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

[Lynne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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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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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.

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