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
faster than any multiply. And aren't there MMX2/SSE abs+add
instructions - that would make abs/add quite fast.
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 resulting output is 'grainy' (same bitrate, no B frames) (and the
rate control is, well, almost non existent - ~2x spikes that'd drive
a hardware player nuts).
Cheers,
Steven Schultz
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