[FFmpeg-devel] [PATCH v3 1/2] avcodec: add adpcm_ima_ssi encoder

[Paul B Mahol]

On 4/12/20, Zane van Iperen <zane at zanevaniperen.com> wrote:
> On Sun, 12 Apr 2020 11:41:40 +0200
> "Paul B Mahol" <onemda at gmail.com> wrote:
>
>> On 4/12/20, Zane van Iperen <zane at zanevaniperen.com> wrote:
>> > On Sat, 11 Apr 2020 22:02:26 +0200
>> > "Paul B Mahol" <onemda at gmail.com> wrote:
>> >
>> >> >
>> >> > Ping.
>> >> >
>> >> > Also, could someone please clarify something for me?
>> >> >
>> >> > When encoding, there's no inherit differences between trellis and
>> >> > non-trellis other then a potentially more-accurate set of
>> >> > nibbles?
>> >> >
>> >> > And the decoder is none the wiser and chews on them both the same
>> >> > way?
>> >> >
>> >> > If I'm right, then this looks to be a decoder issue...
>> >> >
>> >>
>> >> You are wrong, just follow qt adpcm encoder code.
>> >>
>> >
>> > I did.
>> >
>> >
>> > Is it possible, and I don't suggest this lightly, that the trellis
>> > code is bugged, and errors accumulate over time?
>> >
>> > I ask this because all of the encoders (except adpcm_yamaha, which
>> > is different anyway) have either the full sample and/or the step
>> > index periodically written to the bitstream, which is then used to
>> > reset the state machine, thus errors don't accumulate.
>> >
>> > adpcm_ima_ssi doesn't have that, so it relies on the state being
>> > accurate through the entire stream. This would certainly explain the
>> > behaviour I'm seeing in certain cases (see below).
>> >
>> > In both images, the top stream is encoded with '-trellis 0', and the
>> > bottom with '-trellis 1':
>> > - https://0x0.st/iSdS.png
>> >   This one's obvious where the problem is.
>> > - https://0x0.st/iSdQ.png
>> >   This is more subtle, but the entire stream is offset slightly.
>> > It's more noticeable at the start and end.
>> >
>> >
>> > For reference, here's the code I'm using. You'll see it's basically
>> > the same as the adpcm_ima_qt code:
>> >
>> > if (avctx->trellis > 0) {
>> >     FF_ALLOC_OR_GOTO(avctx, buf, n * avctx->channels, error);
>> >
>> >     for (ch = 0; ch < avctx->channels; ch++) {
>> >         adpcm_compress_trellis(avctx, samples + ch, buf + n * ch,
>> >                                c->status + ch, n, avctx->channels);
>> >         c->status[ch].prev_sample = c->status[ch].predictor;
>> >     }
>> >
>> >     for (i = 0; i < n; i++) {
>> >         for (ch = 0; ch < avctx->channels; ch++) {
>> >             put_bits(&pb, 4, buf[n * ch + i]);
>> >         }
>> >     }
>> >     av_free(buf);
>> > }
>> >
>> >
>> > Or maybe I just don't fully understand how trellis works.
>>
>> Yes. that is 100% correct.
>>
>> Also you ignored fact that you do not update nodes for SSI variant in
>> compress trellis.
>
> I probably should have mentioned that yes, I had already added the
> appropriate SSI code to adpcm_compress_trellis().

The one that updates node predictor?
>
>
>
>
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