[non-normative] Explain -srgb formats are not color managed/tagged (#4597)

kainino0x · web-flow · commit 8b5896b399d4 · 2025-10-03T00:20:30.000Z
Fixes #4587
diff --git a/spec/index.bs b/spec/index.bs
@@ -2135,6 +2135,24 @@ WebGPU allows all of the color spaces in the {{PredefinedColorSpace}} enum.
 Note, each color space is defined over an extended range, as defined by the referenced CSS definitions,
 to represent color values outside of its space (in both chrominance and luminance).
 
+<div class=note heading>
+    As described above, {{GPUTexture}}s are not color managed. This includes `-srgb` formats,
+    which despite their are not *tagged* with an sRGB color space (like those described by
+    {{PredefinedColorSpace}} and the CSS color spaces <a value for=color()>srgb</a> and
+    <a value for=color()>srgb-linear</a>).
+
+    However, `-srgb` texture formats *do* have gamma-encoding/decoding properties which are
+    algorithmically close to those used for gamma encoding in {{PredefinedColorSpace/"srgb"}} and
+    {{PredefinedColorSpace/"display-p3"}}. For example, a fragment
+    shader can output an "sRGB-linear"-encoded (physically linear) color value into an `-srgb`
+    format texture, which will gamma-encode the value when it is written.
+    Then, the value in the texture will be correctly encoded for use on a
+    {{PredefinedColorSpace/"srgb"}}-tagged (approximately perceptually-linear) canvas.
+
+    It is similarly possible to take advantage of these properties using
+    {{GPUQueue/copyExternalImageToTexture()}}; see its description for additional information.
+</div>
+
 An <dfn dfn>out-of-gamut premultiplied RGBA value</dfn> is one where any of the R/G/B channel values
 exceeds the alpha channel value. For example, the premultiplied sRGB RGBA value [1.0, 0, 0, 0.5]
 represents the (unpremultiplied) color [2, 0, 0] with 50% alpha, written `rgb(srgb 2 0 0 / 50%)` in CSS.
