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

[James Almer]

On 10/30/2022 3:58 PM, Paul B Mahol wrote:
On 10/30/22, James Almer <jamr...@gmail.com> wrote:
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.

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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