Re: [FFmpeg-devel] [PATCH v2] avcodec/atrac3plus: reorder channels to match the output layout

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

James Almer:
> On 10/31/2022 7:13 PM, Andreas Rheinhardt wrote:
>> James Almer:
>>> The order in which the channels are coded in the bitstream do not
>>> always follow
>>> the native, bitmask-based order of channels both signaled by the WAV
>>> container
>>> and forced by this same decoder. This is the case with layouts
>>> containing an
>>> LFE channel, as it's always coded last.
>>>
>>> Fixes ticket #9964.
>>>
>>> Signed-off-by: James Almer <jamrial@gmail.com>
>>> ---
>>>   libavcodec/atrac3plusdec.c | 13 ++++++++++++-
>>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/libavcodec/atrac3plusdec.c b/libavcodec/atrac3plusdec.c
>>> index ee71645a3c..9e12f21930 100644
>>> --- a/libavcodec/atrac3plusdec.c
>>> +++ b/libavcodec/atrac3plusdec.c
>>> @@ -48,6 +48,17 @@
>>>   #include "atrac.h"
>>>   #include "atrac3plus.h"
>>>   +static uint8_t channel_map[8][8] = {
>>> +    { 0, },
>>> +    { 0, 1, },
>>> +    { 0, 1, 2, },
>>> +    { 0, 1, 2, 3, },
>>> +    { 0, },
>>> +    { 0, 1, 2, 4, 5, 3, },
>>> +    { 0, 1, 2, 4, 5, 8, 3, },
>>> +    { 0, 1, 2, 4, 5, 9, 10, 3, },
>>> +};
>>> +
>>>   typedef struct ATRAC3PContext {
>>>       GetBitContext gb;
>>>       AVFloatDSPContext *fdsp;
>>> @@ -378,7 +389,7 @@ static int atrac3p_decode_frame(AVCodecContext
>>> *avctx, AVFrame *frame,
>>>                             channels_to_process, avctx);
>>>             for (i = 0; i < channels_to_process; i++)
>>> -            memcpy(samples_p[out_ch_index + i], ctx->outp_buf[i],
>>> +           
>>> memcpy(samples_p[channel_map[frame->ch_layout.nb_channels -
>>> 1][out_ch_index + i]], ctx->outp_buf[i],
>>>                      ATRAC3P_FRAME_SAMPLES * sizeof(**samples_p));
>>>             ch_block++;
>>
>> Looking at the last two entries, it seems to me that you simply used the
>> numerical values of the AV_CHAN_* constants, i.e. as if you wanted to
>> write { AV_CHAN_FRONT_LEFT, AV_CHAN_FRONT_RIGHT, AV_CHAN_FRONT_CENTER,
>> AV_CHAN_BACK_LEFT, AV_CHAN_BACK_RIGHT, AV_CHAN_SIDE_LEFT,
>> AV_CHAN_SIDE_RIGHT, AV_CHAN_LOW_FREQUENCY } for the last entry. This is
>> wrong, as it conflates the enum AVChannel domain with the index domain;
>> it will segfault for 6.1 and 7.1, because there are no data planes with
>> indices 8, 9 and 10 in the output frame.
>>
>> The array for 6.1 is { 0, 1, 2, 4, 5, 6, 3 }, for 7.1 it is { 0, 1, 2,
>> 4, 5, 6, 7, 3, } (presuming that your first patch was right). The
>> derivation for 6.1 is as follows: The first channel in atrac is FL,
>> which is also the first channel in AV_CHANNEL_LAYOUT_6POINT1_BACK. So
>> the first entry is 0; the next channel is FR which is the second channel
>> in AV_CHANNEL_LAYOUT_6POINT1_BACK, so the second entry is 1. The next
>> atrac entry is FC, which is also the next entry in
>> AV_CHANNEL_LAYOUT_6POINT1_BACK, so the next entry is 2. The next atrac
>> entry is BL, which is not the next channel in
>> AV_CHANNEL_LAYOUT_6POINT1_BACK (which is lfe), but the one after that.
>> So the next entry is four. Similarly, the next entry is five. The atrac
>> entry after that is BC, which is the next (and last) entry of
>> AV_CHANNEL_LAYOUT_6POINT1_BACK and has index six. It doesn't matter for
>> this that there are several channels in enum AVChannel between BR and
>> BC, as these channels are not present in AV_CHANNEL_LAYOUT_6POINT1_BACK
>> (it would also not matter if there were any gaps in the values of the
>> AV_CHAN_* constants in between). The next (and last) atrac entry is LFE,
>> which is the fourth channel in AV_CHANNEL_LAYOUT_6POINT1_BACK and
>> therefore has index 3.
>>
>> Is it really absolutely guaranteed that atrac only has one channel
>> layout per channel count? It seems to me that adding a const uint8_t
>> *channel_map to the context that gets set like ctx->channel_map = (const
>> uint8_t[]){ 0, 1, 2, 4, 5, 6, 3 } (for 6.1) would be simpler.
> 
> Unless i'm missing something, this would be local to set_channel_params().
> 

Correct.

- Andreas

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