[FFmpeg-devel] [PATCH] avfilter: add ambisonic decoder filter
James Almer
jamrial at gmail.com
Sun Oct 30 21:14:25 EET 2022
On 10/30/2022 3:58 PM, Paul B Mahol wrote:
> On 10/30/22, James Almer <jamrial at gmail.com> wrote:
>> On 10/30/2022 3:29 PM, Paul B Mahol wrote:
>>> On 10/30/22, James Almer <jamrial at gmail.com> wrote:
>>>>
>>>>
>>>> On 10/30/2022 3:19 PM, Paul B Mahol wrote:
>>>>> On 10/30/22, James Almer <jamrial at gmail.com> wrote:
>>>>>> On 10/30/2022 12:34 PM, Paul B Mahol wrote:
>>>>>>> +static const struct {
>>>>>>> + const int order;
>>>>>>> + const int inputs;
>>>>>>> + const int speakers;
>>>>>>> + const int near_field;
>>>>>>> + const int type;
>>>>>>> + const double xover;
>>>>>>> + const AVChannelLayout outlayout;
>>>>>>> + const double *speakers_azimuth;
>>>>>>> + const double *speakers_elevation;
>>>>>>> + const double *speakers_distance;
>>>>>>> +} ambisonic_tab[] = {
>>>>>>> + [MONO] = {
>>>>>>> + .order = 0,
>>>>>>> + .inputs = 1,
>>>>>>> + .speakers = 1,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_MONO,
>>>>>>> + .speakers_azimuth = (const double[1]){ 0. },
>>>>>>> + .speakers_distance = (const double[1]){ 1. },
>>>>>>> + },
>>>>>>> + [STEREO] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 2,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO,
>>>>>>> + .speakers_azimuth = (const double[2]){ -30, 30},
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [STEREO_DOWNMIX] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 2,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout =
>>>>>>> (AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO_DOWNMIX,
>>>>>>> + .speakers_azimuth = (const double[2]){ -90, 90 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [SURROUND] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 3,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_SURROUND,
>>>>>>> + .speakers_azimuth = (const double[3]){ -45, 45, 0 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [L2_1] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 3,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_2_1,
>>>>>>> + .speakers_azimuth = (const double[3]){ -45, 45, 180 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [TRIANGLE] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 3,
>>>>>>> + .type = 1,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_SURROUND,
>>>>>>> + .speakers_azimuth = (const double[3]){ -120, 120, 0 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [QUAD] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 4,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_QUAD,
>>>>>>> + .speakers_azimuth = (const double[4]){ -45, 45, -135, 135 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [SQUARE] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 4,
>>>>>>> + .type = 1,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_4POINT0,
>>>>>>> + .speakers_azimuth = (const double[4]){ 0, -90, 180, 90 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [L4_0] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 4,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_4POINT0,
>>>>>>> + .speakers_azimuth = (const double[4]){ -30, 30, 0, 180 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [L5_0] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 5,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout =
>>>>>>> (AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT0_BACK,
>>>>>>> + .speakers_azimuth = (const double[5]){ -30, 30, 0, -145, 145
>>>>>>> },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [L5_0_SIDE] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 5,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT0,
>>>>>>> + .speakers_azimuth = (const double[5]){ -30, 30, 0, -110, 110
>>>>>>> },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [L6_0] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 6,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_6POINT0,
>>>>>>> + .speakers_azimuth = (const double[6]){ -30, 30, 0, 180,
>>>>>>> -110,
>>>>>>> 110
>>>>>>> },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [L7_0] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 7,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_7POINT0,
>>>>>>> + .speakers_azimuth = (const double[7]){ -30, 30, 0, -145,
>>>>>>> 145,
>>>>>>> -110, 110 },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [TETRA] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 4,
>>>>>>> + .type = 2,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_QUAD,
>>>>>>> + .speakers_azimuth = (const double[4]){ -90, 90, 0, 180 },
>>>>>>> + .speakers_elevation = (const double[4]){ -35.3, -35.3, 35.3,
>>>>>>> 35.3
>>>>>>> },
>>>>>>> + .speakers_distance = same_distance,
>>>>>>> + },
>>>>>>> + [CUBE] = {
>>>>>>> + .order = 1,
>>>>>>> + .inputs = 4,
>>>>>>> + .speakers = 8,
>>>>>>> + .type = 2,
>>>>>>> + .near_field = NF_NONE,
>>>>>>> + .xover = 0.,
>>>>>>> + .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_7POINT1,
>>>>>>
>>>>>> 7.1 defines an LFE channel, which is clearly not intended here, so it
>>>>>> should be either:
>>>>>>
>>>>>> .outlayout = (AVChannelLayout)AV_CHANNEL_LAYOUT_MASK(8,
>>>>>> AV_CH_LAYOUT_QUAD |
>>>>>> AV_CH_TOP_FRONT_LEFT |
>>>>>> AV_CH_TOP_FRONT_RIGHT |
>>>>>> AV_CH_TOP_BACK_LEFT |
>>>>>> AV_CH_TOP_BACK_RIGHT),
>>>>>>
>>>>>> Or the AV_CHANNEL_LAYOUT_CUBE layout (using the exact same bitmask as
>>>>>> above) after the patch i sent just now is committed.
>>>>>
>>>>> CUBE is as real cube in 3d space. No current layout in API can be
>>>>> described correctly.
>>>>
>>>> the TOP_* channels are in a different height layer than the other
>>>> channels, namely above them. The result for this bitmask is a 3D cube
>>>> layout (Left and right speakers both front and back, in two different
>>>> height layers).
>>>
>>> https://en.wikipedia.org/wiki/Ambisonic_reproduction_systems#Cube
>>
>> "If all speakers are placed in room corners", So a 7.1 layout, besides
>> defining one channel as LFE, is also not ideal given that it defines two
>> speaker positions in places other than the room corners, and by no means
>> describes a 3D cube since all speakers are at the same height.
>> The TOP channel ids let you assign four channels to speakers placed in
>> the top room corners.
>
> 7.1 is there just because it have 8 channels. No other reasons.
If you want to just say "there are eight channels", then make the
AVChannelLayout define 8 channels of order UNSPEC. Let the user then
figure out what to do with them. But if you want to actually give each
channel a position, then you need to use the proper IDs to describe the
intended layout.
Using 7.1 here is lying to the user, as it's telling them he needs to
place two speakers at the front, two at the back, two to the sides, and
all at the same height, plus one channel wrongly feeding the sub woofer.
Not to mention what would happen if you try to downmix the stream. swr
would just generate awful results.
Meanwhile, using cube you'll tell the user to place one speaker on every
corner in the room, or a downmixer how to properly handle the channels
to get good results (e.g. on headphones).
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