[FFmpeg-devel] [PATCH] improve DV probe score

[Roman V Shaposhnik]

On Tue, 2009-09-15 at 09:27 +0200, Reimar D?ffinger wrote:
> On Tue, Sep 15, 2009 at 03:54:23AM +0200, Michael Niedermayer wrote:
> > On Mon, Sep 14, 2009 at 03:38:53PM +0200, Reimar D?ffinger wrote:
> > > On Mon, Sep 14, 2009 at 03:06:06PM +0200, Michael Niedermayer wrote:
> > > > On Mon, Sep 14, 2009 at 09:59:31AM +0200, Reimar D?ffinger wrote:
> > [...]
> > > > that said, while i disagree with how you solve this, i
> > > > do not object to the patch, it is an improvment to what we have now
> > > > i just think it can be done quite a bit more solidly, even a check
> > > > of the number of startcodes per byte && the matches being > 3 would
> > > > be better than what its now, also as you return a score >0 for a single
> > > > match, am i missing something or shouldnt a single valid frame have at
> > > > least 10 matches? it seems requireing 3 before a non 0 score is returned
> > > > seems quite reasonable to me
> > > 
> > > How did you come up with the number 10? These checks are exactly like
> > > that used to find the start of the frame, so there should be at most 2
> > > matches per frame, and we have a sample with only one match per frame
> > 
> > static void dv_format_frame(DVVideoContext* c, uint8_t* buf)
> > {
> >     int chan, i, j, k;
> > 
> >     for (chan = 0; chan < c->sys->n_difchan; chan++) {
> >         for (i = 0; i < c->sys->difseg_size; i++) {
> > [...]
> >             buf += dv_write_dif_id(dv_sect_header, chan, i, 0, buf);
> >             buf += dv_write_pack((c->sys->dsf ? dv_header625 : dv_header525), c, buf);
> > 
> > difseg_size being at least 10 in dvdata.h
> 
> Not sure I remember right, but the checks look only for block_number ==
> 0 - I guess mostly because that's what we need for read_header, it
> _might_ make sense to change it for probe - though it means there is a
> higher probability of a match in random data, so not sure if it is
> really an improvement...

In DV the header is duplicated in as many places in the frame as you
have segments. This is done because of tendency that tape has called
bit rot. That said, everything else in FFmpeg assumes fixed location,
so it shouldn't be all the problematic for DV detection code to do
the same.

Thanks,
Roman.