Modern and Future Geometry Rasterizer layout? *spawn*

As far as I'm aware both are limits. AFAIK both AMD and nvidia's rasterizers can take at most one triangle/clock and output at most 16 pixels/clock. So large triangles will take more than one cycle to complete, and small triangles will cause lower than the peak pixel throughput

 

The situation where two backfaced primitives are culled indicates that the hardware can discard up to two primitives per cycle. At some point, every primitive that reaches the fixed-function hardware must be rejected or used to produce a set of threads of some kind corresponding to the pixels it covers.

The old implementation would spend one cycle per primitive regardless of whether it would produce any pixels for rendering. If the hardware can cull two triangles per cycle, a stream of triangles that need to be discarded can be discarded in half the time.
Depending on how many triangles are encountered that can be culled, this can speed up overall processing since there can be fewer cycles spent on triangles that do not produce output pixels. However, if the stream has mostly non-culled triangles, the throughput would be similar to before.

 

Yes, I tried to condense it down a bit. Micropolygons are dreaded by rasterizers, so people came up with compute shader solutions for this, like AMDs Geometry FX. They use CS to cull microgeometry to lessen the burden on rasterizers and geometry engines: https://gpuopen.com/geometryfx/

Also a good read on primitives: https://frostbite-wp-prd.s3.amazonaws.com/wp-content/uploads/2016/03/29204330/GDC_2016_Compute.pdf

 

As I understood this, and please correct me If I am wrong, NVIDIA can rasterize and cull a max of 6 primitives per cycle, as they have a distributed tessellation (PolyMorph) engine that can deal effectively with this. AMD on the other hand can cull 4 primitives but only rasterize about 2.

 

Yes.

No, it's (8)4 for Navi iirc.

 

Navi is still a max of 4, that I am sure of, the question is, 4 culled and 4 rasterized or just 4 culled?

 

No.
Pretty sure it's distributed geometry with 4 culled 4 drawn tris o'clock.

 

Adding to small triangles are bad.

 

This is the reason Epic use a compute rasterizer for Nanite and only bigger triangle use the hardware rasterizer.

 

I can see more than 2 per clock, so that cannot be the hard limit. Probably they are running into some other bottleneck that keeps them from reaching clearly more than 3 or approaching four even.
edit: Just re-read the Whitepaper. AMD says explicitly, that each primitive unit can cull 2 triangles per clock and draw 1 (ouput to rasterizer). Each (of the four) rasterizer can process 1 triangle per clock, test for coverage and emit 16 pixels per clock. I haven't seen culled triangle rates much above 8 GTri/s though, maybe the prim units are not fed quickly enough or the test runs into another bottleneck.

 

PCGH used to run some theoretical tests which included geometry. Do you know why they stopped?

 

No, I left PCGH over 2 years ago.

 

Oh, I wasn't aware you left.

 

That isn't working as easy as that. You don't get to mix multiple polygons in a single wavefront, due to a usually significant data dependency on per-triangle uniform vertex attributes which is handled in the scalar data path. In order to mix like that, you would need to accept a 16x load amplification on the rasterizer output bandwidth as you would have to drop the scalar path and the compacted inputs for a fully vectorised one. There is no cost effective way to afford that amplification with a hardware rasterization, with geometry engines being kept centralized.

EDIT: Maybe we could actually see this in a future architecture, "lone" pixels being caught in a bucket, and then dispatched in batch in a specialized, scalar free variant of the fragment shader program. But that would still require a decentralised geometry engine to better cope with the increased bandwidth requirements, and a higher geometry throughput.

 

Yet, there seems to be some merit to do exactly that (centralizing geo engines), since AMD has done so with the move to RDNA. At least that's what I've gathered from the whitepaper.

 

Why cant GPUs be designed to not shade in quads so that micro polygons don't destroy efficiency?