These changes look good, and I'll push them. I also tried a larger revision to lift the mode tests out of the loop; it's only worth about 10%, but it might set up further improvements.
When I run your example, though, I get much better absolute performance than you're reporting: 34.3 ± 0.2 msec in version 5.3.1, and 8.45 ± 0.1 msec after the changes. That's running `racket' from a command line and on 64-bit Mac OS X, but I get similar results from other machines (and other OSes under VirtualBox on Mac OS X). Any idea why your numbers are so different? At Sun, 16 Dec 2012 11:29:58 -0700, Michael Wilber wrote: > TL;DR: About ~2.8x speedup from using local variables and unsafe > functions. Copying each bitmap row could bring speedup to ~20x, but it > doesn't quite work and I need your help. Pull request at > https://github.com/plt/racket/pull/199 > > Hey there! > > I'm writing some FFmpeg bindings for Racket. It's fast enough to decode > video in real time, but on my machine, set-argb-pixels takes 189.35±1.3 > msec to run for a 500x500 image, which means I'm limited to displaying > frames at ~5fps. > > Here's a toy benchmark to test set-argb-pixels: > https://gist.github.com/4a5661dfad984cfdab19 > > There are some very simple bottlenecks that I've started to address: > > 1. It turns out that the references to b&w? and alpha-channel-local? for > each pixel are slow slow slow. Making them local variables drops the > time down to 124.8±1.0msec. This three-line change gives a speedup > factor of about ~1.5 > > 2. Using unsafe functions everywhere (unsafe-bytes-ref and friends, > unsafe-fx+ and friends) drops it further to 67.05±0.6msec, which is a > speedup factor of ~2.82 over the original on my machine > > A pull request for the above is at > https://github.com/plt/racket/pull/199 > > Now, if we can assume that the input bytes already contain pre-clipped, > premultiplied data, we don't really have to loop through each pixel. If > we copy each row using copy-bytes!, that drops the function to 9.55±6.1 > msec (!) which is a speedup factor of ~20x over the original. > > The problem with that is on my little-endian machine, Cairo expects the > input data in BGRA format, not RGBA, so the colors look wrong. Alas, > this is why Racket's doing all the byte swizzling manually. > > Is there a fast native way of switching the endianness of a byte vector > assumed to contain 32-bit ints? Or some way to do what we want? > > If there's a way to do this, this could make playing simple > low-resolution videos from Racket pretty feasible. > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
