Add a 7.x / 5.x -> 4.0 downmixing routine (for AAC mostly, for a52 this is done by liba52)

modules/audio_filter/channel_mixer/simple.c

@@ -68,9 +68,15 @@ static int Create( vlc_object_t *p_this )
         return -1;
     }
 
-    /* Only conversion to Stereo right now */
+    /* Only conversion to Stereo and 4.0 right now */
     if( p_filter->output.i_physical_channels !=
-        (AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT) ) return -1;
+        (AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT) 
+        && p_filter->output.i_physical_channels !=
+        ( AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT |
+        AOUT_CHAN_REARLEFT | AOUT_CHAN_REARRIGHT) )
+        {
+            return -1;
+        }
 
     /* Only from 7/7.1/5/5.1 */
     if( (p_filter->input.i_physical_channels & ~AOUT_CHAN_LFE) !=
@@ -105,6 +111,9 @@ static void DoWork( aout_instance_t * p_aout, aout_filter_t * p_filter,
     p_out_buf->i_nb_samples = p_in_buf->i_nb_samples;
     p_out_buf->i_nb_bytes = p_in_buf->i_nb_bytes * i_output_nb / i_input_nb;
 
+    if( p_filter->output.i_physical_channels ==
+                            (AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT) )
+    {
         if( p_filter->input.i_physical_channels & AOUT_CHAN_MIDDLELEFT )
         for( i = p_in_buf->i_nb_samples; i--; )
         {
@@ -129,4 +138,41 @@ static void DoWork( aout_instance_t * p_aout, aout_filter_t * p_filter,
 
             if( p_filter->input.i_physical_channels & AOUT_CHAN_LFE ) p_src++;
         }
+    }
+    else
+    {
+        if( p_filter->input.i_physical_channels & AOUT_CHAN_MIDDLELEFT )
+        for( i = p_in_buf->i_nb_samples; i--; )
+        {
+            *p_dest = p_src[6] + 0.5 * p_src[0] + p_src[2] / 6;
+            p_dest++;
+            *p_dest = p_src[6] + 0.5 * p_src[1] + p_src[3] / 6;
+            p_dest++;
+            *p_dest = p_src[2] / 6 +  p_src[4];
+            p_dest++;
+            *p_dest = p_src[3] / 6 +  p_src[5];
+            p_dest++;
+
+            p_src += 7;
+
+            if( p_filter->input.i_physical_channels & AOUT_CHAN_LFE ) p_src++;
+        }
+        else
+        for( i = p_in_buf->i_nb_samples; i--; )
+        {
+            *p_dest = p_src[4] + 0.5 * p_src[0];
+            p_dest++;
+            *p_dest = p_src[4] + 0.5 * p_src[1];
+            p_dest++;
+            *p_dest = p_src[2];
+            p_dest++;
+            *p_dest = p_src[3];
+            p_dest++;
+
+            p_src += 5;
+
+            if( p_filter->input.i_physical_channels & AOUT_CHAN_LFE ) p_src++;
+        }
+
+    }
 }