NormalMap Shader: prevent branching

[WestLangley]

As proposed in #17586 (comment).

/ping @JordyvanDortmont
/ping @sunag

[mrdoob]

Thanks!

[sunag]

@WestLangley Is possible find a way to turn frontFacing a global constant in shader?

#ifdef FRONT_SIDE
bool frontFacing = 1.0;
#elif BACK_SIDED
bool frontFacing = -1.0;
#elif DOUBLE_SIDED
bool frontFacing = dot( cross( S, T ), N ) > 0.0 ? 1.0 : -1.0;
#endif

Thus it can be used in other functions like bump map:

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

Line 38 in e6920cd

fDet *= ( float( gl_FrontFacing ) * 2.0 - 1.0 );

[sunag]

@WestLangley Is possible find a way to turn frontFacing a global constant in shader?

Seems that is was implemented in
https://makc3d.wordpress.com/2015/09/17/alternative-to-gl_frontfacing/
http://jsfiddle.net/8sgdaknm/7/

I think that a face direction (target *= faceDir(-1 or 1), is better that target *= float( boolean ) * 2.0 - 1.0 approach.

[mrdoob]

I think that a face direction (target *= faceDir(-1 or 1), is better that target *= float( boolean ) * 2.0 - 1.0 approach.

Why?

[WestLangley]

Is possible find a way to turn frontFacing a global constant in shader?
#elif DOUBLE_SIDED
bool frontFacing = dot( cross( S, T ), N ) > 0.0 ? 1.0 : -1.0;

We already have access to gl_FrontFacing.

Also, dot( cross( S, T ), N ) is a temporary work-around and a hack.

[sunag]

I am creating a PR for show better the idea.
29f6248

[sunag]

@mrdoob is about to use this approach:
sunag@3b73f1c

[mrdoob]

@sunag yeah, would be nice to have a PR with your suggestion so we can discuss it there.