WebGLRenderer: Use ETC2 for ETC1 textures.

[Mugen87]

Related issue: #27162 (comment)

Description

The idea of this PR is to stop use WEBGL_compressed_texture_etc1 and always rely on WEBGL_compressed_texture_etc by treating ETC1 as ETC2 textures. According to https://www.khronos.org/assets/uploads/developers/library/2012-siggraph-opengl-es-bof/Ericsson-ETC2-SIGGRAPH_Aug12.pdf, slide 10, that should work:

ETC2 is backward compatible with ETC1: If you have an old ETC1 texture, you can load it as an ETC2 texture and the hardware will decode it correctly.

The motivation behind this change is to remove the dependency of RGB_ETC1_Format to gl. compressedTexImage2D() so we can eventually use gl.texStorage*() for ETC1 textures.

This change also includes the WebGL backend of WebGPURenderer.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
671.7 kB (166.7 kB)	671.6 kB (166.7 kB)	-92 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
451.1 kB (109 kB)	451 kB (109 kB)	-92 B

[Mugen87]

@donmccurdy I'm afraid I'm not sure how to test this change since on my devices the KTX examples do not output ETC1. Do you have an idea how we can verify if this PR actually works?

[Mugen87]

To clarify: This is the bit (the RGB_ETC1_Format check) I want to get rid of^^.

[donmccurdy]

@Mugen87 the override in KTX2Loader can be removed with this PR:

three.js/examples/jsm/loaders/KTX2Loader.js

Lines 163 to 164 in a03a74a

	// https://github.com/mrdoob/three.js/pull/22928
	this.workerConfig.etc1Supported = false;

If your devices are still not producing ETC1 after that, I think I could test this on my devices this weekend.

Thank you!

[Mugen87]

Thanks! After removing the override I still get RGBA_BPTC but I can force now ETC1 by setting all flags like bptcSupported in detectSupport() to false except for etc1Supported.

The PR seems to work but it would be good if you could test this change, too!

[donmccurdy]

Confirmed – ETC1 is working in WebGL 2 on my iPhone 11. I believe if you use a KTX2 file encoded with ETC1S / "Basis LZ" (rather than UASTC) then the first transcoding choices, if available, will be ETC2 or ETC1.