Hi Timothy,
I think I've got enough of a handle on the design to attempt a budget
for the dedicated multipliers. Assumptions:
- Four clocks to set up each span
- Interpolate 16 parameters (don't interpolate Z, yes I know it's
currently planned otherwise)
- Assume fog and LOD need 16 bit precision (they might not)
- Assume color is perspective-correct (some think it's ok to relax
this)
- 8 bit precision is good enough for all blending
- Need to double check for any overlooked multiplies
So:
Interpolation correction: 4
(16 interpolants / 4 clocks)
Perspective reciprocal: 2
(1 reciprocal x 2 pixels)
Perspective correction: 30
(15 parameters x 2 pixels)
Not allocated: 4
----
Total: 40 dedicated 18x18 multipliers
This will deliver 2 pixels/clock most of the time, but will stall up to
3 clocks on short spans. Worst case is to fill the screen with single
pixel vertical strips, which will cut the fill rate by 75%. Typical
throughput reduction should be much less, perhaps in the 5-10% range.
If there really are four multipliers left over, they could be applied
to cutting the span setup stall to a single clock.
Comments/flames?
Regards,
Daniel
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