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Clarifies fallback strategy and editorial clean-up #4

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Jul 7, 2017
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Clarifies fallback strategy and editorial clean-up #4

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f8a35d6
Clarified fallback strategy (issue #1)
palemieux Jun 23, 2017
f31e530
Address https://github.com/w3c/png-hdr-pq/pull/4#discussion_r125535857
palemieux Jul 6, 2017
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55 changes: 33 additions & 22 deletions index.html
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Original file line number Diff line number Diff line change
Expand Up @@ -51,9 +51,9 @@

<body>
<section id="abstract">
<p>This specification defines a mechanism for storing images that use the Reference PQ EOTF specified in [[!BT2100-1]] in the Portable
Network Graphics (PNG) format (specified in [[!PNG]]), without modification to existing chunks or the introduction of new
chunks.</p>
<p>This specification defines a mechanism for storing images that use the Reference PQ EOTF specified in [[!BT2100-1]] in the
Portable Network Graphics (PNG) format (specified in [[!PNG]]), without modification to existing chunks or the introduction of
new chunks.</p>
</section>

<section id="sotd">
Expand All @@ -68,28 +68,38 @@ <h2>Introduction</h2>
commonly used for HDR images.</p>

<p>This specification uses the existing <code>iCCP</code> chunk to unambiguously signal the color system of an image that uses
the Reference PQ EOTF specified in [[!BT2100-1]]</p>
the Reference PQ EOTF specified in [[!BT2100-1]]. It also allows graceful processing by decoders that do not conform to this
specification by recommending fallback values for the <code>gAMA</code> chunk, <code>cHRM</code> chunk, and embedded ICC
profile.</p>
</section>

<section id="file">
<h2>PQ HDR PNG File</h2>

<p>The file shall conform to [[!PNG]].</p>
<p>The file SHALL conform to [[!PNG]].</p>

<p>The image type shall be <code>truecolor</code>.</p>
<p>The image type SHALL be <code>truecolor</code>.</p>

<p>The image shall conform to one of the Pixel Encoding Systems specified in the table below.</p>
<p>The image SHALL conform to one of the Pixel Encoding Systems specified in the table below.</p>

<p>The file shall contain an <code>iCCP</code> chunk whose profile name is set according to the table below.</p>
<p>The file SHALL contain an <code>iCCP</code> chunk, constrained as follows:</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 table below.</p>
<ul>
<li>the profile name of the <code>iCCP</code> chunk SHALL be set as specified by the table below according to the Pixel
Encoding System of the image; and</li>

<p>The embedded ICC profile shall be compressed using Method 0, as specified in [[!PNG]].</p>
<li>the embedded ICC profile of the <code>iCCP</code> chunk SHOULD contained one of the ICC profiles specified by the table
below according to the Pixel Encoding System of the image; and</li>

<li>if present, he embedded ICC profile SHALL be compressed using Method 0, as specified in [[!PNG]].</li>
</ul>

<p>The <code>cHRM</code> chunk and the <code>gAMA</code> chunk of the file SHOULD be set as specified by the table below
according to the Pixel Encoding System of the image.</p>

<table id="hdr-systems" style="font-size:small;">
<caption>
HDR Systems
Pixel Encoding Systems
</caption>

<thead>
Expand All @@ -98,7 +108,7 @@ <h2>PQ HDR PNG File</h2>

<th><code>iCCP</code> Chunk Profile Name</th>

<th><code>iCCP</code> Chunk Embedded ICC Profile</th>
<th><code>iCCP</code> Chunk Embedded ICC Profile(s)</th>

<th><code>gAMA</code> Chunk</th>

Expand All @@ -113,7 +123,7 @@ <h2>PQ HDR PNG File</h2>
<td><code>ITUR_2100_PQ_FULL</code></td>

<td>
<a href="icc/ITUR_2100_PQ_FULL.icc">ITUR_2100_PQ_FULL.icc</a>
<a href="icc/ITUR_2100_PQ_FULL.icc">ITUR_2100_PQ_FULL.icc</a>
</td>

<td>15000</td>
Expand Down Expand Up @@ -183,16 +193,17 @@ <h2>PQ HDR PNG File</h2>
</tbody>
</table>

<p class="note">The <code>gAMA</code> and embedded ICC profile are provided solely for compatibility with processors that do
not conform to this specification.</p>
<p class="note">The <code>gAMA</code> chunk, <code>cHRM</code> chunk, and embedded ICC profile specified in the table above are
not necessarily accurate, and, even if accurate, their processing is not required to reproduce the image as intended by the
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Not necessarily? They are known to be inaccurate.
Suggest "... in the table above are inaccurate, due to limitations in ICC V4 with HDR images. However, their processing is not required ..."

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@svgeesus Yes, at this point, all are inaccurate. I used not necessarily to keep the door open for some of these parameters to be accurate if/when more rows are added to the table in the future.

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I prefer to clearly state that the current ones are known to be very inaccurate.

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Ok. cHRM is in fact accurate (it reflects Rec 2020 primaries), so I plan to revise the note to state that ICC and gamma are inaccurate.

author. They are provided for graceful fallback for implementations that do not recognize the semantics associated with a
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Positioning these as fallback is good. I don't know if it is graceful however. You end up with an SDR image with a peak luminance of 80 cd/m^2,, right? So the whole thing looks super dim?

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@svgeesus svgeesus Jul 4, 2017 •
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(That isn't a change request btw, just that I would like to understand the fallback better.)

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You end up with an SDR image with a peak luminance of 80 cd/m^2,, right?

Well SDR systems typically scale peak sRGB luminance to their peak luminance, so I would expect the same to apply here.

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I meant in the case where a system with HDR capability uses the fallback, because it does not follow this spec..

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I meant in the case where a system with HDR capability uses the fallback, because it does not follow this spec..

I do not think it is possible to predict exactly what HDR systems will do when processing an ICC profile that was not designed for HDR systems... some might map the 80 nits of the ICC profile to what peak SDR luminance would be in SDR mode, others might map it to 80 nits and others might map it to peak HDR luminance.

<code>iCCP</code> Chunk Profile Names defined by this specification. Future versions of this specification might include
additional ICC profiles, e.g. to reflect improvements in ICC profile capabilities.</p>
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Like that part, it allows a V5 profile to be added later which actually describes the image.


<p class="note">This specification does not constrain bit depth.</p>
</section>

<section class='appendix' class="informative">
<h2>Privacy and Security Considerations</h2>

See Section 13.4 at [[!PNG]].
</section>

<section class="informative">
<h2>Privacy and Security Considerations</h2>See Section 13.4 at [[!PNG]].
</section>
</body>
</html>