, b) → (r, g, b)
</p>
</item>
<item>
<p>
Convert from sRGB color space to <bibref ref="bt2100_1"/> color space:
</p><p>
[(0.62740389593470, 0.32928303837789, 0.04331306568741),
(0.06909728935823, 0.91954039507545, 0.01136231556630),
(0.01639143887515, 0.08801330787723, 0.89559525324763)] ∙ (r, g, b) → (r, g, b)
</p>
</item>
<item>
<p>
Normalize to 10,000 cd∙m<sup>-2</sup>:
</p><p>
(r, g, b) / 10000 cd∙m<sup>-2</sup> → (r, g, b)
</p>
</item>
<item>
<p>
Apply inverse PQ EOTF specified in <bibref ref="bt2100_1"/>:
</p><p>
(PQ(r), PQ(g), PQ(b)) → (r, g, b)
</p><p>
with PQ(L) = [(c1 + c2 ∙ L<sup>m1</sup>) / (1 + c3 ∙ L<sup>m1</sup>)]<sup>m2</sup> and m1 = 0.1593017578125, m2 = 78.84375, c1 = 0.8359375, c2 = 18.8515625, and c3 = 18.6875.
</p>
</item>
<item>
<p>
Apply opacity:
</p><p>
(1-a) ∙ (r, g, b) → (r, g, b)
</p>
</item>
<item>
<p>
Apply 10-bit full-range quantization:
</p><p>
(Q(r), Q(g), Q(b)) → (r, g, b)
</p><p>
where Q(N) = floor(1023 ∙ N + 0.5)
</p>
</item>
<item>
<p>
Composite to yield (R<sub>c</sub>, G<sub>c</sub>, B<sub>c</sub>)
</p><p>
(clamp(r + R), clamp(g + G), clamp(b + B)) → (R<sub>c</sub>, G<sub>c</sub>, B<sub>c</sub>)
</p><p>
1 + (1 - a) ∙ (A - 1) → A<sub>c</sub>
</p><p>
where clamp(x) = {x, if x ∈ [0, 1023]} or {1023, if x > 1023}
</p>
</item>
</olist>

</inform-div1>

<div1 id="png-hdr-signaling">
<head>Using ITU BT.2100 PQ full-range images with the PNG Format</head>
<p><emph>This appendix is normative.</emph></p>
<div2>
<head>Introduction</head>
<p>The <code>gAMA</code> chunk of the Portable Network Graphics (PNG) format (specified in <bibref ref="png"/>) parameterizes the transfer function of the image as a power law.
As such, it cannot model the PQ transfer function specified in <bibref ref="bt2100_1"/>, which is commonly used for HDR images.</p>
<p>The following specifies a means of storing iITU BT.2100 PQ full-range images in the PNG format, without
modification to existing chunks or the introduction of new chunks.</p>
</div2>


<div2>
<head>HDR PNG File</head>
<p>
The file shall conform to <bibref ref="png"/>.
</p><p>
The image type shall be <code>truecolor</code>.
</p><p>
The file shall contain an <code>iCCP</code> chunk with a profile name equal to
one of the values in <specref ref="hdr-systems"/>.
</p><p>
The embedded ICC profile of the <code>iCCP</code> chunk, the <code>cHRM</code> chunk and the <code>gAMA</code> chunk should be set according to the profile
name, as specified in <specref ref="hdr-systems"/>.
</p>
<note role="clarification">
<p>
The <code>gAMA</code>, <code>cHRM</code> and embedded ICC profile are provided
for compatibility with processors that do not conform to this Appendix.
</p>
</note>
<p>
The embedded ICC profile shall be compressed using Method 0, as specified in <bibref ref="png"/>.
</p>
<table id="hdr-systems" role="common">
<caption>HDR Systems</caption>
<thead>
<tr>
<th>Pixel Color Specifications</th>
<th><code>iCCP</code> Chunk Profile Name</th>
<th><code>iCCP</code> Chunk Embedded ICC Profile</th>
<th><code>gAMA</code> Chunk</th>
<th><code>cHRM</code> Chunk</th>
</tr>
</thead>
<tbody>
<tr>
<td>Full-range PQ as specified in <bibref ref="bt2100_1"/></td>
<td><code>TTML2_COLOR_ITUR2100PQFull</code></td>
<td><loc href="icc/TTML2_COLOR_ITUR2100PQFull.icc">TTML2_COLOR_ITUR2100PQFull.icc</loc></td>
<td>15000</td>
<td>

<table>
<thead>
<tr><th>Parameter</th><th>Value</th></tr>
</thead>
<tbody>
<tr><th>White point x</th><th>31270</th></tr>
<tr><th>White point y</th><th>32900</th></tr>
<tr><th>Red x</th><th>70800</th></tr>
<tr><th>Red y</th><th>29200</th></tr>
<tr><th>Green x</th><th>17000</th></tr>
<tr><th>Green y</th><th>79700</th></tr>
<tr><th>Blue x</th><th>13100</th></tr>
<tr><th>Blue y</th><th>4600</th></tr>
</tbody>
</table>

</td>
</tr>
</tbody>
</table>
<note role="clarification">
<p>
The value of the <code>gAMA</code> chunk is selected for a close match to the PQ EOTF below 100 nits.
</p>
</note>

</div2>

</div1>

&acknowledgements;
</back>
</spec>
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