Benedict Bridgwater wrote: > > Justin Schoeman wrote: (regarding RGB/YUV conversion) > > > > I actually benchmarked these two versions a while ago, and it turns out > > that on a reasonably new PC (K6/PII or up), fixed point arithmetic is > > quite a bit faster than tables - It turns out that a fixed point > > multiply is a lot quicker than a memory access (especially when taking > > the heavy cache usage of image processing into account). On older PCs > > (Pentium/K5/etc) the table version is quicker. > > Justin, do you remember if this difference would show in a simple > conversion loop benchmark, or did it have to be in context of an actual > image conversion with corresponding cache usage? > > Ben I didn't test it in simple loops, but the difference will definitely be more marked with real images (due to cache contention for the look-up tables). As a rough estimate, I would expect the two routines to come out approximately equal on simple conversion loops (a L-1 cache hit is about as expensive as an integer multiply on newer CPUs). One area where tables can help is the final downscale and clamp operation. Conditional jumps based on (relatively) random data can be very hard on deeply pipelined CPUs, while a table lookup will usually hit L-1 cache... -justin
