Re: [Algorithms] portal engines in outdoor environments

[jason w. <jas...@po...>]

> But _something_ still has to do the readback. The start of the chips'
> pipeline still needs to know the results at the framebuffer end before it
> can decide whether or not to start drawing the triangles. It doesn't
matter
> whether the CPU does it or some bit in the T&L section of the chip - it's
> the same issue - something has to wait for pixels to go most of the way
down
> the rendering pipeline. And that's a performance lose.

Right, of course the rejection logic has to be able to read the relavent
z's. But, it doesn't have to read the most immediate state of the z.. the
state a few polygons ago will do fine for a conservative rejection. So I
don't see where a stall happens, or where there's a performance lose. Maybe
the gain is reduce, when you think about how the rejection means there are
skips in the flow from AGP RAM to the cards local storage/instruction bus,
but as I understand it, that's all controled by DMA's from the card anyhow,
so not a big deal.

It doesn't rely on frame2frame conherance (which I feel is often a bad
thing). Perhaps it would maybe be best with a heirarchical z system.

> Added to which, one of the growing bottlenecks is the AGP bus, and doing
it
> this way doesn't help that at all.

Not directly, well.. not unless your primatives are so large that the
rejection happens before the primative has entirely streamed into the cards
local FIFO or whatever. However, done right, it could potencially reduce
state changes or texture downloads, aleavating some bus bandwidth. Hardware
seems to be moving to complexe primatives that take more local processing to
reduce to fragments anyhow, compressing the information that goes accross
the bus, and the more overhead is associated with each member primative of a
primative array, the more useful a rejection system like this becomes.

So I totally agree that what I suggest isn't a ready to impliment scheme
that should be in nVidia's next card, but I do think that early rejection of
polygon or higher level primatives is a good thing, something that hardware
should be moving toward. This is the same principle in use in the PVR series
of chips, and it certainly seems to work well enough in the dreamcast. I
don't think you should have to resort to the PVR's odd rendering interface
to get the gains of early rejection and deferred texturing tho, and I don't
see how hinting for early rejection can be a bad thing.

Especially considering that we're quickly reaching the limits of what fill
rate can be supported by available memory technologies (at the right price
that is). And tho embeded dram seems to answer that, it and other possible
technologies haven't exactly materialized.