What happens to NURBS at the hardware stage?

[Jason210]

Do 3D cards work only with vertices and triangles, or can they work with NURBS data? If the former, when and where does conversion take place?

Thanks in advance,
Jason

 

[arjan de lumens]

AFAIK, no modern GPU is capable of handling NURBS in hardware (even though OpenGL has supported them for ages) - they are tessellated into triangles in software at the driver level and only then passed on to the GPU.

 

[Grall]

Arjan,

The NVTeaPot demo (which I had on my harddrive for a while but deleted in a fit of tidyingup-itis) ran faster with the hardware NURB option on my GF3 than the software mode. So some kind of support must exist.

*G*

 

[ERP]

It's possible that NVidia, convert the NURB's into Beziers and use their hardware to tesselate the result.
Given the amout of CPU setup required to draw a Bezier on NV2x I wouldn't expect it to be much faster than doing the tesselation with the CPU unless the curve density was low and the polygon density high.

 

[Xmas]

You mean this one? http://glygrik.hotbox.ru/

It seems to be Bézier patches, not NURBS. NVidia provides an extension (NV_evaluators) for rational polynomial surfaces (up to order 8 IIRC). It seems to be a software implementation, though.

 

[ERP]

It seems to be Bézier patches, not NURBS. NVidia provides an extension (NV_evaluators) for rational polynomial surfaces (up to order 8 IIRC). It seems to be a software implementation, though.

FWIW this matches what the hardware can do.
But it requires significant CPU setup to get the data into a format suitable for hardware acceleration. Unless the rendered number of tris is much larger than the patch count I really can't see it being much of a win performance wise.

 

[Tagrineth]

...which is why that feature's support was pulled