[MPlayer-users] PSNR Question Completed!

[Struan Bartlett]

Sorry 'bout my previous incomplete email (sent by a rogue keypress).

I'm comparing the PSNRs generated with different lavcopts when encoding 
a 60s chunk of a DVD.

I don't understand why the PSNR *decreases* when I crop the black 
borders surrounding the picture. I would have expected that, following 
the crop, for a given bitrate the PSNR would increase, as there would be 
available more bits per pixel.

-oac copy -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=10000:psnr:vhq:vpass=1

Pos:  60.0s   1500f (99%)  14fps Trem:   0min  19mb  A-V:0.040 [2217:448]

Video stream: 2217.400 kbit/s  (277174 bps)  size: 16619410 bytes  59.960
secs  1500 frames

Audio stream:  448.000 kbit/s  (56000 bps)  size: 3388000 bytes  60.500 secs
PSNR: Y:48.18, Cb:49.54, Cr:50.71, All:48.73

But:

-oac copy -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=10000:psnr:vpass=1:vhq
-vop crop=720:426:0:74

Pos:  60.0s   1500f (99%)  14fps Trem:   0min  18mb  A-V:0.040 [2152:448]

Video stream: 2152.284 kbit/s  (269035 bps)  size: 16131372 bytes  59.960
secs  1500 frames

Audio stream:  448.000 kbit/s  (56000 bps)  size: 3388000 bytes  60.500 secs
PSNR: Y:46.54, Cb:48.19, Cr:49.34, All:47.15

Please can anyone explain why this might be?

Struan.