Clarify luminance gain prose (#1117).

w3c · Sep 8, 2019 · cbd2015 · cbd2015
1 parent e56a0ef
commit cbd2015
Showing 1 changed file with 6 additions and 4 deletions.
diff --git a/spec/ttml2.xml b/spec/ttml2.xml
@@ -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
 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
+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
 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>
@@ -11551,7 +11553,7 @@ images whose processing generates <bibref ref="srgb"/> pixel values, and an abso
 only the <bibref ref="srgb"/> pixel values are transformed.</p></note>
 <p>When applying <att>tts:luminanceGain</att>, the linear optical output values
 (C<sub>r</sub>, C<sub>g</sub>, C<sub>b</sub>) of the <loc href="#terms-component">components</loc> of a pixel of a region are defined as
-80 ∙ <att>tts:luminanceGain</att> ∙ (r, g, b), where (r, g, b) are the normalized linear <bibref ref="srgb"/> <loc href="#terms-component">components</loc> of the pixel.
+80 cd m<sup>-2</sup> ∙ <att>tts:luminanceGain</att> ∙ (r, g, b), where (r, g, b) are the normalized linear <bibref ref="srgb"/> <loc href="#terms-component">components</loc> of the pixel.
 </p>
 <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 supported value.</p>