KHR_materials_volume extension doesn't state intended color spaces for properties

[hybridherbst]

While looking into a few color management problems I've come across at least one extension where the intended color space is not specified.

These readmes explicitly mention intended color space:

• https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_lights_punctual/README.md
  • light color is linear
• https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/Specification.adoc#392-metallic-roughness-material
  • baseColorFactor and baseColor are sRGB
  • emissiveFactor and emissive are sRGB (which I think has been discussed as problematic a number of times)
  • vertex color is linear
  • everything else is linear
• https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_clearcoat/README.md
  • everything is linear

These don't:

• https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_volume/README.md#properties
  • thicknessTexture is likely intended as linear data
  • attenuationColor is likely intended as sRGB (?)

[lexaknyazev]

The base spec and all extensions follow the same rules:

• All JSON numbers, e.g., base color factor, specular color factor, clearcoat factor, thickness factor, are linear multipliers.
• Non-color textures, e.g., metallic roughness texture, transmission texture, contain linear values, i.e., an 8-bit value of 0x80 means 0.502.
• Color textures, e.g., base color texture, sheen color texture, specular color texture, contain sRGB-encoded values, i.e., an 8-bit value of 0x80 means 0.216.

Volume attenuation color is a bit unique because it is not texturable but the general rule still applies: JSON numbers are linear. We should clarify the extension spec.

/cc @emackey

[donmccurdy]

I really wish we could use the terms "Linear", "sRGB" or "Rec. 709", and "D65" when describing these factors. I understand that we're dancing around a tricky distinction — multiplicands whose product is a Linear-Rec709-D65 color. But I don't think "linear multiplier" is giving the reader enough context to understand that. Particularly as more tools begin supporting additional linear color spaces.