Re: [FFmpeg-devel] [PATCH] avfilter: add ambisonic decoder filter

[James Almer]

On 10/30/2022 3:29 PM, Paul B Mahol wrote:
On 10/30/22, James Almer <jamr...@gmail.com> wrote:


On 10/30/2022 3:19 PM, Paul B Mahol wrote:
On 10/30/22, James Almer <jamr...@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.
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