So I'm teaching myself GLSL and am trying to figure out why it's suppose to be faster than the fixed function pipeline.
The reason I am having a problem is that from my understanding , the shaders you create are replacing sections of the pipeline that were there before. So, how's simply providing your own version speeding things up?
The only thing I can think is if you tried to supply say your own lighting equation before, you would have to do the calculation on the CPU, but now you can do the calculations on the GPU which will be faster.
Am I understanding this correctly?
Answer
Shaders you create aren't going to be your own version of fixed-function pipeline(FFP), but a custom vertex- and pixel-manipulating operations to achieve something cool and complex.
Many things you do via programmable pipeline(PP) will work faster than their possible FFP implementations, because PP reduces the number of passes or amount of combiner and cubemap magic required to render these hypothetical things in FFP.
Imagine implementing such a common thing as per-pixel lighting in FFP with only interpolated vertex data and sample texture in your hands. It's not even possible to do it "honestly", only hacks for special cases depending on faithful precalculated cubemaps and some serious blending. With PP it becomes a matter of colorizing a dot product between light direction and vertex normal.
All in all, PP turns slow and impossible into fast and possible. But if you decide to write a shader to implement the same algorithms used in FFP, you would find out that FFP will be slightly faster because it's very hardware-optimised.
No comments:
Post a Comment