[racket] H264 Codec in Racket

From: Neil Toronto (neil.toronto at gmail.com)
Date: Thu Jun 7 17:25:03 EDT 2012

On 06/06/2012 03:21 PM, Anurag Mendhekar wrote:
> I'm considering writing an H.264 encoder/decoder in Racket. Has anyone
> tried such a thing before?
> Codecs require a lot of bit-whacking and the h264 standard is
> particularly convoluted. Efficiencies are obtained in C in many
> different and usually complex ways. Codec experts usually recommend
> starting at C/++ and go to assembly to extract further performance.

The best way to get good performance currently is to write in Typed 
Racket and let its optimizer convert checked code to unchecked code. For 
example, it'll replace `fxand' with `unsafe-fxand'. Use the Performance 
Report tool to find out exactly how the optimizer transforms your code 
and get suggestions for speed improvements.

You won't be able to operate on bytes directly, so you'll have to use 
fixnums. You'll only be able to operate on 3 bytes at a time on 32-bit 
platforms, or 7 bytes at a time on 64-bit platforms. (Racket avoids 
allocating space for fixnums by encoding them in a pointer with a type 
tag, which uses up a few bits.) If the algorithm allows operating on 
more bytes at a time, try shoving them all into an integer, which can be 
any bit width, and use integer bitwise ops like `bitwise-and'.

Neil ⊥

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