Media: downmix 7.1 AAC in the order FilmCraft now delivers it (#1701)

FilmCraft reorders AAC channel configuration 12 (7.1) to WAVE/SMPTE order,
as it does 3 to 6, so 7.1 AAC's left channel was mixed as the centre. The
downmix oracle test gains 6.1 (a program config element, left in AAC's
order) and 7.1 AAC fixtures.

crates/media/src/downmix.rs

@@ -65,8 +65,10 @@ pub fn stereo_gains(count: usize, aac: bool) -> Vec<Pair> {
 }
 
 /// The gains for AAC as FilmCraft's decoder hands it over: channel configurations 3 to 6 (3 to 6
-/// channels) are already in WAV/SMPTE order (L R C [Cs] / [LFE] Ls Rs), everything else (mono,
-/// stereo, 7 and 8 channels, program-config layouts) in AAC's element order.
+/// channels) and 12 (7.1, 8 channels) are already in WAV/SMPTE order (L R C [Cs] / [LFE] Ls Rs,
+/// L R C LFE Lb Rb Ls Rs), everything else (mono, stereo, program-config layouts such as the 6.1
+/// ffmpeg writes) in AAC's element order. Configuration 11 (6.1) and 7 (7.1 with front wides)
+/// can't be told from those by the channel count and are read in the order of the commoner one.
 pub fn decoded_aac_gains(count: usize) -> Vec<Pair> {
     match count {
         // C L R Cs, reordered to L R C Cs: the same gains, permuted.
@@ -74,7 +76,7 @@ pub fn decoded_aac_gains(count: usize) -> Vec<Pair> {
             let g = stereo_gains(4, true);
             [1, 2, 0, 3].iter().filter_map(|&i| g.get(i).copied()).collect()
         }
-        3 | 5 | 6 => stereo_gains(count, false),
+        3 | 5 | 6 | 8 => stereo_gains(count, false),
         _ => stereo_gains(count, true),
     }
 }
@@ -118,9 +120,9 @@ mod tests {
     }
 
     #[test]
-    fn decoded_aac_is_in_wav_order_for_three_to_six_channels() {
-        // FilmCraft hands AAC 3.0 to 5.1 over as L R C [LFE] Ls Rs, the order of the plain layouts.
-        for n in [3, 5, 6] {
+    fn decoded_aac_is_in_wav_order_for_three_to_six_and_eight_channels() {
+        // FilmCraft hands AAC 3.0 to 5.1 and 7.1 over in the order of the plain layouts.
+        for n in [3, 5, 6, 8] {
             assert_eq!(decoded_aac_gains(n), stereo_gains(n, false), "{n} channels");
         }
         let g = decoded_aac_gains(4);
@@ -128,7 +130,7 @@ mod tests {
         assert!(g[0][0] > 0.0 && g[0][1] == 0.0 && g[1][1] > 0.0 && g[1][0] == 0.0, "L R first");
         assert!(g[2][0] > 0.0 && g[2][0] == g[2][1], "the centre is third and feeds both sides");
         assert!(g[3][0] > 0.0 && g[3][0] < g[2][0], "the rear centre is last, below the centre");
-        for n in [1, 2, 7, 8, 9] {
+        for n in [1, 2, 7, 9] {
             assert_eq!(decoded_aac_gains(n), stereo_gains(n, true), "{n} channels");
         }
     }

crates/media/tests/downmix.rs

@@ -94,8 +94,18 @@ fn cases() -> Vec<Case> {
     let (n71, g71) = seven_one();
     let k5 = 1.0 / (1.0 + 2.0 * K);
     let kq = 1.0 / (1.0 + K);
+    let k61 = 1.0 / (1.5 + 2.0 * K);
     vec![
         Case { file: "dm51.m4a", layout: "5.1", names: n51, codec: &["-c:a", "aac", "-b:a", "384k"], gains: g51.clone(), tol: 0.15 },
+        Case {
+            file: "dm61.m4a",
+            layout: "6.1",
+            names: &["FL", "FR", "FC", "LFE", "BC", "SL", "SR"],
+            codec: &["-c:a", "aac", "-b:a", "448k"],
+            gains: vec![[k61, 0.0], [0.0, k61], [k61 * K, k61 * K], [0.0, 0.0], [k61 / 2.0, k61 / 2.0], [k61 * K, 0.0], [0.0, k61 * K]],
+            tol: 0.15,
+        },
+        Case { file: "dm71.m4a", layout: "7.1", names: n71, codec: &["-c:a", "aac", "-b:a", "512k"], gains: g71.clone(), tol: 0.15 },
         Case { file: "dm51.ac3.mp4", layout: "5.1", names: n51, codec: &["-c:a", "ac3", "-b:a", "640k"], gains: g51.clone(), tol: 0.15 },
         Case { file: "dm51.wav", layout: "5.1", names: n51, codec: &["-c:a", "pcm_s16le"], gains: g51.clone(), tol: 0.02 },
         Case { file: "dm51.flac", layout: "5.1", names: n51, codec: &["-c:a", "flac"], gains: g51, tol: 0.02 },