(lack of) Clipping Planes on WebGL 1 and 2 Spec

[André Venâncio]

Hi everyone.

I'm new to the Group, and I'm  wondering if there's a reason (and what that reason might be) why WebGL1.0 or WebGL 2.0 didn't implement clipping planes (gl_ClipDistance)  in the standard?

I would think that if the clipping was done at the vertex shader level then there would be less calls to the fragment shader - hence better performance, as opposite to discard pixels on the fragment shader.

Also I've read in quite a few places that shaders with "discard" can't be optimised by the GPU as they normally would. (Not totally sure about this though).

Thank you

[Ken Russell]

Hi André,

In OpenGL ES, on which WebGL is based, gl_ClipDistance is provided via an extension: https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_clip_cull_distance.txt .

We can expose it as an extension to WebGL. From this report:

http://opengles.gpuinfo.org/gles_listreports.php?extension=GL_EXT_clip_cull_distance

it looks like it's supported on Mesa, NVIDIA GPUs, and newer Adreno GPUs.

Could you please file an issue requesting it here: https://github.com/KhronosGroup/WebGL/issues .

Thanks,

-Ken

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[André Venâncio]

Hey Ken.

thanks for your reply.

I've requested it. https://github.com/KhronosGroup/WebGL/issues/2556

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[Gabby Getz]

Hi Andre,

If you are interested in the approach using discard in the fragment shader, I recently added clipping planes to CesiumJS using that method. The code changes are here.

Thanks!

Gabby

[André Venâncio]

Hi Gabby.

Thank you kindly for your reply. I've implemented this myself so I dont need the code. I just want to improve performance on my engine. :)