View Single Post
Old 18th October 2005, 12:06   #42  |  Link
unmei
frying subs
 
unmei's Avatar
 
Join Date: Jan 2003
Location: ch-2500
Posts: 891
What surprised me a bit is that the adaptive quantisation seems to work by "upping" the quality on bright areas (as compared to lower the quality in dark areas like i thought XviD did it).
I suspected this after doing a CQ (21) test with and without AQ. And while the one withou came out at 800 kbit/s the AQ one was 1400kbit/s (!). So i played the file and set ffdshow to show quantizers - and no longer surprisingly - a P frame supposed to be Q21 has the highest quantizer at 21 and the lowest one 12. A I frame (supposedly 18) even had blocks with quant 10 (that is the lowest allowed by the set Q range!).

[edit]
Encoding with CQ 26 and AQ gave me a file with almost the same bitrate as Q21 and no AQ (800.52 vs 800.75 kbit/s). Interestingly the one with out AQ has a mean PNSR almost 1dB higher and global PSNR ~1.2dB higher than the one with AQ.

Also AQ seems to considerably raise the number of direct B macroblocks (2%->14%) and lower the number of skip B macroblocks (72%->42%). This is comparing the encodes with the same CQ (the one with adapted CQ 26 has 12% direct and 62% skip).
[/edit]
__________________
-nyo

Last edited by unmei; 18th October 2005 at 12:41.
unmei is offline   Reply With Quote