Re: [FFmpeg-devel] [PATCH v2 8/8] aacdec: add a decoder for AAC USAC (xHE-AAC)

[Marton Balint <cus@passwd.hu>]

On Tue, 21 May 2024, Lynne via ffmpeg-devel wrote:

> On 21/05/2024 09:16, Marton Balint wrote:
>>
>>
>>  On Sun, 19 May 2024, Lynne via ffmpeg-devel wrote:
>>
>>>  On 19/05/2024 21:39, Marton Balint wrote:
>>>> 
>>>>
>>>>   On Sun, 19 May 2024, Lynne via ffmpeg-devel wrote:
>>>>
>>>>>   This commit adds a decoder for the frequency-domain part of USAC.
>>>>>
>>>>   [...]
>>>> 
>>>>>
>>>>>   +/* Finish later */
>>>>>   +static const enum AVChannel usac_ch_pos_to_av[64] = {
>>>>>   +    [0] = AV_CHAN_FRONT_LEFT,
>>>>>   +    [1] = AV_CHAN_FRONT_RIGHT,
>>>>>   +    [2] = AV_CHAN_FRONT_CENTER,
>>>>>   +    [3] = AV_CHAN_LOW_FREQUENCY,
>>>>>   +    [4] = AV_CHAN_BACK_LEFT, // unsure
>>>>>   +    [5] = AV_CHAN_BACK_RIGHT, // unsure
>>>>>   +    [6] = AV_CHAN_FRONT_LEFT_OF_CENTER,
>>>>>   +    [7] = AV_CHAN_FRONT_RIGHT_OF_CENTER,
>>>>>   +    [8] = 0, /* rear surround left is missing */
>>>>>   +    [9] = 0, /* rear surround right is missing */
>>>>>   +    [10] = AV_CHAN_BACK_CENTER,
>>>>>   +    [11] = AV_CHAN_SURROUND_DIRECT_LEFT,
>>>>>   +    [12] = AV_CHAN_SURROUND_DIRECT_RIGHT,
>>>>>   +    [13] = AV_CHAN_SIDE_LEFT, // fairly sure
>>>>>   +    [14] = AV_CHAN_SIDE_RIGHT, // fairly sure
>>>>>   +    [15] = AV_CHAN_WIDE_LEFT, // somewhat confident
>>>>>   +    [16] = AV_CHAN_WIDE_RIGHT, // somewhat confident
>>>>>   +    [17] = AV_CHAN_TOP_FRONT_LEFT,
>>>>>   +    [18] = AV_CHAN_TOP_FRONT_RIGHT,
>>>>>   +    [19] = AV_CHAN_TOP_FRONT_CENTER,
>>>>>   +    [20] = AV_CHAN_TOP_BACK_LEFT,
>>>>>   +    [21] = AV_CHAN_TOP_BACK_RIGHT,
>>>>>   +    [22] = AV_CHAN_TOP_BACK_CENTER,
>>>>>   +    [23] = AV_CHAN_TOP_SIDE_LEFT,
>>>>>   +    [24] = AV_CHAN_TOP_SIDE_RIGHT,
>>>>>   +    [25] = AV_CHAN_TOP_CENTER,
>>>>>   +    [26] = AV_CHAN_LOW_FREQUENCY, // actually LFE2
>>>>>   +    [27] = AV_CHAN_BOTTOM_FRONT_LEFT,
>>>>>   +    [28] = AV_CHAN_BOTTOM_FRONT_RIGHT,
>>>>>   +    [29] = AV_CHAN_BOTTOM_FRONT_CENTER,
>>>>>   +    [30] = 0, /* top left surround is missing */
>>>>>   +    [31] = 0, /* top right surround is missing */
>>>>>   +};
>>>>
>>>>   Some comment would be nice about the source of this table (which
>>>>  document,
>>>>   which table).
>>>>
>>>>   It looks very similar to the ISO channel positons used in mov_chan. I
>>>>   think we follow this mapping in most cases:
>>>>
>>>>   Left  Surround is SIDE_LEFT
>>>>   Right Surround is SIDE_RIGHT
>>>>   Rear Surround Left  is BACK_LEFT
>>>>   Rear Surround Right is BACK_RIGHT
>>>>
>>>>   So in your table [4] and [5] should be SIDE, [8] and [9] should be
>>>>  BACK.
>>>>   [26] can be AV_CHAN_LOW_FREQUENCY_2, we do have that.
>>>>
>>>>   Yes, Left/Right Surround and Left/Right Side Surround will be the same,
>>>>   but those are not present in commonly used layouts at the same time.
>>>>
>>>>   Regards,
>>>>   Marton
>>>>   _______________________________________________
>>>>   ffmpeg-devel mailing list
>>>>   ffmpeg-devel@ffmpeg.org
>>>>   https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>>>>
>>>>   To unsubscribe, visit link above, or email
>>>>   ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>>>
>>>  Source of the table is ISO/IEC 23003-3, Table 74 — bsOutputChannelPos:
>>>
>>>  0 L left front FL front left
>>>  1 R right front FR front right
>>>  2 C center front FC front centre
>>>  3 LFE low frequency enhancement LFE1 low frequency effects-1
>>>  4 Ls left surround LS left surround
>>>  5 Rs right surround RS right surround
>>>  6 Lc left front center FLc front left centre
>>>  7 Rc right front center FRc front right centre
>>>  8 Lsr rear surround left BL back left
>>>  9 Rsr rear surround right BR back right
>>>  10 Cs rear center BC back centre
>>>  11 Lsd left surround direct LSd left surround direct
>>>  12 Rsd right surround direct RSd right surround direct
>>>  13 Lss left side surround SL side left
>>>  14 Rss right side surround SR side right
>>>  15 Lw left wide front FLw front left wide
>>>  16 Rw right wide front FRw front right wide
>>>  17 Lv left front vertical height TpFL top front left
>>>  18 Rv right front vertical height TpFR top front right
>>>  19 Cv center front vertical height TpFC top front centre
>>>  20 Lvr left surround vertical height rear TpBL top back left
>>>  21 Rvr right surround vertical height rear TpBR top back right
>>>  22 Cvr center vertical height rear TpBC top back centre
>>>  23 Lvss left vertical height side surround TpSiL top side left
>>>  24 Rvss right vertical height side surround TpSiR top side right
>>>  25 Ts top center surround TpC top centre
>>>  26 LFE2 low frequency enhancement 2 LFE2 low frequency effects-2
>>>  27 Lb left front vertical bottom BtFL bottom front left
>>>  28 Rb right front vertical bottom BtFR bottom front right
>>>  29 Cb center front vertical bottom BtFC bottom front centre
>>>  30 Lvs left vertical height surround TpLS top left surround
>>>  31 Rvs right vertical height surround TpRS top right surround
>>>
>>>  Third field is "Loudspeaker position", last field is "Loudspeaker
>>>  position according to IEC 100/1706/CDV/IEC 62574 (TC100)", each prefixed
>>>  with an abbreviation.
>>>
>>>  I've added the source to the table comment in the code.
>>>
>>>  I've also fixed the SIDE/BACK/LFE2 issue in my github repo I linked
>>>  earlier.
>>
>>  Thanks. Later in the code when you actually use this I can see that you
>>  are creating a native layout:
>>
>>>  +    channel_config_idx = get_bits(gb, 5); /* channelConfigurationIndex
>>>  */
>>>  +    if (!channel_config_idx) {
>>>  +        /* UsacChannelConfig() */
>>>  +        uint8_t channel_pos[64];
>>>  +        uint8_t nb_channels = get_escaped_value(gb, 5, 8, 16); /*
>>>  numOutChannels */
>>>  +        if (nb_channels >= 64)
>>>  +            return AVERROR(EINVAL);
>>>  +
>>>  +        av_channel_layout_uninit(&ac->oc[1].ch_layout);
>>>  +        for (int i = 0; i < nb_channels; i++)
>>>  +            channel_pos[i] = get_bits(gb, 5); /* bsOutputChannelPos */
>>>  +
>>>  +        ac->oc[1].ch_layout.order = AV_CHANNEL_ORDER_NATIVE;
>>>  +        ac->oc[1].ch_layout.nb_channels = nb_channels;
>>>  +        ac->oc[1].ch_layout.u.mask = 0;
>>>  +
>>>  +        for (int i = 0; i < nb_channels; i++)
>>>  +            ac->oc[1].ch_layout.u.mask |= 1 <<
>>>  usac_ch_pos_to_av[channel_pos[i]];
>>>  +
>>>  +        av_channel_layout_copy(&avctx->ch_layout, &ac->oc[1].ch_layout);
>>>  +    } else {
>>
>>  Probably you should create a custom layout here, because the channels are
>>  not necessary in native order. We already have a relatively simple way to
>>  do that and to fall back to native layouts if possible, here is an example
>>  copied from mov_chan:
>>
>>  ret = av_channel_layout_custom_init(ch_layout, channels);
>>  if (ret < 0)
>>      return ret;
>>  for (i = 0; i < channels; i++) {
>>       enum AVChannel id = layout_map[i].id;
>>       ch_layout->u.map[i].id = (id != AV_CHAN_NONE ? id : AV_CHAN_UNKNOWN);
>> }
>>  return av_channel_layout_retype(ch_layout, 0,
>>  AV_CHANNEL_LAYOUT_RETYPE_FLAG_CANONICAL);
>>
>>  So you should adapt this accodingly to aac.
>>
>>  Regards,
>>  Marton
>>  _______________________________________________
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>>  ffmpeg-devel@ffmpeg.org
>>  https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>>
>>  To unsubscribe, visit link above, or email
>>  ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>
> According to the spec:
>
>>  In case of multiple channel elements the index i of
>>  bsOutputChannelPos[i] indicates the position in which the channel
>>  appears in the bitstream.
>
> So the channels will always be in native order, as far as I understand.
>

AV_CHANNEL_ORDER_NATIVE expects channels in the order of the channel IDs 
in the AVChannel enum. Surely that is not the case here, unless the 
decoder reorders the channels. Or what if an output position appears 
multiple times?

Regards,
Marton
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