C++ shader optimization question

Question

Could someone explain me the pretty basics of pixel and vertex shader interaction.

The obvious things are that vertex shaders receive basic vertex properties and then repass some of them to the actual pixel shader.

But how does the actual vertex->pixel transition happens? I know that obviously all types of pipelines include the rasterizer change, which is capable of interpolating the vertex parameters and can apply textures based on the certain texture coordinates.

And as far as I understand those are also interpolated (not quite sure about this moment, heard something about complex UV derivative math, but I assume that we can say that they are being interpolated).

So, here are some "targeted" questions.

How does the pixel shader operate? I mean that pixel shader obviously does some actions "per pixel", but due to the unobvious vertex->pixel transition this yields some questions.

Can I assume that if I evaluate matrix - vector product once in my pixel shader, it would be evaluated once when the image is rasterized? Or would it be better to evaluate everything that's possible in my vertex shader and then pass it to the pixel shader?

Also, if someone could point articles / abstracts on this topic, I would really appreciate that.

Thank you.

UPDATE

I thought it actually doesn't matter, because the interaction should be pretty same everywhere. I'm developing visualization applications and games for desktops, using HLSL / GLSL / Nvidia CG for shaders and mostly C++ as the base language.

You might get better response if you explain what platform/API you are attempting to use. You're probably missing a topic tag or two as well. As it stands, I don't even know what you're talking about. — Edward Strange
why is this tagged C++? The shaders are, by definition, writte in a shader language such as GLSL or Cg, and not in C++? — jalf
Actually, the only thing wrong with the tags is that the C++ doesn't really belong. Modern 3D hardware does shading on-board and that code is typically written in High Level Shader Language (HLSL). I only have a primitive understanding of shaders which I'll offer here and leave the answer section to those with more knowledge: A vertex shader projects 3D points onto the 2D rendering surface. A pixel shader determines what color a given pixel takes on. — Tergiver

jalf jalf · Accepted Answer · 2010-11-24T20:12:03

The vertex shader is executed once for every vertex. It allows you to transform the vertex from world space coordinates (or whichever other coordinate system it might be in) into screenspace coordinates.

That is, if you have a triangle, each vertex is transformed, so it ends up with a position on the screen.

And given these positions, the rasterizer determines which pixels are covered by the triangle spanned by those three vertices.

And then, for each pixel inside the triangle, the pixel shader is invoked. The output from the vertex shader is usually interpolated for each pixel, so pixels close to vertex v0 will receive values very close to those computed by the vertex shader for v0.

And this means that everything you do in the pixel shader is executed once per pixel covered by the primitive being rasterized

C++ shader optimization question

1 Answers