On Fri, 19 Dec 2003, Steven M. Schultz wrote: > On Fri, 19 Dec 2003, Trent Piepho wrote: > > > On Fri, 19 Dec 2003, Andrew Stevens wrote: > > > > Is SAD really any faster to calculate than variance? SAD uses an absolute > > value-add operation while variance is multiply-add. Multiply-add is usually > > the most heavily optimized operation a cpu can perform. > > Au contraire. Multiply is a lot slower than abs(). All abs() has > to do is flip a sign bit (effectively) and that's going to be a lot
In floating point, all you have to do is flip a sign bit. But with integers, it's not so easy. There is no instruction for absolute value in MMX, you have to use a four instruction sequence and two registers. Slower than squaring a value, which only takes two instructions. Though you can cleverly combine an unsigned subtraction and absolute value operation into four instructions total, and perform it on eight unsigned bytes at a time. So you can compute an absolute value of differences quite a bit faster under MMX than I was thinking you could. Clearly SAD would be faster than variance. Another advantage of SAD is that you can find an intermediate result easier than with variance. That way you can short-circuit the SAD calculation if you have already reached the best SAD already found. > faster than any multiply. And aren't there MMX2/SSE abs+add > instructions - that would make abs/add quite fast. I finally found a mmx2 reference, and you're right about that. MMX2 added psadbw, packed sum of absolute differences. If you have 8-bit unsigned data it makes computing SAD pretty darn easy, you can find the SAD of 8 pixels in one instruction. Though why did mpeg2enc use variance in the first place? Maybe it's a better estimator than SAD for motion compensation fit? > At any rate I checked out ffmpeg's mpeg2 encoding vs mpeg2enc on > my G4 Powerbook. Yes, ffmpeg has a big speed advantage (~2x) but The level of altivec optimizations ffmpeg vs mpeg2 is probably an important factor in any speed difference, and one that wouldn't matter for other CPUs, which the level of MMX/MMX2/SSE optimizations makes a large difference. ------------------------------------------------------- This SF.net email is sponsored by: IBM Linux Tutorials. Become an expert in LINUX or just sharpen your skills. Sign up for IBM's Free Linux Tutorials. Learn everything from the bash shell to sys admin. Click now! http://ads.osdn.com/?ad_id=1278&alloc_id=3371&op=click _______________________________________________ Mjpeg-users mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/mjpeg-users
