which shader stage must write gl_ClipDistance[x]

Question

It is available as output variable in all shaders except fragment shader. So which shader stage must write it? Is its value taken from the last shader stage that wrote it?

Also please explan what is the purpose of having the value gl_ClipDistance in fragment shader?

Answer 1

Considering it is really only useful for clipping... you need to write it in the last stage of vertex processing in your GLSL program. Currently there is only one stage that does not fall under the category of vertex processing, so whatever comes immediately before the fragment shader needs to output this.

If you are using a geometry shader, that would be where you write it. Now, generally in a situation like this you might also write it in the vertex shader that runs before the geometry shader, passing it through. You don't have to do anything like that, but that is typical. Since it is part of gl_PerVertex, it is designed to be passed through multiple vertex processing stages that way.

Name

gl_ClipDistance — provides a forward-compatible mechanism for vertex clipping

Description

[...]

The value of gl_ClipDistance (or the gl_ClipDistance member of the gl_out[] array, in the case of the tessellation control shader) is undefined after the vertex, tessellation control, and tessellation evaluation shading stages if the corresponding shader executable does not write to gl_ClipDistance.

If you do not write to it in the final vertex processing stage, then it becomes undefined immediately before clipping occurs.

Answer 2

As long as you only work with vertex and fragment shaders, you write it in the vertex shader. According to the GLSL spec, geometry and tessellation shaders can write it as well.

The fragment shader can read the value. Based on the way I read the documentation, it would give you the interpolated value of the clip distance for your fragment.