Alpha rejection material cost..

[syedhs]

Artist have gotten lazy and they simply use material with alpha_rejection in all meshes regardless of the texture transparency. A lot of the textures are not transparent - so the question is, is it worthwhile to remove 'alpha_rejection' for texture with no transparency in it? Will there be any speed increase or fps smoothness? Currently, the speed is not that great and there are fps hiccups here and there.

[dark_sylinc]

Well, the benefits depends on the bottlenecks and performance patterns.

However, yes alpha_rejection isn't free.
It prevents early depth from kicking in. So if you've got heavy pixel shaders and you're rendering front to back, you won't get the performance boost of having the Z Buffer kill the pixel shaders before they even get to run for the pixels that are hidden.
It also may disable Z compression.

It's still however, much faster than doing regular alpha blending. And if there's no heavy overdraw or pixel shaders are cheap, then there's not a noticeable difference.

On tilers (i.e. most mobile GPUs; no desktop GPU); alpha_rejection can be very expensive as the GPU must deal it like a special case. In some cases it may even be better to do alpha blending instead (depends on a lot of factors).

[syedhs]

Thank you dark_sylinc for the reply.

It prevents early depth from kicking in. So if you've got heavy pixel shaders and you're rendering front to back, you won't get the performance boost of having the Z Buffer kill the pixel shaders before they even get to run for the pixels that are hidden.

My scene is pretty simple.. all materials are just slapped with diffuse texture (and they are either solid or have transparency).
I have done simple test.. ie comment all the lines in the materials containing 'alpha_rejection'.. and I have got nice boost.. 35 fps to 45~47.. probably the actual FPS will come down to the worst 42fps..

It's still however, much faster than doing regular alpha blending.

Yes the alpha blending has already been eliminated altogether!

Probably I will end up with C++ code that reparse material files to remove the alpha_rejection when the texture doesn't contain transparency.

On the other hand, I read that DXT1 can contain 1 bit alpha, probably I can squeeze something here..