[Ffmpeg-devel] ratecontrol advice

Dan Finkelstein df1
Fri Jul 29 00:51:56 CEST 2005

Hi --

I have also noticed the same phenomena where the h263 encoder will take a 
number of seconds to converge to a sharp image.  My observation is that 
when a quick movement is made, the image becomes quite blurry and will 
slowly converge.  I've also had trouble picking parameters in the encoder 
so that the frames are encoded up to a desired maximum data rate and the 
quality remains decent.  (I'm using ffmpeg from cvs a couple of weeks back.)

If you happen across a solution to this, could you please pass it my way ! 
I'll be glad to test it...


At 07:12 PM 7/25/2005, Matthew Hodgson wrote:
>Hi folks,
>After a bit of a hiatus, I finally got 'round to trying to cap frame sizes
>generated by ffmpeg in 1-pass h.263 encoding.  Unfortunately, things did
>not go smoothly - I tried Loren's advice of:
>Loren Merrit wrote:
>>But if you don't mind modifying a few lines of code, look in
>>modify_qscale() line 435 or so. Currently that limits per-frame bits
>>based on the number of bits left in the VBV buffer; you could put in your
>>own cap similarly. (Warning: anything done by a 1pass encode will be an
>>approximate/soft limit, since you don't know in advance excatly how many
>>bits a given QP will take. So you'll need to fudge the limit a little if
>>you care about strict compliance.)
>...but unfortunately the predicted size of a frame given by qp2bits(rce, 
>q) in modify_qscale seems even more inaccurate than I feared.  The problem 
>seems to be that the estimate of the frame size has to converge over time 
>- which is fine for a longish talking-head style video; after ~60 frames 
>it gets pretty accurate and you can detect potential large framesizes and 
>clobber their QP.
>However, when a video is made up of a series of different still images 
>(e.g. non-scrolling credits) it takes much longer (~120 frames or 
>so).  And whilst it's converging, it guesses the framesize pretty badly - 
>32 bits rather than the actual 14296 for the first frame, 500 bits rather 
>than 25480 for the 30th frame, 153315 bits rather than 74776 for the 77th 
>frame, etc. (However, frame 91 is almost right (30767 v. 26312), and by 
>frame 121 it's pretty close (30546 v. 30144)).
>Am I using the wrong metric for estimating the end frame size in 1-pass 
>encoding?   And does anyone know which of the various counters & 
>predictors starts off initialised incorrectly, causing this convergence effect?
>Alternatively, I need a way to tell the ratecontrol to encode a sudden 
>change in static image as a small I + several incremental P frames - 
>rather a single huge monolithic I frame and a subsequent string of 'empty' 
>P frames.  Is there any way to force ffmpeg to encode in this way?
>Rich Felker wrote:
>>On Tue, May 10, 2005 at 12:55:44AM -0700, Loren Merritt wrote:
>>>(Warning: anything done by a 1pass encode will be an approximate/soft 
>>>limit, since you don't know in advance excatly how many bits a given QP
>>>  will take. So you'll need to fudge the limit a little if you care
>>>about strict compliance.)
>>While true, this isn't an inherent limitation, just a flaw in lavc's rate
>>control engine. It's easy to implement strict limits in 1pass encoding:
>>just repeatedly reencode the frame at different values of qp until you
>>find one that gives the size you want. With binary search it shouldn't
>>even be that slow...
>I've also tried going down this line of attack, but it seems that ffmpeg 
>doesn't make multiple executions of the encode_thread for a given frame 
>very easy - all the rtp callbacks and bitstream output happen directly 
>from within the thread.  I take this to mean that I have to completely 
>isolate the encode_thread and buffer all its sideeffects in order to run 
>it in a sandbox to see how big its output is going to be, and then re-run 
>it with a higher QP as needed.  This seems relatively tricky - is there a 
>better way of doing it, or is this the only way to go?
>I've also tried doing a two-pass encode and fiddling the stats file 
>between passes by boosting the perceived i_tex/p_tex/var on frames which 
>were too big, hoping that the next pass would overcompensate and shrink 
>them down. This doesn't seem to work at all - i'm assuming that some kind 
>of blurring or smoothing is annihilating my hacky tweaks, or I'm 
>completely misunderstanding how the multipass encoding is mean to work.
>any suggestions would be enormously appreciated.
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