Added support for 3f->stereo/mono downmixing (close #2926).

(cherry picked from commit 9a55c487)

modules/audio_filter/channel_mixer/simple.c

 /*****************************************************************************
- * simple.c : simple channel mixer plug-in (only 7/7.1/5/5.1 -> Stereo for now)
+ * simple.c : simple channel mixer plug-in
  *****************************************************************************
  * Copyright (C) 2002, 2006 the VideoLAN team
  * $Id$
@@ -33,6 +33,7 @@
 #include <vlc_aout.h>
 #include <vlc_filter.h>
 #include <vlc_block.h>
+#include <assert.h>
 
 /*****************************************************************************
  * Module descriptor
@@ -61,6 +62,7 @@ vlc_module_end ()
 #define AOUT_CHANS_STEREO_MIDDLE (AOUT_CHAN_MIDDLELEFT | AOUT_CHAN_MIDDLERIGHT )
 
 #define AOUT_CHANS_2_0          AOUT_CHANS_STEREO_FRONT
+#define AOUT_CHANS_3_0          ( AOUT_CHANS_STEREO_FRONT | AOUT_CHAN_CENTER )
 #define AOUT_CHANS_4_0          ( AOUT_CHANS_STEREO_FRONT | AOUT_CHANS_STEREO_REAR )
 #define AOUT_CHANS_4_0_MIDDLE   ( AOUT_CHANS_STEREO_FRONT | AOUT_CHANS_STEREO_MIDDLE )
 #define AOUT_CHANS_5_0          ( AOUT_CHANS_4_0 | AOUT_CHAN_CENTER )
@@ -104,6 +106,8 @@ static void DoWork( aout_instance_t * p_aout, aout_filter_t * p_filter,
     const bool b_input_5_0 = !b_input_7_0 &&
                              ( (i_input_physical & AOUT_CHANS_5_0) == AOUT_CHANS_5_0 ||
                                (i_input_physical & AOUT_CHANS_5_0_MIDDLE) == AOUT_CHANS_5_0_MIDDLE );
+    const bool b_input_3_0 = !b_input_7_0 && !b_input_5_0 &&
+                             (i_input_physical & ~AOUT_CHAN_LFE) == AOUT_CHANS_3_0;
     int i_input_nb = aout_FormatNbChannels( &p_filter->input );
     int i_output_nb = aout_FormatNbChannels( &p_filter->output );
     float *p_dest = (float *)p_out_buf->p_buffer;
@@ -127,7 +131,7 @@ 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
+        else if( b_input_5_0 )
         for( i = p_in_buf->i_nb_samples; i--; )
         {
             *p_dest = p_src[4] + 0.5 * p_src[0] + 0.33 * p_src[2];
@@ -137,6 +141,18 @@ static void DoWork( aout_instance_t * p_aout, aout_filter_t * p_filter,
 
             p_src += 5;
 
+            if( p_filter->input.i_physical_channels & AOUT_CHAN_LFE ) p_src++;
+        }
+        else if( b_input_3_0 )
+        for( i = p_in_buf->i_nb_samples; i--; )
+        {
+            *p_dest = p_src[2] + 0.5 * p_src[0];
+            p_dest++;
+            *p_dest = p_src[2] + 0.5 * p_src[1];
+            p_dest++;
+
+            p_src += 3;
+
             if( p_filter->input.i_physical_channels & AOUT_CHAN_LFE ) p_src++;
         }
     }
@@ -162,6 +178,16 @@ 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( b_input_3_0 )
+        for( i = p_in_buf->i_nb_samples; i--; )
+        {
+            *p_dest = p_src[2] + p_src[0] / 4 + p_src[1] / 4;
+            p_dest++;
+
+            p_src += 3;
+
+            if( p_filter->input.i_physical_channels & AOUT_CHAN_LFE ) p_src++;
+        }
         else
         for( i = p_in_buf->i_nb_samples; i--; )
         {
@@ -173,6 +199,9 @@ static void DoWork( aout_instance_t * p_aout, aout_filter_t * p_filter,
     }
     else
     {
+        assert( p_filter->output.i_physical_channels == AOUT_CHANS_4_0 );
+        assert( b_input_7_0 || b_input_5_0 );
+
         if( b_input_7_0 )
         for( i = p_in_buf->i_nb_samples; i--; )
         {
@@ -310,11 +339,12 @@ static bool IsSupported( const audio_format_t *p_input, const audio_format_t *p_
         return false;
     }
 
-    /* Only from 7/7.1/5/5.1/2.0
+    /* Only from 7/7.1/5/5.1/3/3.1/2.0
      * XXX 5.X rear and middle are handled the same way */
     if( (p_input->i_physical_channels & ~AOUT_CHAN_LFE) != AOUT_CHANS_7_0 &&
         (p_input->i_physical_channels & ~AOUT_CHAN_LFE) != AOUT_CHANS_5_0 &&
         (p_input->i_physical_channels & ~AOUT_CHAN_LFE) != AOUT_CHANS_5_0_MIDDLE &&
+        (p_input->i_physical_channels & ~AOUT_CHAN_LFE) != AOUT_CHANS_3_0 &&
          p_input->i_physical_channels != AOUT_CHANS_2_0 )
     {
         return false;