[FFmpeg-devel] [Patch] Scale filter should use multiples of 2

[Michael Niedermayer]

On Wed, Jul 07, 2010 at 11:02:01PM +0200, Stefano Sabatini wrote:
> On date Wednesday 2010-07-07 15:53:16 -0400, Daniel G. Taylor encoded:
> > On 07/01/2010 10:27 AM, Stefano Sabatini wrote:
> > >>Basically, I don't know how I can do this easily, and writing a
> > >>bunch of code doing it a stupid way seems like a waste of time, so
> > >>any advice would be appreciated if we do want to go this way.
> > >
> > >libavutil/eval.h, check how it is done in the libavfilter-soc overlay
> > >and setpts filters. This requires to add eval evalutation to the scale
> > >filter, check the recent patch by Baptiste on which I commented
> > >yesterday.
> > 
> > Thanks for the help here Stefano.
> > 
> > I've run into a bit of a snag I'd say... See attached patch to
> > reproduce what I'm running into, but basically:
> > 
> >  * av_parse_expr and such use a comma as a delimiter for function
> >    arguments
> >  * avfilter's graph parsing uses a comma as a delimiter for filters
> > 
> > Because of that it means anytime you put a comma into the filter
> > arguments it thinks you are passing in the next filter. If we are
> > going to allow functions and such as filter arguments it might be a
> > good idea to change the syntax for specifying filters, maybe using a
> > pipe character like "|" or something, e.g. -vf
> > "scale|unsharp|...|crop".
> > 
> > Attached patch works for stuff like -vf "scale=320:240" or -vf
> > "scale=W/2:H/2" and such but if you try -vf "scale=W:round(H, 16)"
> > you'll immediately see the issue.
> > 
> > Thoughts?
> 
> Possible solutions:
> 
> 1) change the character currently used, which is ",", and which is
> very likely to appear in expressions inside the filter args.
> 
> '|' is an option, note that we didn't choose '|' in the first place
> because it's a special sh character, for example this doesn't work in
> sh:
> -vf scale|unsharp|crop
> 
> you need:
> -vf "scale|unsharp|crop"
> 
> For me this doesn't look like a big issue, but we should wart the
> users in the docs.
> 
> The requirement is that the choosen character cannot compare inside
> the filter args.
> 
> 2) make the separator definable by the user, for example we could have
> something like:
> -vfd "%" -vf "scale % unsharp % ... % crop".
> 
> 3) Implement an escaping mechanism. Since at the first level of
> escaping there is just one special character, we may use '\' to escape
> the ',', for example:
> 
> scale="W:round(H\, 16), unsharp, crop"
> 
> The problem with this approach is that combined with the shell
> escaping mechanism this can easily became an escaping hell.
> 
> So I tend to prefer the option 2, and use by default the value "|" for
> the separator.

how do you pass a string to a filter? you need escaping anyway
a eval expression is just a string ...

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is not what we do, but why we do it that matters.
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