Fix normal map rendering issue on Adreno GPUs #15850

[JordyvanDortmont]

This fixes #15850 by checking if the material is double sided, if so we use an alternative for computing the possible normal inversion with gl_FrontFacing.

The same issue was identified in #10331 and @bhouston also commented the fix I implemented on the blog post that was already being referenced in the shader code.

[mrdoob]

Thanks!

[Mugen87]

@JordyvanDortmont Please notice that this change was reverted via #17958.

[JordyvanDortmont]

@Mugen87 Thanks for notifying me. We'll fork and patch the shader.

[Mugen87]

We'll fork and patch the shader.

It should be no problem to replace the shader chunk with Material.onBeforeCompile() if necessary. Also notice the discussion from here #17804 (comment). Using BufferGeometryUtils.computeTangents() and setting Material.vertexTangents to true actually solved the rendering glitch with the respective wheels model.

[JordyvanDortmont]

It should be no problem to replace the shader chunk with Material.onBeforeCompile() if necessary.

Yes, I intended to patch it this way as well, instead of forking.

Also notice the discussion from here #17804 (comment). Using BufferGeometryUtils.computeTangents() and setting Material.vertexTangents to true actually solved the rendering glitch with the respective wheels model.

So just to be sure, if BufferGeometryUtils.computeTangents() is called on the geometry with the material that's double-sided and set Material.vertexTangents to true, rendering on an Adreno GPU should work?

For both cases we can check if there's an Adreno GPU and apply a fix.

[Mugen87]

So just to be sure, if BufferGeometryUtils.computeTangents() is called on the geometry with the material that's double-sided and set Material.vertexTangents to true, rendering on an Adreno GPU should work?

Yes. However, BufferGeometryUtils.computeTangents() currently requires an indexed-geometry but this can be fixed.

[Mugen87]

@WestLangley What do you think about generating an index in BufferGeometryUtils.computeTangents() if not present, use it for the tangent computation and then remove it at the end of the method?

[WestLangley]

@Mugen87 If it doesn't require and index to produce reasonable results -- and I do not think it does -- then I'd just operate on the triangle soup directly.

I'll file a PR and add VertexTangentsHelper.

[mrdoob]

In case anyone lands here, the Adreno bug is only when gl_FrontFacing is inside a function: #21205