WebGL differs from OpenGL preprocessor on same graphics stack

Question

I just come upon an interesting effect by Chrome's use of the GLSL compiler. The statement

#define addf(index) if(weights[i+index]>0.) r+=weights[i+index]*f##index(p);

does not compile stating

preprocessor command must not be preceded by any other statement in that line

It seems that the ## syntax is unsupported. However, on the same platform (eg. Linux 64bit, Nvidia GPU) the same shader compiles and runs fine. Why this? I thought the shader compiler is part of the GPUs driver stack and would be used in both cases. So why this different experience?

Answer 1

Actually WebGL is also quoted as "OpenGL ES 2.0 for the Web", so there are some differences to OpenGL.

The WebGL spec ( https://www.khronos.org/registry/webgl/specs/1.0/ ) tells us: "A WebGL implementation must only accept shaders which conform to The OpenGL ES Shading Language, Version 1.00."

Looking into the GLSL ES 1.0 spec ( https://www.khronos.org/registry/gles/specs/2.0/GLSL_ES_Specification_1.0.17.pdf ) I found:

Section 3.4 defines the preprocessor and also states "There are no number sign based operators (no #, #@, ##, etc.), nor is there a sizeof operator."

So whatever the browser's implementation does internally, it follows the standard :)