[XviD-devel] Revolutionary idea for speeding up 1st pass ;->

[Christoph Lampert]

...or maybe not.

Just a quick idea I had yesterday: 

The quantizers most frequently used for high quality without Bframes are
(as far as I know) below 10, with emphasis on 3, 4 and 5. First pass is
usually done with fixed quant 2. 

Now during quantization, this fixed 2 is passed by value to each and every
routine. The same for the other values: The same value is passed to every
of the 6 blocks of the 1000 macroblocks of a frame, unless adaptive
quantization is active. 

Couldn't we create separate (de)quant routines (or more) for the most
frequently used quantizers? This would save 1 register for the quant
values, and also sholud be easier to optimize (for the programmer or
compiler) because division/multiplication/rounding and/or table lookup
is constant and could be simplified or completely removed. 

Since we have function pointers anyway, these pointers could just be
updated at the beginning of each frame to point to the right fixed
quantizer routines. Then it would need 31 different routines, of
course, which should then rather be generate by a script (partial
evaluation).
Otherwise, we would have to check e.g. for Quant<=5 and then call
QuantizeSpecial[Quant](); or QuantizeGeneral(Quant); 

For H263-quant, the result would be that simple, that maybe just table
lookup would work, too (if this is faster at all, which I don't know), at
least in the most frequently used interval for small coefficients. 

Instead, maybe const value quantization would become that simple that it
could be integrated into (i)DCT. Those act on the same memory, which would
then be SIMD registers, right? 

gruel