[FFmpeg-devel] [PATCH] adpcm: Store trellis nodes in a heap structure

[Michael Niedermayer]

On Wed, Nov 10, 2010 at 11:42:48AM +0200, Martin Storsj? wrote:
> On Wed, 3 Nov 2010, Martin Storsj? wrote:
> 
> > 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.
> > 
> > This doesn't give identical results to the initial version, since the 
> > nodes from the previous round are used for doing the next step in the 
> > order they're stored in the array, which is different.
> > 
> > Instead of chekcing all the frontier/2 leaf nodes to find the worst one, 
> > patch #3 just picks one of the leaf nodes and tries replacing that one. By 
> > picking a different one of the leaf nodes each time, we more or less 
> > achieve the same thing. (If only one spot would be tested, only the path 
> > from that node up to the root would be updated once the heap is full.)
> > 
> > The last patch removes the search for nodes with an equal sample value to 
> > the one currently inserted, since it's an O(frontier) operation, which 
> > speeds things up immensely, without notably affecting the quality.
> > 
> > Some numbers, runtime:
> > Original: 16.1 s
> > After patch #1: 14.8 s
> > After patch #3: 10.9 s
> > After patch #4: 5.6 s
> > 
> > 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.
> 
> Updated benchmarks of this code - the sample in question is a 30 second 
> clip, 44 kHz mono. Now I've done the testing with adpcm_ima_wav as codec,

can that clip be downloaded somewhere?


[...]

> Also, for reference, the same input with different trellis sizes, after 
> patch #4:

it would be interresting to also see this with pre #4 so one can compare it


> 
> 0:
> user    0m0.037s
> stddev:   40.03 PSNR: 64.28 MAXDIFF: 2874 bytes:  2646016/  2649218
> 
> 1:
> user    0m0.191s
> stddev:   37.48 PSNR: 64.85 MAXDIFF: 3026 bytes:  2646016/  2649218
> 
> 2:
> user    0m0.433s
> stddev:   35.22 PSNR: 65.39 MAXDIFF: 2681 bytes:  2646016/  2649218
> 
> 3:
> user    0m0.880s
> stddev:   33.87 PSNR: 65.73 MAXDIFF: 2643 bytes:  2646016/  2649218
> 
> 4:
> user    0m1.684s
> stddev:   32.96 PSNR: 65.97 MAXDIFF: 2650 bytes:  2646016/  2649218
> 
> 5:
> user    0m3.252s
> stddev:   32.34 PSNR: 66.13 MAXDIFF: 2467 bytes:  2646016/  2649218
> 
> 6:
> user    0m6.044s
> stddev:   31.69 PSNR: 66.31 MAXDIFF: 1907 bytes:  2646016/  2649218
> 
> 7:
> user    0m11.906s
> stddev:   31.57 PSNR: 66.34 MAXDIFF: 2105 bytes:  2646016/  2649218
> 
> 8:
> user    0m22.960s
> stddev:   31.35 PSNR: 66.40 MAXDIFF: 1782 bytes:  2646016/  2649218
> 
> // Martin
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-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Observe your enemies, for they first find out your faults. -- Antisthenes
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