WebGLCubeRenderTarget: Use CubeTexture.

[Mugen87]

This PR makes WebGLCubeRenderTarget use a CubeTexture.

I've realized this allows a nice refactoring in WebGLTextures. setTextureCubeDynamic() becomes obsolete.

[Mugen87]

It seems when applying an instance of WebGLCubeRenderTarget (or its CubeTexture) to Scene.background, the background is now flipped (webgl_materials_cubemap_dynamic). Investigating...

[Mugen87]

It seems these checks are problematic:

uniforms.flipEnvMap.value = envMap.isCubeTexture ? - 1 : 1;

The engine should not set flipEnvMap to -1 for CubeTextures referring to a WebGLCubeRenderTarget.

[Mugen87]

Ah, the removed comments from #18438 explain the need for further refactoring^^.

[Mugen87]

@mrdoob @WestLangley Do you have already a preferred solution for this use case? Maybe a flag CubeTextures.needsFlip that controls this operation?

[Mugen87]

Well, at least it seems the flag approach works^^.

[WestLangley]

This is an excellent refactoring, IMHO. Everything seems to be working.

TBH, I am not a fan of ._needsFlipEnvMap, but I do not have a better suggestion.

CubeTexture extends Texture

	this._needsFlipEnvMap = true;

WebGLCubeRenderTarget extends WebGLRenderTarget

	this.texture = new CubeTexture()

	this.texture._needsFlipEnvMap = false;

[Mugen87]

@mrdoob What do you think about this PR? I would love to see setTextureCubeDynamic() and isWebGLCubeRenderTargetTexture go away 😇 . And using CubeTexture in WebGLCubeRenderTarget is good for #20111 (comment), too.

[mrdoob]

Yes! Lets give this a try!

[mrdoob]

Thanks!

[mrdoob]

TBH, I am not a fan of ._needsFlipEnvMap, but I do not have a better suggestion.

Would flipXZ be better? (Considering we already have flipY) 🤔

[Mugen87]

BTW: I would like to add a comment to the property that explains why it's there. I understand that it's necessary to flip the env map when just using CubeTexture and not cube render target. However, I'm not sure I fully understand why this detail could not be implemented elsewhere.

[mrdoob]

I used to know, but I forgot...

[WestLangley]

Would flipXZ be better?

I do not think so. What is in the PR is better.

[Mugen87]

I used to know, but I forgot...

Well, it seems the flipping happens since the early days^^.

946cc6a#diff-c0e88b98497597a015ecf238e91ac3a0R890

[WestLangley]

@WestLangey said:

By convention -- likely based on the RenderMan spec from the 1990's -- cube maps are specified by WebGL (and three.js) in a coordinate system in which positive-x is to the right when looking up the positive-z axis -- in other words, in a left-handed coordinate system. By continuing this convention, preexisting cube maps continued to render correctly.

three.js uses a right-handed coordinate system. So environment maps used in three.js appear to have px and nx swapped. (This is the case for every three.js cube map example.)

[Mugen87]

I hope you don't mind if I copy this into the code base for clarification 😇 .