Remove sRGB inline decoding.

I’m currently working on a PR to remove the inline sRGB decode from the shader. We want to do this to avoid wrong texture filtering which was discussed in earlier issues. The removal also simplifies the code base at various places (especially the WebGL 2 code paths).

The idea is to use SRGB8_ALPHA8 with WebGL 2 and a fallback for WebGL 1. In order to use SRGB8_ALPHA8, textures have to be in RGBA format and must use unsigned byte which is true for most color textures. However, there are a couple of issues with that approach and I want to discuss them step by step. The first problem is processing compressed textures holding sRGB encoded values like in:

https://github.com/mrdoob/three.js/blob/d10d1be864f5e08c9d13dc098dd07af61c39de53/examples/webgl_loader_texture_basis.html#L53-L58

How should we handle this use case? Decoding compressed texels values on the CPU is by definition no option. If you do a separate render pass and decode the texture into a render target, we would loose the compression (unless we compress it again which is no good idea either).

As far as I understand we can only support compressed textures with sRGB values if formats like SRGB8_ALPHA8_ASTC_4x4_Format are used. Or we have to keep the inline decode. Are there any other options?

[Mugen87]

I’ve added a solution here: https://github.com/mrdoob/three.js/pull/23129/commits/57ac4776ac7328f7fd239c5b24b61a96171e8419

Should be easy to revert when the Chromium bug is solved.

Just for the protocol: VideoTexture does not use gl.texStorage2D() right now and with the above commit no SRGB8_ALPHA8 format. The sRGB decode happens inline and only if the texture is assigned to the map property.

[Mugen87]

Can WebGL not do sRGB conversion for compressed formats other than ASTC? 😕

Yes, it can. However, three.js does not support them yet. After looking at the extensions the following sRGB formats could be added:

• WEBGL_compressed_texture_etc
  • COMPRESSED_SRGB8_ETC2
  • COMPRESSED_SRGB8_PUNCHTHROUGH_ALPHA1_ETC2.
  • COMPRESSED_SRGB8_ALPHA8_ETC2_EAC
• WEBGL_compressed_texture_s3tc_srgb
  • COMPRESSED_SRGB_S3TC_DXT1_EXT
  • COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT
  • COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT
  • COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT
• EXT_texture_compression_bptc
  • COMPRESSED_SRGB_ALPHA_BPTC_UNORM_EXT