sRGB-related improvements

[mosra]

sRGB/S3TC

After reading The sRGB learning curve (great article, by the way) I was investigating sRGB support in OpenGL and found out that the old EXT_texture_sRGB extension (part of GL 2.1) had an interaction with EXT_texture_compression_s3tc and provided additional sRGB/DXT compression formats. It might be a good idea to provide these in the TextureFormat and CompressedPixelFormat enums:

    COMPRESSED_SRGB_S3TC_DXT1_EXT                  0x8C4C
    COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT            0x8C4D
    COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT            0x8C4E
    COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT            0x8C4F

OpenGL ES is a bit different. The EXT_sRGB ES extension says the following and the S3TC/SRGB support is then added by NV_sRGB_formats (ES2+).

3) Should we allow the SRGB_*_S3TC_DXT* or any of the other COMPRESSED
formats if the implementation supports any of those formats?

No since all hardware doesn't support this.  It can be added as a separate
extension if needed.

No such thing in WebGL, sadly. WebGL: WEBGL_compressed_texture_s3tc_srgb

• [x] Add missing SRGB/S3TC formats -- 02538d566f704324761cc587e989ac013121d4b6

S3TC/SRGB support in DDS importer

• [x] Add SRGB/DXT support to DDS importer (DXGI format 72, e.g.) -- mosra/magnum-plugins@e05e532f0731a887cd3719b09742968bc3e50de3

sRGB-related features

• [x] sRGB PixelFormat values -- 40b8815b9da1841b885bb2e7ac20aa8f0c3417c1
• [ ] importers importing images as sRGB (detecting sRGB from PNG/JPEG/... metadata)
  • [ ] And converters embedding the information about sRGB into the output
  • [ ] Or at least supporting the Srgb pixel formats :sweat_smile:
• [ ] importers distinguishing between vertex colors as sRGB and vertex colors as linear (e.g. a 8-bit PLY vertex colors are probably sRGB (spec?! blender exports them as such), while 16-bit vertex colors linear)
  • [ ] glTF specifies vertex colors to be linear (https://github.com/KhronosGroup/glTF/issues/1638)
  • [ ] Is there even a vertex format that could describe sRGB colors?!
  • [ ] MeshData::colorsAsArray() taking that into account to always return linear
  • [ ] e.g. a glTF 8-bit vertex colors can be
• [ ] Some opaque type for 8-bit sRGB color representation so we can easily use it with Debug and Configuration (at the moment it's being cast to Color[34]ub which wrong on many levels)
• [ ] Ability to query color encoding on a GL framebuffer attachment
• [ ] Update GL clear color based on whether sRGB conversion on the framebuffer is enabled or not (sRGB conversion is enabled by default on ES if the framebuffer is sRGB-capable, so it should query sRGB support and then enable it by default)
• [ ] Enable GL framebuffer sRGB by default on desktop as well?
• [ ] Provide ability to do implicit sRGB conversion in the builtin shaders for webgl? Since there's no other way to do it: https://www.khronos.org/webgl/public-mailing-list/public_webgl/1707/msg00018.php (but that would play bad with blending or MSAA)

sRGB-related extensions

• [ ] NV_generate_mipmap_sRGB (ES2, I think ES3 doesn't have this restriction anymore)
• [x] EXT_texture_sRGB_R8, EXT_texture_sRGB_RG8 (desktop, ES3+) -- 40b8815b9da1841b885bb2e7ac20aa8f0c3417c1

sRGB conversion utilities

The idea with sRGB is that it only makes sense to do all operations (filtering, interpolating, ...) in linear and use sRGB only as a "compression format" and never do anything with it except for converting from and back to it. Thus it seems to me like it would make sense to have some conversion functions on Math::Color: assuming everything stored in that class is linear and provide sRGB conversion functions that take or return "untyped" Math::Vector3. And, similarly as OpenGL does it, treat alpha channel always as linear (thus copying it without conversion).

• [x] Color3 Color3::fromSrgb(const Vector3&), Vector3 Color3::toSrgb()
• [x] Color4 Color4::fromSrgbAlpha(const Vector4&), Vector4 Color4::toSrgbAlpha()
• [x] _srgb, _srgba, srgbf, _srgbaf color literals
• [ ] Color[34]::fromSrgb(const char*) for conversion from hexadecimal string representation (provide a linear one as well?)
• [ ] Batch float-to-8bit-sRGB conversion utility (equivalent to Math::packInto() / unpackInto(), basically)

Correct sRGB usage (in examples and elsewhere)

• [ ] sRGB-correct rendering of examples
  • [ ] and their webgl ports (in a shader? see above)
  • [x] sRGB-correct Vulkan Triangle example
  • [ ] blog that
• [ ] What's ImGui using? https://github.com/ocornut/imgui/issues/1724
• [ ] Verify that images from http://www.ericbrasseur.org/gamma.html load and scale with a correct output

Questionable

• [ ] Deprecate _rgb, _rgba, _rgbf, _rgbaf literals and the Color3ub / Color4ub types because they are not sRGB? OTOH it should be possible to save a packed 8bit linear color somehow. It's also better to leave them in for cases where the workflow is not sRGB-correct and yet it's still needed to properly load colors etc.

[mosra]

Some (scary!) information about colorspaces in DDS files: http://stackoverflow.com/questions/13491131/what-are-the-exact-color-spaces-of-dxgi-formats

[mosra]

sRGB support in Color[34] classes done in 2b97e5336022841e1ff4d122d5acea4b10dc276f.

[mosra]

Just discovered that WebGL has WEBGL_compressed_texture_s3tc_srgb.

[mosra]

Added http://www.ericbrasseur.org/gamma.html to the list, great set of test images. Chrome on my HiDPI screen is apparently not doing the right thing, it seems.

[Squareys]

Ha! Chome on Windows 10 without HiDPI fails the test also 😆

[mosra]

Nah, actually it seems to be doing the correct thing: http://www.ericbrasseur.org/gamma_dalai_lama.html

[mosra]

• sRGB variants of S3TC compression formats added in https://github.com/mosra/magnum/commit/02538d566f704324761cc587e989ac013121d4b6
• sRGB PixelFormat values and one and two-component sRGB texture formats added in https://github.com/mosra/magnum/commit/40b8815b9da1841b885bb2e7ac20aa8f0c3417c1