[gst-devel] GStreamer presets (feedback requested)

[Christian Fredrik Kalager Schaller]

Hi,
So based on the feedback from Mike and Daniel I took a look at the
current presets. It seems we already do use non-bitrate settings for
quality for everything but a few video codecs. So I guess we should
simply fix those (including ffenc_mpeg4). 

For audio however I notice that most codecs only seem to have a bitrate
setting to tweak, only Vorbis of the ones I looked at got a 'Quality'
setting we could use instead. As far as I understand bitrate is relative
to the number of channels. So if you use the same bitrate for a stereo
stream and a 5.1, then the quality of sound for each of the 6 channels
in 5.1 would be quite bad (or the stereo bitrate would be silly high).
Not sure how to fix that apart from maybe having presets called

"Quality Normal 6-Channels"
"Quality Normal Stereo"

Tim suggested on IRC that to avoid the kind of confusion that MikeS
mentioned we should probably call the Profiles something little less
'human readable', like Quality/Normal/6-Channels for instance.

Mike mentioned he would like standardized property values instead of the
presets. Which also goes into another question which is if we are to at
some point try to put the media format profiles into the caps (like
making 'Low Complexity' for AAC a caps)

I talked to Wim a bit more about this today and he didn't see how either
standardized properties or profiles in caps where really viable
solutions. So for the time being I will continue with assuming that
presets will be our solution for these issues.

Also if a system is put in place which allows us to mark certain presets
as mutually exclusive I wouldn't mind adding support for that in the prs
files. It is worth nothing though that the way things work now is that
nothing will 'fail' if you apply 'mutually exclusive' presets. What
happens is that if one preset sets cabac to false and 
another to true, the final value will be of the last preset applied.

Christian

On Thu, 2009-06-25 at 10:23 +0200, Daniel G. Taylor wrote:
> On Tue, 2009-06-23 at 19:06 +0100, Christian Fredrik Kalager Schaller
> wrote:
> > Hi,
> 
> Hey,
> 
> > I have been working for a while on defining GStreamer level presets
> > based on the current plugin level preset system. The general document
> > for those presets can be found here:
> > http://gstreamer.freedesktop.org/wiki/PresetDesign
> 
> This looks pretty good in my opinion, as discussed on IRC.
> 
> > My current set of .prs files can be found here:
> > http://cgit.freedesktop.org/~uraeus/transmageddon/tree/presets
> 
> I think the .prs files are also looking good. As discussed before, I
> think it would be good to use the textual names rather than the
> numerical ones when available, e.g. pass=pass1 instead of pass=17
> because it isn't easy to guess what 17 stands for.
> 
> I also think that any quality based setting needs to have the pass set
> to quantizer or quality mode, e.g. for x264enc we want the Normal
> Quality preset to include e.g. pass=qual quantizer=21 or something
> similar. The Main Profile could then be used to restrict the settings
> and the Pass 1 profile could be used to make it an average bitrate-based
> encode rather than a quality one, which is useful for many devices. So
> for example:
> 
> Load Normal Quality
>  * pass=qual
>  * quantizer=21
>  * dct8x8=true
>  * ref=3
>  * bframes=3
>  * me=umh
>  * subme=6
>  * threads=0
> 
> Load Baseline Profile
>  * dct8x8=false
>  * bframes=0
>  * cabac=false
> 
> Load Pass 1
>  * pass=pass1
> 
> Final settings on the element (x264enc Normal Quality, Baseline Profile,
> Pass 1):
>  * pass=pass1
>  * quantizer=21 (unused)
>  * dct8x8=false
>  * ref=3
>  * bframes=0
>  * cabac=false
>  * me=umh
>  * subme=6
>  * threads=0
> 
> After this you are responsible for setting a sensible bitrate based on
> either the output dimensions/framerate or the device you are encoding to
> (which may also need vbv limits like the iPod).
> 
> So the basic idea is to have the quality presets set the maximum
> possible quality and have other presets based on profiles or multi-pass
> to restrict those settings. This means that of course the order of
> loading presets matters and should be communicated to users and
> developers in some sane way.
> 
> Some other random thoughts that I had while we talked via IRC:
> 
>  * DivX Home Theater NTSC/PAL profiles should be available for MPEG4 
>    ASP encoders (xvid, ffenc_mpeg4, etc)
>  * xvidenc really needs to output video/mpeg4 or something so that is 
>    shows up as an encoder if you request MPEG4 video
> 
> Since you mentioned the device profiles are still a work in progress I
> won't talk about them here, other than to say that they do still need
> some work (I'm modifying the Arista preset format somewhat to
> accommodate things like subtitle rendering for various devices). I guess
> this will all be discussed at some point once the presets are in
> GStreamer trunk.
> 
> Take care,
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