On Sat, Feb 22, 2003 at 09:12:24PM +0100, Attila Kinali wrote:
Heyo,
I recently had a discussion with Jonas about the usage of vqscale in the first pass of a two pass encoding. So i wanted to test it and got following results:
Source: VIDEO: [MP43] 320x240 24bpp 23.98 fps 615.4 kbps (75.1 kbyte/s)
--- 1.pass: vbitrate=615 / 2.pass: vbitrate=615:
1.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615 2.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=2:vbitrate=615
Video stream: 615.120 kbit/s (76889 bps) size: 381096793 bytes 4956.394 secs 118836 frames Audio stream: 128.000 kbit/s (15999 bps) size: 79295296 bytes 4955.956 secs PSNR: Y:44.30, Cb:47.20, Cr:47.93, All:45.14 --- --- 1.pass: vqscale=5 / 2.pass: vbitrate=615
1.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615:vqscale=5 2.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615
Video stream: 615.094 kbit/s (76886 bps) size: 381080897 bytes 4956.394 secs 118836 frames Audio stream: 128.000 kbit/s (15999 bps) size: 79295296 bytes 4955.956 secs PSNR: Y:44.02, Cb:46.98, Cr:47.74, All:44.87 ---
Now, the two results seem to be pretty much the same from size and PSNR, but a analysis plotpsnr.pl shows, that the second test (with vqscale) has the quantisizer size most time at 4 and less (just a few times at the beginning at 5 and 6) while the first test has it at 5 and less.
If i'm not mistaken, then the file size and the quality of an encoding is mostly dictated by the used quantisizer values (assuming that the same MVs are choosen)
But why is there so less difference in the file size (beside the file with the lower quantisizer is smaller!) and why doesnt the PSNR differ at all ?
It does!! 0.27 is a fairly large difference in PSNR!! BTW, you should try with vqscale=2 on the first pass and see if that works well, rather than vqscale=5. The latter will encode the whole first pass at very low quality so it probably won't estimate complexity well. Rich