[MEncoder-users] Terrible results with x264

[belcampo]

Grozdan wrote:
> On Wed, Dec 29, 2010 at 12:02 AM, belcampo <belcampo at zonnet.nl> wrote:
>> James Hastings-Trew wrote:
>>> On 10-12-23 11:55 AM, belcampo wrote:
>>>> I would like to learn how dumb I am. My observations:
>>>> qp gives the most 'constant' quality, less quality variations than crf
>>>> qp encodes 'significantly' faster than crf.
>>> You have things a little mixed up. QP does not mean constant quality - it
>>> means constant quantizer - its only there for testing purposes and will
>>> produce non-optimal output. CRF is what some other encoders (like Handbrake)
>>> call "Constant Quality" - it adjusts the quantizers on a frame by frame
>>> basis to produce a consistant quality across the video - always what you
>>> want.
>> By using -crf I see this figures come by, it doesn't make the impression to
>> me that the quality is 'constant'.
>> frame=38285 fps= 35 q=31.0 size=  186156kB time=1531.90 bitrate=
>> 995.5kbits/s   frame=38297 fps= 35 q=30.0 size=  186316kB time=1532.48
>> bitrate= 996.0kbits/s   frame=38313 fps= 35 q=29.0 size=  186380kB
>> time=1532.86 bitrate= 996.1kbits/s   frame=38327 fps= 35 q=28.0 size=
>> 186476kB time=1533.44 bitrate= 996.2kbits/s   frame=38342 fps= 35 q=29.0
>> size=  186572kB time=1534.02 bitrate= 996.3kbits/s   frame=38359 fps= 35
>> q=29.0 size=  186684kB time=1534.78 bitrate= 996.4kbits/s   frame=38376 fps=
>> 35 q=27.0 size=  186748kB time=1535.36 bitrate= 996.4kbits/s frame=38391
>> fps= 35 q=28.0 size=  186812kB time=1535.94 bitrate= 996.4kbits/s
>> frame=38410 fps= 35 q=28.0 size=  186908kB time=1536.70 bitrate=
>> 996.4kbits/s   frame=38428 fps= 35 q=28.0 size=  187004kB time=1537.47
>> bitrate= 996.4kbits/s   frame=38446 fps= 35 q=26.0 size= 187068kB
>> time=1538.05 bitrate= 996.4kbits/s   frame=38466 fps= 35 q=26.0 size=
>>  187196kB time=1538.82 bitrate= 996.6kbits/s   frame=38497 fps= 35 q=14.0
>> size=  187307kB time=1539.78 bitrate= 996.5kbits/s   frame=38537 fps= 35
>> q=16.0 size=  187403kB time=1541.31 bitrate= 996.0kbits/s frame=38558 fps=
>> 35 q=31.0 size=  187531kB time=1542.85 bitrate= 995.7kbits/s   frame=38574
>> fps= 35 q=27.0 size=  187691kB time=1543.42 bitrate= 996.2kbits/s
>> frame=38610 fps= 35 q=13.0 size=  187825kB time=1544.34 bitrate=
>> 996.3kbits/s   frame=38648 fps= 35 q=15.0 size= 187921kB time=1546.11
>> bitrate= 995.7kbits/s   frame=38663 fps= 35 q=31.0 size=  188145kB
>> time=1547.07 bitrate= 996.3kbits/s
>>
>> I didn't specify a bitrate, but it almost looks like cbr.
>> q varies from 31 to 13, which is  a very high variation in my opimion. Am I
>> misinterpreting matters ??
> 
> This is exactly how crf works. It varies the quantize on a per frame
> basis, so it may decrease it if it "thinks" a frame needs a lower
> quantizer to look good and increase it if it "thinks" that a higher
> quantizer on a specific frame will give the same quality as on a
> previous frame, which may have required a lower one.
I thought that the 'q-value' was meant as an indication as 'quality 
relative to the original'.
> 
> qp just slaps one quantizer on all frames, regardless if a frame may
> require a (much) lower quantizer to look good, something that qp won't
> adjust for, and regardless if a specific quantizer is overkill for a
> frame so it'll just waste bitrate for nothing.
> 
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