[FFmpeg-user] Actual bitrate is lower than target

James Darnley james.darnley at gmail.com
Wed Jul 15 01:29:54 CEST 2015

On 2015-07-15 01:17, Wesley Wen wrote:
> On Wed, Jul 15, 2015 at 6:49 AM, James Darnley <james.darnley at gmail.com>
> wrote:
>> On 2015-07-14 23:00, Wesley Wen wrote:
>>> -g 29.9002997
>> This is an integer parameter.
>>> [libx264 @ 0x7ff1e8816e00] frame I:71    Avg QP: 2.19  size: 46261
>>> [libx264 @ 0x7ff1e8816e00] frame P:2040  Avg QP: 2.51  size: 11560
>>> [libx264 @ 0x7f948a822800] frame I:71    Avg QP: 1.80  size: 48058
>>> [libx264 @ 0x7f948a822800] frame P:2040  Avg QP: 2.04  size: 12990
>> Your video is just too simple to reach 7M with the settings you've
>> chosen.  This shows that you're pushing against the minimum QP of 0.
> Do you mean source video is too simple? And I don't really understand QP,
> could you help explain more and give me some suggestion?

Yes.  Your video is easy to compress to a small size.  Little motion,
little noise, little detail.  The obvious example to explain this is a
static image.  Simply speaking, the encoder just has to compress it once
and then say "repeat".  QP is roughly the quantiser used, if that means
any more to you (probably not).  It can't go lower than zero.  If it
can't go an lower, the video stream cannot be made bigger.

If, for some inexplicable reason, you want to inflate the size you can
do any or all of the following:
- use "faster" options
- scale to a bigger size
- add noise
- use bitstream padding.  I recall that there was an option to pad the
bitstream but that may have been in the x264 command line encoder and
not ffmpeg.

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