Global constant faceDirection instead of gl_FrontFacing

[sunag]

Still I did not make any deep-test... but I can finish. Apparently work for normal/bump maps.

Considering that Adreno GPUs not work property gl_FrontFacing, otherwise, it can be more simple.

#17598

This is a WIP

/ping @WestLangley @mrdoob

[sunag]

Considering that Adreno GPUs not work property gl_FrontFacing, otherwise, it can be more simple.

This is affirmation is true?
If yes, I think that this will prevent other bugs related, maybe not published in github.
If no, I think that the code can be more simple.

/ping @WestLangley @mrdoob

three.js/src/renderers/shaders/ShaderChunk/normalmap_pars_fragment.glsl.js

Line 41 in 26df5e2

// Workaround for Adreno GPUs gl_FrontFacing bug. See #15850 and #10331

#15850 (comment)

[makc]

fwiw, I have seen like 2 other drivers having problems with gl_FrontFacing, and heard of 3rd one. what I did was

#extension GL_OES_standard_derivatives : enable
vec3 fdx = dFdx(v_position.xyz);
vec3 fdy = dFdy(v_position.xyz);
vec3 faceNormal = normalize(cross(fdx,fdy));
vec3 normal = v_normal;
if (dot (normal, faceNormal) < 0.0) {
	normal *= -1.0;
}

instead of

vec3 normal = v_normal;
if (!gl_FrontFacing) {
    normal *= -1.0;
}

[WestLangley]

Still I did not make any deep-test... but I can finish.

Thanks, @sunag. I think this is definitely worth further study.

[makc]

hmm, funny thing here. my workaround above should be the same as in #17804 (comment) but that one is causing problems for @WestLangley 🤔

[Mugen87]

Notice that #17958 has removed the Adreno workaround.

FWIW: While studying the shader code of Sketchfab, I've noticed that they use gl_FrontFacing in the shader, too. AFAIK they don't use hacks like dot( cross( S, T ), N ), so introducing a global constant seems not necessary. Hence, I vote to close the PR.

[sunag]

...so introducing a global constant seems not necessary. Hence, I vote to close the PR.

@Mugen87 I think so, this PR is too outdated... Maybe in future simplify the operation a *= ( float( gl_FrontFacing ) * 2.0 - 1.0 ) for a *= faceDir can be an option for optimisation. closing...