Re: [Algorithms] Terrain performance comparisons

[Jonathan B. <jo...@nu...>]

Tom Forsyth wrote:

> Shader LOD is less compelling to me than geometry LOD. If you have
> pretty much any sort of LOD - even static levels - then you're going to
> be drawing (very) roughly constant-area triangles. So your
> vertex-processing load per pixel is roughly constant. And obviously your
> pixel load per pixel is constant (excepting overdraw, which you can't do
> much about using LOD - you need occlusion algos, which is orthogonal to
> the problem).

I think we are thinking about shader LOD for different reasons!

It sounds to me like you're saying, the purpose of shader LOD is to make
far-away pixels cheaper to draw, and that's not really so important in the
grand scheme of things.  (Correct me if I am misunderstanding you).  

That is not at all why I think shader LOD is important.  

The phenomenon I am worried about is that if you don't merge shaders,
then there is an artificial limiter on how much you can LOD your geometry,
because you can't merge triangles that use different shaders.  So what you
end up with is, past a certain distance, a lot of small batches of
triangles that just won't reduce any further.  Your LOD is prevented
from operating.  And it's really slow to submit all those batches to the 
hardware, due to all the context switching.

I am not worried about the amount of time it takes to actually render a
single pixel (which is why I was dreaming about a world in which we use
one shader for everything, and just data-drive its parameters).

   -Jonathan.