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Clarify luminance gain prose (#1117). #1156

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Clarify luminance gain prose (#1117). #1156

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cbd2015
Clarify luminance gain prose (#1117).
skynavga Sep 8, 2019
858e508
Address https://github.com/w3c/ttml2/pull/1156#discussion_r322867807.
skynavga Sep 19, 2019
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8 changes: 5 additions & 3 deletions spec/ttml2.xml
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Expand Up @@ -11485,12 +11485,14 @@ of the paragraph.</p>
</div3>
<div3 id="style-attribute-luminanceGain">
<head>tts:luminanceGain</head>
<p>The <att>tts:luminanceGain</att> attribute is used to specify the absolute luminance of a region when
<p>The <att>tts:luminanceGain</att> attribute is used to determine the absolute luminance of a region when
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I think it is specifying, not determining. However to compute the end value, some additional calculation is needed.

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No, it doesn't specify the absolute luminance. It specifies a relative gain to apply to a reference absolute luminance to determine the desired absolute luminance, which is why I changed specified to determined in this context.

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I don't understand the difference between those two things @skynavga . Isn't it only a difference in the unit of luminance?

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@skynavga skynavga Sep 9, 2019 •
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tts:luminanceGain specifies a unit-less gain multiplier which, when multiplied by 80 cd/m2, determines, i.e., is used to calculate, an absolute luminance; tts:luminanceGain does not specify a value that is equal to absolute luminance, in which case it would have been incorrectly named (as a gain), and should be renamed to tts:luminance.

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Are you making the distinction between an attribute value that includes a units component vs one that does not?

Since there's no established symbol for the "80cd.m-2" unit, it's hard for authors to write it down. I've been thinking of it as something that has these units which just happen to be defined by specification instead of including them in the value. But it still ends up with units one way or another. I think there's a term for this kind of "dancing around terminology distinctions", and it's probably not very complimentary! We probably shouldn't spend too long doing it. (we already have)

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No. By units and unit-less, I am talking physics, not ttml syntax. If we had an attribute that specified absolute luminance, then it would be called tts:luminance not tts:luminanceGain, and it would specify a number that has an implied physical unit of candelas per square meter, e.g., tts:luminance="80" would mean a specified absolute luminance of 80 cd/m2. But that is not what we have, so we can't say that tts:luminanceGain specifies an absolute luminance. It expresses a multiplier (like ttp:frameRateMultiplier) that is multiplied by 80 cd/m2 to obtain an absolute luminance.

its presentation requires a greater dynamic range than that provided by the <bibref ref="srgb"/> color space,
and when that presentation requires an absolute luminance value.</p>
<note role="clarification">
<p>The <bibref ref="srgb"/> color space specifies an absolute reference white point luminance of 80 cd∙m<sup>-2</sup>.
As a result, colors specified in the <bibref ref="srgb"/> space cannot be
<p>Absolute luminance is determined by means of a gain multiplier applied to the absolute reference white
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There's another layer of potential confusion here, since some presentation systems may not be able to present the specified absolute luminance. In this case I think we're providing a rule for calculating the used value, which may then be further modified to become an actual value based on the capabilities and settings of the presentation device.

Using the phrasing "Absolute luminance is determined by" does not make it clear which value is being specified or determined.

Therefore I'd suggest that we express this in terms of the used value being computed by multiplying the value of the attribute by the 80cd.m-2 specified by [[SRGB]] as an alternative to the wording proposed here.

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Here, it is the computed value that is determined which happens to be the same as the used value; however, the actual value may be different. This is the default reading that applies to all of our style definitions, so we don't bother to repeat it under each style.

point luminance of 80 cd∙m<sup>-2</sup> of the <bibref ref="srgb"/> color space (and not the actual white luminance
of some output display device).</p>
<p>Without the ability to specify this gain, colors specified in the <bibref ref="srgb"/> space cannot be
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I suggest writing:

_ applied to the constant absolute reference white point luminance of 80 cd∙m-2 of the [SRGB] color space (and not the actual white luminance of the ultimate output display device, which can vary across devices and viewing environments and configurations)._

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This expansion doesn't add anything to the original parenthetical, i.e., "ultimate" is redundant, and "which can vary across ..." is obviously implied.

reliably composited with high-dynamic range (HDR) content intended for display with significantly
greater peak luminance, such as content that uses the perceptual quantizer electro-optical transfer function (PQ EOTF)
specified in <bibref ref="bt2100_1"/>. Specifically,</p>
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