6 replies to this topic

[cdgray]

Hi,

While I was working on implementing shadows in my graphics framework, I came to realize that multiplass rendering is the way forward as it makes coneptual sense (that is, it is clean and easy to understand). In a rendering system which utilizes an effects system (similar to the DirectX effects file), multi-pass rendering is used heavily.

However, it is generally true that some multi-pass effects could be performed in only one pass and is usually faster.

My question is, are the current GPU vendors optimizing for multi-pass rendering? Is it bad practise to use it?

Thanks!

[Reedbeta]

Do things in one pass where you can. Multipassing involves writing to the frame buffer (or a texture) and then reading from it again, which is slower than just running one long shader. Of course, some effects require multiple passes, at least until general-purpose shaders with branches and loops become commonplace and fast.

[infernus]

Reedbeta said:

Do things in one pass where you can. Multipassing involves writing to the frame buffer (or a texture) and then reading from it again, which is slower than just running one long shader. Of course, some effects require multiple passes, at least until general-purpose shaders with branches and loops become commonplace and fast.

I've seen the word "branches" many places now and I'm realy curious about it..
What are "branches"?
I know there is a similar word for the things pointing out from a tree..
But what does branches have to do with enything?

[SigKILL]

Branches is usally if's and stuff..

-si

[Ed Mack]

This is 'branching' in pseudocode:

somevar = 4;
compare somevar to othervar;
branch if equal somewhere;
somevar--;
somewhere:

In C, this would read

if (somevar != othervar) {
  somevar--;
}

[Reedbeta]

Branching is anything where the program flow is affected by the value of a variable or similiar. This includes if statements, switches, and also loops.

[KneeDragr]

We have experimented with multipass vs branching pixel shaders and found that multipass is faster. However I was at a recent dev event and from what I understand, improvements to the shader compiler may reverse this.