CODEWITHSUNDEEP
openglgeometryfragment-shader
wondra
wondra

Reputation: 3583

Access world-space primitive size in fragment shader

It is essential for my fragment shader to know the world-space size of the primitive it belongs to. It is intended to be used solely for rendering rectangles (=triangles pair).
Naturally, I can compute its width and height on CPU and pass it as uniform value, but using such shader can be uncomfortable in long run - one have to remember what and how to compute it or search documentation. Is there any "automated" way of finding primitive size?
I have an idea of using a kind of pass-through geometry shader for doing this (since it is the only part of pipeline I know of that have access to whole primitive), but would that a good idea?

Upvotes: 1

Views: 338

Answers (1)

datenwolf
datenwolf

Reputation: 162164

Is there any "automated" way of finding primitive size?

No because the concept of "primitive size" depends entirely on you, namely how your shaders work. As far as OpenGL is concerned, there's not even such a thing as world space. There's just clip space and NDC space and your vertex shaders take a formless bunch of data, called vertex attributes, and throw it out into clip space.

Since the vertex shader operates on a per-vertex base and don't see the other vertices (except if you pass them in as another vertex attribute, with a shifted index; if doing it that way, the output varying must be flat per vertex and the computation being skipped for 0 != vertex_ID % 3 for triangles) the only viable shader stages to do fully automate this is using a geometry or tesselation shader, doing that preparative calculation, emitting the result as a scalar vertex attribute.

Upvotes: 1

Related Questions