Add unpackHDRHeadroom attribute for tone mapping

[ccameron-chromium]

HDR images can be drawn an infinite number of ways, often unrepresentable as a single bitmap (e.g, gainmap images). There exists a general problem wherein an HDR image must be put into pixels in a buffer. At the moment the HDR image is put into pixels in a buffer, this infinite number of ways of representing it must be collapsed into a single representation.

This problem exists in 2D canvas at drawImage time (see analogous issue) and in WebGPU at copyExternalImageToTexture and importExternalTexture time (see analogous issue).

This problem also exists in WebGL at tex[Sub]Image time. This explainer proposes to add an unpackHDRHeadroom attribute to WebGLRenderingContextBase, indicating the amount of tone mapping (if any) to perform when populating the texture's pixels.

Unlike 2D canvas and WebGPU, WebGL has a few special "opt out of color space conversion" situations (e.g, UNPACK_COLORSPACE_CONVERSION_WEBGL ). The unpackHDRHeadroom attribute will be ignored in the same situations where unpackColorSpace is ignored.

[lexaknyazev]

There's also colorSpaceConversion: "none" for createImageBitmap.

[ccameron-chromium]

Yes, the colorSpaceConversion:"none" will give access to the raw pixels with no tone mapping applied.

[ladyhavoc]

I have a strong disagreement with unpackHDRHeadroom being log2 magnitude, I believe it should be linear to avoid confusing web developers (it becomes logical that if you specify srgb-linear the RGB values should be <= linear unpackHDRHeadroom); there is no precedent for a log2 value in any API used on the web (even texture mipmaps go the opposite direction and serve a slightly different purpose).

This spec is implementable either way, but I can't find a justification that sticks for me on why this should be log2.

[ccameron-chromium]

There are more preferences for linear being voiced here. The motivation for log2 is "that's how the standards define HDR headroom", but we can side-step that by using unpackLinearHDRHeadroom or unpackLinearizedHDRHeadroom.