[FFmpeg-devel] parameter ranges bug / minrate q

tripp eliared
Fri Jul 18 20:47:01 CEST 2008


--- On Thu, 7/17/08, Michael Niedermayer <michaelni at gmx.at> wrote:

> > so i assume that there's the recommendation because,
> > setting -minrate != -maxrate ratecontrol is unable to meet target bitrate?
> > and currently -minrate is only useful in setting cbr encodes,
> > with minrate=maxrate?
> > if so is it to change?
> interresting speculations
> Now while i do not exactly remember why this was added originally 
> i do not think it means much more now than that
> min_rate>0 is a waste of bits.

isn't that it's design? to waste bits if required?

regarding speculations,
i believe i'd tried two 2-pass encodings,
one with and one without -minrate,
with -minrate it was off target, undershoot, probably ~5%

tried again,
and for the first time ever, got this error:
[lavc rc] Error: 2pass curve failed to converge
and on another attempt (bitarate being major change):
[lavc rc] Error: bitrate too low for this video

with removal of -minrate, both encodes ran fine.

> > a word in the docs would be nice.
> send a patch

fair enough.
i'll give it a try,
still i think it better if a coder does it.
also while looking at comments in avcodec.h
i saw b_quant_offset had the b_quant_factor description 

patches attached.

> [...]
> > 3. speaking of,
> > setting -rc_eq "1" does not deliver a cbr encode
> > nor does -rc_eq "tex" do the opposite from what i gather.
> yes, this is correct.

mencoder documentation states it does though, 
i assume they got it from somewhere.
even i thought i'd seen it in some ffmpeg doc (can't find anything now though)

main ratecontrol equation
	constant bitrate
	constant quality""

and on a completely different note,
wouldn't it be better for the -fs paramater to be in MB?


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