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Conform cICP chunk to Rec. ITU-T h.273 (#134)
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* Conform to Rec. ITU-T h.273 (#129)
Clean-up cICP chunk

* Remove misleading example
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@palemieux
palemieux authored Jun 23, 2022
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Expand Up @@ -159,6 +159,12 @@
date: "2004-11-30",
href: "https://standards.globalspec.com/std/892300/SMPTE%20ST%20170M"
},
"SMPTE RP 2077": {
title: "Full-Range Image Mapping",
publisher: "Society of Motion Picture and Television Engineers",
date: "2013-01-01",
href: "https://doi.org/10.5594/SMPTE.RP2077.2013"
},
"TIFF 6.0": {
"href": "https://www.loc.gov/preservation/digital/formats/fdd/fdd000022.shtml",
"title": "TIFF Revision 6.0",
Expand Down Expand Up @@ -554,6 +560,10 @@ <h2 id="3Definitions">Definitions</h2>
are scaled to the range 0 to 1.
</dd>

<dt><dfn>full-range image</dfn></dt>
<dd>Image where reference black and white correspond to sample values
<code>0</code> and <code>2<sup>bit depth</sup> - 1</code>, respectively.</dd>

<!-- Maintain a fragment named "3greyscale" to preserve incoming links to it -->
<dfn id="3greyscale">
<dt>greyscale</dt></dfn>
Expand Down Expand Up @@ -637,6 +647,10 @@ <h2 id="3Definitions">Definitions</h2>
<dd>data compression algorithm described by Ziv and Lempel in
their 1977 paper [[Ziv-Lempel]].</dd>

<dt><dfn>narrow-range image</dfn></dt>
<dd>Image where reference black and white do not correspond to sample values
<code>0</code> and <code>2<sup>bit depth</sup> - 1</code>, respectively.</dd>

<!-- Maintain a fragment named "3networkByteOrder" to preserve incoming links to it -->
<dfn id="3networkByteOrder">
<dt>network byte
Expand Down Expand Up @@ -4131,115 +4145,59 @@ <h2><span class="chunk">cICP</span>
99 73 67 80
</pre>

<p>If the <span class="chunk">cICP</span> chunk is present, the
image samples have a specifically defined colour space, transfer function, and signal
range that should be displayed using the specified rendering intent defined in
[[ITU-T H.273]].
</p>
<p>If present, the <span class="chunk">cICP</span> chunk specifies the colour
space of the image using the code points specified in [[ITU-T H.273]]. This
colour space SHOULD be used when processing the image, including by a decoder
when rendering the image.</p>

<p>The cICP chunk contains:</p>
<p>The following specifies the syntax of the <span class="chunk">cICP</span> chunk:</p>

<table class="Regular" summary=
"This table defines the cICP chunk">
<table id="cICP-chunk-syntax" class="simple">
<tr>
<td class="Regular">Colour Primaries</td>
<td class="Regular">2 bytes</td>
<td>Colour Primaries</td>
<td>1 byte</td>
</tr>

<tr>
<td class="Regular">Transfer Function</td>
<td class="Regular">2 bytes</td>
<td>Transfer Function</td>
<td>1 byte</td>
</tr>

<tr>
<td class="Regular">Matrix Coefficients</td>
<td class="Regular">2 bytes</td>
<td>Matrix Coefficients</td>
<td>1 byte</td>
</tr>


<tr>
<td class="Regular">Full Range Flag</td>
<td class="Regular">1 byte</td>
<td>Video Full Range Flag</td>
<td>1 byte</td>
</tr>

</table>

<p>Each of the fields of the <span class="chunk">cICP</span> chunk corresponds
to the parameter of the same name in [[ITU-T H.273]].</p>

<p>Coding-Independent Code Points are defined in [[ITU-T H.273]] and
are typically embedded in video content and streams to define an explicit
video rendering. The most common renderings for video imagery are defined in
[[ITU-R BT.709]] and
and [[ITU-R BT.2100]].</p>
<p>The <code>Matrix Coefficients</code> value is equal to <code>0</code>.</p>

<p>The cICP chunk contains four decimal values corresponding to
the colour primaries, transfer function, matrix coefficients and video signal
range flag for the source imagery. </p>
<aside class="note">A <code>Matrix Coefficients</code> value othen than
<code>0</code> signals a transformation between colour difference and RGB
representations, which is not used in this International Standard since PNG
images are RGB images.</aside>

<p>Note: In Coding-Independent Code Points
[[ITU-T H.273]], the
third video coding point defines matrix coefficients which are used to describe
conversions from other colour representations to RGB. PNG image samples are
explicitly RGB, so the matrix coefficients value must be set to "0" which is
labeled in [[ITU-T H.273]]
as "RGB Identity". With PNG, decoders will always assume a matrix coefficient
value of "0" but this value will remain in PNG for forward compatibility.</p>
<p>The <code>Video Full Range Flag</code> value MUST be either <code>0</code>
or <code>1</code>.</p>

<aside class="note">If <code>Video Full Range Flag</code> value is
<code>1</code>, then the image is a <a>full-range image</a>. The vast majority
of computer graphics and web images, including those used in traditional PNG
workflows, are <a>full-range images</a>. If <code>Video Full Range
Flag</code> value is <code>0</code>, then the image is a <a>narrow-range
image</a>. Such images are found in video workflows and the interpretation of
sample values smaller (larger) than reference black (white) depend on the
application. [[SMPTE RP 2077]] specifies mapping between <a>full-range
images</a> and <a>narrow-range images</a>.</aside>

<table class="Regular" summary=
"This table defines the values in the cICP chunk that allow for an accurate rendering
of source content with a video rendering intent">
<tr>
<td class="Regular">0</td>
<td class="Regular">Colour Primaries</td>
<td class="Regular">Identifies the colour primaries of the source picture as
defined in [[ITU-T H.273]]</td>
</tr>
<p>The <span class="chunk">cICP</span> chunk MUST come before the IDAT chunk.</p>

<tr>
<td class="Regular">1</td>
<td class="Regular">Transfer Characteristics</td>
<td class="Regular">Identifies the reference opto-electronic transfer
characteristic function or inverse reference electro-optical transfer
characteristic function as defined in [[ITU-T H.273]]</td>
</tr>

<tr>
<td class="Regular">2</td>
<td class="Regular">Matrix Coefficients</td>

<td class="Regular">Identifies the matrix coefficients used in deriving luma and
chroma signals from the green, blue and red as defined in [[ITU-T H.273]]</td>.
As noted earlier, since PNG is explicitly RGB, this value must always be set to
"0" which is labeled as "RGB Identity" in [[ITU-T H.273]].

</td>
</tr>

<tr>
<td class="Regular">3</td>
<td class="Regular">Full Range Flag</td>
<td class="Regular">Identifies a Full-Range scaling when this value is "1" as defined in
[[ITU-T H.273]].</td>
Currently, PNG is "RGB ONLY" and typically lossless and therefore a value of "1"
is the default to indicate the the use of full-range signal scaling. There are
some use-cases where levels below or above nominal signal level (0 or 100%)
should be preserved, and therefore the signal range would be defined as "narrow
range" which is typically used in YC<sub>B</sub>C<sub>R</sub> video imagery. For
"narrow range" the Full-Range Flag would be set to "0". In the future, if PNG is
updated to include the YC<sub>B</sub>C<sub>R</sub> colour representation,
narrow-range signal scaling would be the default. In the future, if PNG is
updated to include IC<sub>T</sub>C<sub>P</sub> the full-range flag would be set
to a value of "1" since it uses the PQ transfer function and therefore doesn't
benefit from the existence of sub-blacks or super-whites.
</td>
</tr>


</table>
<p>The cICP chunk must come before the IDAT chunk.</p>

<p>When the cICP chunk is present, PNG decoders that recognize it shall ignore
the following chunks:</p>
<p>When the <span class="chunk">cICP</span> chunk is present, decoders that
recognize it SHALL ignore the following chunks:</p>
<ul>
<li>iCCP</li>
<li>ICCN </li>
Expand All @@ -4248,83 +4206,37 @@ <h2><span class="chunk">cICP</span>
<li>sRGB </li>
</ul>

<aside class="example"><span class="chunk">cICP</span> chunk field values for
a common sRGB <a>full-range image</a>:
<pre>
1 13 0 1
</pre>
</aside>

<table class="Regular" summary=
"The table below provides an example of the COLPRIMS, TRANSFC, MATCOEFFS, VIDFRNG
values for cICP">

<p>Below is an example for the use of the cICP:</p>

<tr>
<th colspan="2"><a href="#11COLPRIMS"><span class=
"chunk">COLPRIMS</span></a> </th>
</tr>

<tr>
<td class="Regular">COLPRIMS</td>
<td class="Regular">9</td>
</tr>

<tr>
<th colspan="2"><a href="#11TRANSFC"><span class=
"chunk">TRANSFC</span></a> </th>
</tr>

<tr>
<td class="Regular">TRANSFC</td>
<td class="Regular">16</td>
</tr>

<tr>
<th colspan="2"><a href="#11MATCOEFFS"><span class=
"chunk">MATCOEFFS</span></a> </th>
</tr>

<tr>
<td class="Regular">MATCOEFFS</td>
<td class="Regular">0</td>
</tr>

<tr>
<th colspan="2"><a href="#11VIDFRNG"><span class=
"chunk">TRANSFC</span></a> </th>
</tr>

<tr>
<td class="Regular">VIDFRNG</td>
<td class="Regular">1</td>
</tr>

</table>



<p>Here is an example of source content that uses the BT.2100 colour primaries [[ITU-R BT.2100]],
with the PQ transfer function, Matrix Coefficients (using RGB Identity)and Full-Range signal scaling</p>

<aside class="example"><span class="chunk">cICP</span> chunk field values for a
<a>full-range image</a> that uses the colour primaries and the PQ transfer
function specified at [[ITU-R BT.2100]]:
<pre>
9 16 0 1
</pre>
</aside>

<p>Here is an example of source content that uses the BT.2100 colour primaries,
with the HLG transfer function, Matrix Coefficients (using RGB Identity)and Full-Range signal scaling</p>

<aside class="example"><span class="chunk">cICP</span> chunk field values for a
<a>full-range image</a> that uses the colour primaries and the HLG transfer
function specified at [[ITU-R BT.2100]]:
<pre>
9 18 0 1
</pre>
</aside>

<p>Here is an example of source content that uses BT.709 colour primaries [[ITU-R BT.709]],
with the BT.709 transfer function, Matrix Coefficients (using RGB Identity)and Full-Range signal scaling</p>

<aside class="example"><span class="chunk">cICP</span> chunk field values for a
<a>narrow-range image</a> that uses the colour primaries and the transfer
function defined at [[ITU-R BT.709]]:
<pre>
1 1 0 1
1 1 0 0
</pre>
</aside>


<p>When the <span class="chunk">cICP</span> chunk is used, a
decoder should be capable of producing the proper rendering intent as described in
[[ITU-T H.273]] and
it's associated recommendations.</p>
</section>

<section>
Expand Down Expand Up @@ -4364,7 +4276,6 @@ <h2><span class="chunk">iCCN</span>
<td class="Regular">n bytes</td>
</tr>
</table>

<p>The iCCN profile name may be any convenient name for referring to
the profile. It is case-sensitive. Profile names shall contain printable
characters. Leading, trailing, and consecutive spaces are not permitted.</p>
Expand Down

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