[FFmpeg-devel] [PATCH] adpcm: Store trellis nodes in a heap structure
Wed Nov 3 18:24:32 CET 2010
On Wed, 3 Nov 2010, Michael Niedermayer wrote:
> On Wed, Nov 03, 2010 at 02:33:03PM +0200, Martin Storsj? wrote:
> > Hi,
> > As pointed out by Michael when reviewing the G.722 trellis encoder, the
> > stored trellis nodes could be stored in a heap-like structure, instead of
> > in a straight sorted array.
> > Currently, when inserting a new trellis node, a linear search (which in
> > itself perhaps could be sped up by converting it to a binary search) is
> > used to find the spot where it should be inserted, and then all later
> > node pointers are moved back one step with memmove. Since only a subset of
> > all evaluated nodes are stored, the worst one is removed once the array is
> > full.
> > Instead of doing this, the attached patch set stored the node pointers in
> > a heap structure, by first adding all evaluated nodes to a heap, as long
> > as they all fit. Once they don't all fit, we check through all the
> > frontier/2 leaf nodes to find the worst one, replace that one with the
> > current and restore the heap property.
> why dont you store the heap fliped? that is with the worst node at the base
> that way removial of it is much faster
Yes, that's a good idea, Loren suggested that too. I'll try it, together
with doing the testing with adpcm_ima_wav instead of adpcm_ima_qt, since
that gives longer runs which gives the trellis much better possibility to
actually gain something.
> > Output from tiny-psnr:
> > No trellis:
> > stddev: 101.13 PSNR: 56.23 MAXDIFF: 7183 bytes: 4865398/ 4865408
> > -trellis 5, original code:
> > stddev: 81.78 PSNR: 58.08 MAXDIFF: 4798 bytes: 4865398/ 4865408
> > After patch #1:
> > stddev: 81.70 PSNR: 58.08 MAXDIFF: 4798 bytes: 4865398/ 4865408
> > After patch #3:
> > stddev: 80.77 PSNR: 58.18 MAXDIFF: 4766 bytes: 4865398/ 4865408
> > After patch #4:
> > stddev: 80.94 PSNR: 58.17 MAXDIFF: 4524 bytes: 4865398/ 4865408
> > So even if patch #3 and #4 in theory should worsen the output slightly,
> > they actually seem to improve the result in this case, since other nodes
> > happen to be stored/thrown away. The main point is that it doesn't seem to
> > harm the output quality significantly while improving the runtime
> > performance massively.
> It would be insterresting to know why they improve quality
Yes - my uneducated guess is that we simply keep/throw away other (roughly
equally good/bad) nodes which happen to be more beneficial later, which
can't really be predicted at that point.
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