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Brian Paul wrote:

> Another thought is to move some traditional per-fragment operations into 
> the fragment program itself.  Alpha test, for example, may no longer 
> exist in OpenGL 3.0; it would be up to the user to encode the test in 
> their shader.
> 
> With software rendering, we have the potential to do almost everything 
> in the fragment shader.  One idea I had is to add a new input register 
> to fragment programs which is the current framebuffer color.  We could 
> use that to do blending/masking/etc. in the program.  Normally, this 
> would be hidden, but it could be exposed as an interesting new OpenGL 
> extension (and something unique to Mesa).

The other option is to add another shader stage for post-fragment
operations.  This is roughly analogous to how geometry shaders are
implemented.  When I've thought about this in the past, I had only
considered using it to replace alpha blending and stencil operations.

I don't see any reason why we couldn't also implement alpha and stencil
testing in the same stage.  I'm a bit nervous about including depth
testing.  It seems like that would cause issues with early Z test.
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