[XviD-devel] On the compression ratio of xvidcore
Christoph Lampert
christoph.lampert at gmail.com
Sat Nov 22 16:37:09 CET 2008
Hi,
MPEG-4 doesn't have a fixed compression ratio, it depends
on the setting of the quantization parameters. The stronger
the quantization, the higher the compression but the worse
the picture quality.
The default in xvid_encraw is a quantizer of 2, which is
smallest value (highest quality) that makes sense. You can
encode with other compression ratios either by specifying
a pre-defined output bitrate (e.g. 100kb/s) :
./xvid_encraw -single -bitrate 100
or a fixed quantization parameter (here Q ):
./xvid_encraw -single -bitrate 100 -zq 0 Q
Cheers,
Christoph
lingan wrote:
> Hello, dear:
> I'm studying to use xvidcore to encode yuv images into mpeg4, But
> I find the compression ratio I get is quite low. For example, I use
> the following command to encode a yuv file :
> p9322 at wllab1:~/program863/xvidcore-1.1.3/examples$ ./xvid_encraw -i
> coastguard_cif.yuv -type 0 -w 352 -h 288 -frames 1000 -o
> coastguard.mpeg4 -max_key_interval 10
> then :
> p9322 at wllab1:~/program863/xvidcore-1.1.3/examples$ ls -lh coastguard*
> -rw-r--r-- 1 p9322 p9322 44M 2008-10-31 17:35 coastguard_cif.yuv
> -rw-r--r-- 1 p9322 p9322 6.1M 2008-11-21 22:35 coastguard.mpeg4
>
> you see? the compression ratio is only about 7.
> this is part of the encoding output info:
> xvid_encraw - raw mpeg4 bitstream encoder written by Christoph Lampert
> 2002-2003
>
> 0: key=2, time= 21, len= 36006 | type=I, quant= 2, len= 36006
> 1: key=0, time= 24, len= 19250 | type=P, quant= 2, len= 19250
> 2: key=0, time= 28, len= 20667 | type=P, quant= 2, len= 20667
> 3: key=0, time= 23, len= 20431 | type=P, quant= 2, len= 20431
> 4: key=0, time= 29, len= 22397 | type=P, quant= 2, len= 22397
> 5: key=0, time= 23, len= 20005 | type=P, quant= 2, len= 20005
> 6: key=0, time= 28, len= 21632 | type=P, quant= 2, len= 21632
> 7: key=0, time= 27, len= 21329 | type=P, quant= 2, len= 21329
> 8: key=0, time= 23, len= 20117 | type=P, quant= 2, len= 20117
> 9: key=0, time= 24, len= 20940 | type=P, quant= 2, len= 20940
> 10: key=2, time= 11, len= 36691 | type=I, quant= 2, len= 36691
> 11: key=0, time= 34, len= 18852 | type=P, quant= 2, len= 18852
> 12: key=0, time= 35, len= 21458 | type=P, quant= 2, len= 21458
> 13: key=0, time= 34, len= 20337 | type=P, quant= 2, len= 20337
> the resulted P frames are not much larger than I frames.
>
> Then I use xvidcore to encode the webcam stream, the output info is as
> follows:
> frame: key=2,len= 14441 | type=^@, quant= 0, stat_len= 0
> frame: key=0,len= 0 | type=^@, quant= 0, stat_len= 0
> frame: key=0,len= 8593 | type=I, quant= 2, stat_len= 14441
> frame: key=0,len= 2676 | type=B, quant= 4, stat_len= 2676
> frame: key=2,len= 14325 | type=P, quant= 2, stat_len= 8593
> frame: key=0,len= 8105 | type=I, quant= 2, stat_len= 14325
> frame: key=0,len= 2813 | type=B, quant= 4, stat_len= 2813
> frame: key=2,len= 14290 | type=P, quant= 2, stat_len= 8105
> frame: key=0,len= 6932 | type=I, quant= 2, stat_len= 14290
> ......
>
> The second column is the length of coded frame. My webcam is 320x240,
> so the compression ratio is about 320x240x3/10000=23. And strangely,
> the volume of P frames is even larger than I frames sometimes.
>
> It's said that mpeg4 can achieve at most 400 compression ratio.
> Generally, it can achieve 6,70 compression ratio easily. But why the
> CR I get is so small ? Any suggestions?
>
> The version is xvidcore-1.1.3, and the OS is debian linux-2.6.21.7.
> Thank you very much!!
> _______________________________________________
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> XviD-devel at xvid.org
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>
>
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