[Public WebGL] sRGB textures

[Florian Bösch]

*Motivation*

It is widely acknowledged that it's important to perform calculations to do
with radiance in linear space:
http://http.developer.nvidia.com/GPUGems3/gpugems3_ch24.html

If calculations for radiance are not done in linear space odd effects can
happen, such as textures brightening on minification and being incorrectly
summed up altogether: http://hsivonen.iki.fi/png-gamma/

Transferring/Processing all values in linear space would be
preferrable. HDR images (such as PNGs that use alpha as an exponent) are a
one solution to this. However this has several drawbacks.

   - Half or Single precision floating point textures are large to transfer.
   - Using alpha for exponent only works with PNGs and eliminates the alpha
   channel.
   - Blending (also anti-aliasing) will be done in gamma space when
   manually outputting gamma corrected colors.
   - Manual gamma correction is not a "drop in" solution.

sRGB solves these issues:

   - reads from sRGB textures are first converted to linear space and all
   calculations prior to handoff to the fragment program (mipmapping,
   anisotropy, linear interpolation etc.) are done in linear space.
   - writes to sRGB textures are treated as being in linear space encoding
   to sRGB space happens after fragment program evaluation and blending.
   - Shaders need not be modified for gamma, merely texture formats have to
   be switched.

sRGB also has these drawbacks:

   - repeated de/encoding can introduce its own banding
   - sRGB assumes consumer grade CRT monitors gamut
   - Apple systems are not (by default) configured to run in sRGB space.
   - mipmap generation is not supported
   - does not support compressed textures (ES extension restriction,
   desktop defines sRGB for compressed textures).

*Specifications*

   - 2002: Direct3D 9.0:
   http://msdn.microsoft.com/en-us/library/windows/desktop/bb173460(v=vs.85).aspx
   - 2007: OpenGL Extension for textures:
   http://www.opengl.org/registry/specs/EXT/texture_sRGB.txt
   - 2008: OpenGL Extension for framebuffers:
   http://www.opengl.org/registry/specs/ARB/framebuffer_sRGB.txt
   - 2011: OpenGL ES Extension for framebuffers/textures:
   http://www.khronos.org/registry/gles/extensions/EXT/EXT_sRGB.txt
   - 2012: OpenGL ES 3.0 core textures: 3.8.16 sRGB texture color
   conversion
   http://www.khronos.org/registry/gles/specs/3.0/es_spec_3.0.0.pdf
   - 2012: OpenGL ES 3.0 core framebuffers: 4.1.8 sRGB Conversion
   http://www.khronos.org/registry/gles/specs/3.0/es_spec_3.0.0.pdf

*Support*

   - Direct3D *99.43%* (Direct3D 9 or above)
   http://store.steampowered.com/hwsurvey
   - OpenGL* 63%* (framebuffer&texture EXTs)
   http://feedback.wildfiregames.com/report/opengl/
   - OpenGL ES: *94 of 783* devices http://www.glbenchmark.com/result.jsp ,
   full list http://codeflow.org/download/EXT_sRGB.txt

*Mirror Extension Proposal*
https://github.com/KhronosGroup/WebGL/pull/112
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