3

I am new to directx, but have been surprised that most examples I have seen the world matrix and view matrix are multiplied as part of the vertex shader, rather than being multiplied by the CPU and the result being passed to the shader.

For rigid objects this means you multiply the same two matrices once for every single vertex of the object. I know that the GPU can do this in parallel over a number of vertices (don't really have an idea how many), but isn't this really inefficient, or am I just missing something? I am still new and clueless.

2 Answers2

5

In general, you want to do it on the CPU. However, DirectX 9 has the concept of "preshaders", which means that this multiplication will be done on the CPU up-front. This has been removed for newer APIs, but it might be very well relevant for the examples you're looking at.

Moreover, modern GPUs are extremely fast when it comes to ALU operations compared to memory access. Having a modestly complex vertex shader (with a texture fetch maybe) means that the math required to do the matrix multiplication comes for free, so the authors might have not even bothered.

Anyway, the best practice is to pre-multiply everything constant on the CPU. Same applies for moving work from the pixel shaders into the vertex shaders (if something is constant across a triangle, don't compute it per-pixel.)

Anteru
  • 19,042
  • 12
  • 77
  • 121
1

Well, that doesn't sound clueless to me at all, you are absolutely right!

I don't know exactly what examples you have been looking at, but in general you'd pass precalculated matrices as much as possible, that is what semantics like WORLDVIEW (and even more appropriate for simple shaders, WORLDVIEWPROJECTION) are for.

Exceptions could be cases where the shader code needs access to the separate matrices as well (but even then I'd usually pass the combined matrices as well)... or perhaps those examples where all about illustrating matrix multiplication. :-)

Paul-Jan
  • 16,746
  • 1
  • 63
  • 95