af_lavrresample: fudge some channel layout conversion

Prevents channels from being dropped, e.g. when going 7.1 -> 7.1(wide)
and similar cases. The reasoning here is that channel layouts over HDMI
don't work anyway, and not dropping a channel and playing it on a
slightly "wrong" (but expected) speaker is preferable to playing silence
on these speakers.

Do this to remove issues with ao_coreaudio. Frankly I'm not sure whether
our mapping (between CA and mpv/FFmpeg speakers) is correct, but on the
other hand due to the reasons stated above it's not all that meaningful.

1 files changed, 33 insertions, 0 deletions

diff --git a/audio/filter/af_lavrresample.c b/audio/filter/af_lavrresample.c
index f7f448cad0..477443cefc 100644
--- a/audio/filter/af_lavrresample.c
+++ b/audio/filter/af_lavrresample.c
@@ -179,6 +179,37 @@ bool af_lavrresample_test_conversion(int src_format, int dst_format)
            check_output_conversion(dst_format) != AV_SAMPLE_FMT_NONE;
 }
 
+static struct mp_chmap fudge_pairs[][2] = {
+    {MP_CHMAP2(FLC, FRC), MP_CHMAP2(SL,  SR)},
+    {MP_CHMAP2(SL,  SR),  MP_CHMAP2(FLC, FRC)},
+    {MP_CHMAP2(SDL, SDR), MP_CHMAP2(SL,  SR)},
+    {MP_CHMAP2(SL,  SR),  MP_CHMAP2(SDL, SDR)},
+};
+
+// Modify out_layout and return the new value. The intention is reducing the
+// loss libswresample's rematrixing will cause by exchanging similar, but
+// strictly speaking incompatible channel pairs. For example, 7.1 should be
+// changed to 7.1(wide) without dropping the SL/SR channels. (We still leave
+// it to libswresample to create the remix matrix.)
+static uint64_t fudge_layout_conversion(struct af_instance *af,
+                                        uint64_t in, uint64_t out)
+{
+    for (int n = 0; n < MP_ARRAY_SIZE(fudge_pairs); n++) {
+        uint64_t a = mp_chmap_to_lavc(&fudge_pairs[n][0]);
+        uint64_t b = mp_chmap_to_lavc(&fudge_pairs[n][1]);
+        if ((in & a) == a && (in & b) == 0 &&
+            (out & a) == 0 && (out & b) == b)
+        {
+            out = (out & ~b) | a;
+
+            MP_VERBOSE(af, "Fudge: %s -> %s\n",
+                       mp_chmap_to_str(&fudge_pairs[n][0]),
+                       mp_chmap_to_str(&fudge_pairs[n][1]));
+        }
+    }
+    return out;
+}
+
 // mp_chmap_get_reorder() performs:
 //  to->speaker[n] = from->speaker[src[n]]
 // but libavresample does:
@@ -297,6 +328,8 @@ static int configure_lavrr(struct af_instance *af, struct mp_audio *in,
     if (map_out.num > out_lavc.num)
         mp_audio_set_channels(&s->pool_fmt, &map_out);
 
+    out_ch_layout = fudge_layout_conversion(af, in_ch_layout, out_ch_layout);
+
     // Real conversion; output is input to avrctx_out.
     av_opt_set_int(s->avrctx, "in_channel_layout",  in_ch_layout, 0);
     av_opt_set_int(s->avrctx, "out_channel_layout", out_ch_layout, 0);