[FFmpeg-user] H.264 file size

[Daniel Zens]

Hi everyone,

first excuse my english. I rarely write in english. ;-)

I've developed an application with ffmpeg, which grabs images from a 
camera and encode them with the H.264 Codec from ffmpeg.

My application works well, but after testing a little bit, I was 
wondering about the results.

I've grabbed two scenes with a length of 40 seconds. Scene one has 
nearly no motion. Scene two has very much motion.

Encoding those scenes with MPEG1, I got for scene two a file that was 
bigger than the file of scene one.

That seems legit, because in scene one are less changes to encode than 
in scene two.

But when I use H.264 for encoding, the file of scene two is smaller than 
in scene one.

In scene one are many macroblocks skipped, but the I-frames have much 
more 4x4 macroblocks compared to scene two.

So, the I-Frames size in scene one is much bigger than in scene two, and 
so the scene one file is also bigger than the scene two file.

But I don't unterstand, why H.264 divide the I-frames in so much 
macroblocks, when there is no motion. Why is the no motion file bigger 
than the motion file?

Here is some data output from the codec of the two scenes. I used the 
high profile and level 3.0. Bitrate and all other settings are in both 
scenes the same.

Scene One, less motion, filesize about 2152 kb:
[libx264 @ 0x7f0ffc1ce540] frame I:103 Avg QP:24.79 size: 19355
[libx264 @ 0x7f0ffc1ce540] frame P:512 Avg QP:28.03 size: 316
[libx264 @ 0x7f0ffc1ce540] frame B:410 Avg QP:33.10 size: 116
[libx264 @ 0x7f0ffc1ce540] consecutive B-frames: 20.0% 80.0%
[libx264 @ 0x7f0ffc1ce540] mb I I16..4: 29.4% 41.7% 28.9%
[libx264 @ 0x7f0ffc1ce540] mb P I16..4: 0.3% 0.1% 0.0% P16..4: 7.5% 0.3% 
0.3% 0.0% 0.0% skip:91.5%
[libx264 @ 0x7f0ffc1ce540] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 4.7% 0.0% 
0.0% direct: 0.1% skip:95.1% L0:12.4% L1:87.1% BI: 0.4%
[libx264 @ 0x7f0ffc1ce540] final ratefactor: 23.93
[libx264 @ 0x7f0ffc1ce540] 8x8 transform intra:41.3% inter:75.7%
[libx264 @ 0x7f0ffc1ce540] coded y,uvDC,uvAC intra: 58.9% 72.7% 38.2% 
inter: 0.1% 3.3% 1.0%
[libx264 @ 0x7f0ffc1ce540] i16 v,h,dc,p: 6% 40% 1% 53%
[libx264 @ 0x7f0ffc1ce540] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 30% 29% 7% 5% 
4% 4% 6% 5% 10%
[libx264 @ 0x7f0ffc1ce540] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 28% 43% 6% 3% 
3% 4% 5% 4% 5%
[libx264 @ 0x7f0ffc1ce540] i8c dc,h,v,p: 50% 28% 16% 5%
[libx264 @ 0x7f0ffc1ce540] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x7f0ffc1ce540] ref P L0: 82.2% 2.7% 11.0% 4.1%
[libx264 @ 0x7f0ffc1ce540] ref B L0: 75.7% 24.3%
[libx264 @ 0x7f0ffc1ce540] kb/s:429.87

Scene two, much motion, filesize about 2034 kb:
[libx264 @ 0x7f99841ce540] frame I:103 Avg QP:32.64 size: 10292
[libx264 @ 0x7f99841ce540] frame P:522 Avg QP:36.86 size: 1467
[libx264 @ 0x7f99841ce540] frame B:400 Avg QP:40.37 size: 642
[libx264 @ 0x7f99841ce540] consecutive B-frames: 22.0% 78.0%
[libx264 @ 0x7f99841ce540] mb I I16..4: 28.9% 51.6% 19.6%
[libx264 @ 0x7f99841ce540] mb P I16..4: 3.5% 6.4% 0.6% P16..4: 21.0% 
3.0% 1.3% 0.0% 0.0% skip:64.2%
[libx264 @ 0x7f99841ce540] mb B I16..4: 0.7% 1.6% 0.0% B16..8: 17.8% 
1.6% 0.2% direct: 0.5% skip:77.6% L0:34.6% L1:63.6% BI: 1.8%
[libx264 @ 0x7f99841ce540] final ratefactor: 31.53
[libx264 @ 0x7f99841ce540] 8x8 transform intra:55.5% inter:88.1%
[libx264 @ 0x7f99841ce540] coded y,uvDC,uvAC intra: 43.6% 45.9% 6.7% 
inter: 3.3% 4.9% 0.0%
[libx264 @ 0x7f99841ce540] i16 v,h,dc,p: 20% 37% 4% 39%
[libx264 @ 0x7f99841ce540] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 21% 19% 14% 8% 
7% 7% 8% 8% 8%
[libx264 @ 0x7f99841ce540] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 28% 32% 10% 5% 
5% 5% 6% 5% 5%
[libx264 @ 0x7f99841ce540] i8c dc,h,v,p: 70% 16% 12% 3%
[libx264 @ 0x7f99841ce540] Weighted P-Frames: Y:12.5% UV:0.6%
[libx264 @ 0x7f99841ce540] ref P L0: 70.4% 13.6% 11.4% 4.2% 0.4%
[libx264 @ 0x7f99841ce540] ref B L0: 88.9% 11.1%
[libx264 @ 0x7f99841ce540] kb/s:406.32

Greetings!