Fast fullscreen quad rendering in Direct3D 11?

Question

For the last few weeks, I've been trying to port a DX9 implementation of HDR rendering (tone mapping, bloom, stars, etc.) over to DX11. I believe I've got all features working but I'm not getting good enough performance. I'd like to be able to render the whole effect in under 4ms on a fairly low-powered GPU, but using D3D11 Queries I'm noticing that it takes 0.5ms to just render a fullscreen quad with a solid color, and 1.0ms to render a fullscreen texture! And because tone-mapping is the only part of the effect that uses a fullscreen texture, this makes it the most expensive!

I'm already doing some optimisations with my limited graphics knowledge, I've disabled blending and depth-testing, I make sure that the texture sampler uses sensible filtering settings, and I'm pretty sure that the effects of any state-changes are negligible. I've heard that rendering 1 oversized triangle instead of 2 can yield some improvements, but I'm not sure if that will help me in this situation. Basically, does anyone have any suggestions to speed up rendering of a textured quad?

Answer 1

So after experimenting a little more with Intel GPA following Sean Middleditch's comment, I noticed that messing around with state changes, including sampler states, seemed to give me no improvement whatsoever.

But sampler states got me thinking about texture formats, so I had another look at the DirectX documentation for a texture format that might be more optimal. Previously I was using R16G16B16A16 textures for all my render surfaces, but DXGI has the 32-bit packed format R11G11B10, which, according to some forum threads, is particularly well-suited to HDR rendering. I changed both my source and target textures to use this format, and BOOM, tone-mapping now takes about 0.65ms on my integrated gpu with no apparent loss of quality, which seems good enough for me.

Clearly I'd underestimated the importance of a compact texture format - I'd guess it has numerous benefits to caching and simplifying arithmetic, but I'd be interested to hear more detailed theory if anyone would like to comment. Worth trying if anyone else gets stuck on the same issue.

Answer 2

Maybe you can save some time if you don't bind a vertex buffer and use SV_VERTEXID (which is created automatically by the IA stage) to build your quad in the vertex shader, something like:

struct VSOUT{
 float4 pos:SV_POSITION;
 float2 tex:TEXCOORD0;
};

VSOUT main(uint vI: SV_VERTEXID)
{
 VSOUT Out = VSOUT(0);
 Out.tex=float2(vI%2,vI%4/2);
 Out.pos=float4((Out.tex.x-0.5f)*2,-(Out.tex.y-0.5f)*2,0,1);
 return Out;
}

The vertex shader will output a full screen quad this way if you call a draw for 4 vertices with a TRIANGLESTRIP topology;

Probably you can also save some time if you try out different rendertarget formats and resoulutions.

If you can have depth or stencil testing it can also save you some time in some scenarios.