Create a map assigning two bits to every output bin. During the atomic
flush, compute a threshold for discarding writes altogether that would
keep us under 2% error - discard 1 of every 2 writes if we've already
accumulated 64 writes (hotspot value 1), 7 of 8 if we're above 256
(hotspot value 2), or 31 of 32 at 2048 (hotspot value 3). Pack this
value into a read-only buffer that can often be cached at L2, and for
particularly concentrated flames (which historically choke cuburn), L1.
During writeback, discard writes at the apporpriate rate. During the
flush of the integer accumulator to the float, scale the integer
accumulators by the discard rate.
This works because for most flames, there's not a lot of interesting
stuff in the middle regimes; either stuff is very well defined, in which
case we pretty much know exactly what the color is going to be
(remember, the max 2% relative error gets log-scaled as well), or it's
loosely defined so we should keep it at full accuracy.
Of course, a 10x boost is best-case-ish - a long, high-res render. I
realized though that I really didn't care about low quality stuff and
should go for broke optimizing this for my use case, which is
ridiculously high res HDR stuff. (On pathological flames, on the other
hand, 10x is conservative; this easily gives us 100x.)
Build an array of one-bit flags for every pixel (addressed as u32 data).
If we have accumulated at least 64 points for that pixel, set the flag;
thereafter only write 1/16 (and multiply subsequent points that do get
written by 16).
The theory is, after 64 points, the color is pretty much locked in; this
lets us crank SPP up to get excellent coverage in dark areas but the
bright ones don't matter so much since they're fully resolved. Still
needs a lot of tuning to get peak performance, and the trigger threshold
may need to be scaled along with the render size. It also will likely
not scale as well to higher resolutions, because we rely on L2 cache to
make this fast.
This is useful for doing color manipulation on output renders, either by
hand in color grading software or automatically using renormalization
based on image statistics.
I'm not sure what's going wrong; the math still holds up at higher
densities, but when you crank up the samples-per-pixel count the
accumulators start overflowing stochastically, and when they do
they dump nonsense into the output. Until I have time, take a small
perf hit by flushing much more often.
Steven Robertson