From 579d743159c665b643e5c53d7e0cf27e89043ac4 Mon Sep 17 00:00:00 2001
From: cigaes <cigaes@b3059339-0415-0410-9bf9-f77b7e298cf2>
Date: Sun, 26 Sep 2010 08:45:40 +0000
Subject: [PATCH] audio: auto-add downmix filter when stereo output is
 requested
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When -channels 2 [default] is specified and the audio decoder used
does not support internal downmixing, automatically add a pan filter
after the decoder to downmix to stereo.

Patch by Clément Bœsch, ubitux gmail com

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@32356 b3059339-0415-0410-9bf9-f77b7e298cf2
---
 libaf/af.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/libaf/af.c b/libaf/af.c
index 5238e6bc85..0190a75f5e 100644
--- a/libaf/af.c
+++ b/libaf/af.c
@@ -410,6 +410,26 @@ static int fixup_output_format(af_stream_t* s)
     return AF_OK;
 }
 
+/**
+ * Automatic downmix to stereo in case the codec does not implement it.
+ */
+static void af_downmix(af_stream_t* s)
+{
+    static const char * const downmix_strs[AF_NCH + 1] = {
+        /*                FL       FR       RL       RR          FC          LF         AL      AR */
+        [3] = "pan=2:" "0.6:0:" "0:0.6:"                     "0.4:0.4",
+        [4] = "pan=2:" "0.6:0:" "0:0.6:" "0.4:0:"  "0:0.4",
+        [5] = "pan=2:" "0.5:0:" "0:0.5:" "0.2:0:"  "0:0.2:"  "0.3:0.3",
+        [6] = "pan=2:" "0.4:0:" "0:0.4:" "0.2:0:"  "0:0.2:"  "0.3:0.3:"   "0.1:0.1",
+        [7] = "pan=2:" "0.4:0:" "0:0.4:" "0.2:0:"  "0:0.2:"  "0.3:0.3:"              "0.1:0:" "0:0.1",
+        [8] = "pan=2:" "0.4:0:" "0:0.4:" "0.15:0:" "0:0.15:" "0.25:0.25:" "0.1:0.1:" "0.1:0:" "0:0.1",
+    };
+    const char *af_pan_str = downmix_strs[s->input.nch];
+
+    if (af_pan_str)
+        af_append(s, s->first, af_pan_str);
+}
+
 /* Initialize the stream "s". This function creates a new filter list
    if necessary according to the values set in input and output. Input
    and output should contain the format of the current movie and the
@@ -421,6 +441,7 @@ static int fixup_output_format(af_stream_t* s)
    The return value is 0 if success and -1 if failure */
 int af_init(af_stream_t* s)
 {
+    struct MPOpts *opts = s->opts;
   int i=0;
 
   // Sanity check
@@ -436,6 +457,10 @@ int af_init(af_stream_t* s)
 
   // Check if this is the first call
   if(!s->first){
+    // Append a downmix pan filter at the beginning of the chain if needed
+    if (s->input.nch != opts->audio_output_channels
+        && opts->audio_output_channels == 2)
+      af_downmix(s);
     // Add all filters in the list (if there are any)
     if (s->cfg.list) {
       while(s->cfg.list[i]){