[vlc-devel] [PATCH] Added Central Channel Filter module

[Vedanta Nayak]

Hello, I am an applicant for GSoc and this is my first time sending a
patch on the vlc-devel list, so please excuse any errors.
I have implemented an upmixing filter for stereo audio which makes a
central channel by averaging the left and the right channels and
dividing by a factor of sqrt(2).
---
 modules/audio_filter/Makefile.am |   6 +-
 modules/audio_filter/center.c    | 139 +++++++++++++++++++++++++++++++
 2 files changed, 143 insertions(+), 2 deletions(-)
 create mode 100644 modules/audio_filter/center.c

diff --git a/modules/audio_filter/Makefile.am b/modules/audio_filter/Makefile.am
index 309074c75b..f17630cc2a 100644
--- a/modules/audio_filter/Makefile.am
+++ b/modules/audio_filter/Makefile.am
@@ -33,7 +33,8 @@ libspatializer_plugin_la_SOURCES = \
 	audio_filter/spatializer/revmodel.hpp \
 	audio_filter/spatializer/spatializer.cpp
 libspatializer_plugin_la_LIBADD = $(LIBM)
-
+libcenter_plugin_la_SOURCES = audio_filter/center.c
+libcenter_plugin_la_LIBADD  = $(LIBM)
 audio_filter_LTLIBRARIES = \
 	libaudiobargraph_a_plugin.la \
 	libchorus_flanger_plugin.la \
@@ -46,7 +47,8 @@ audio_filter_LTLIBRARIES = \
 	libscaletempo_plugin.la \
 	libscaletempo_pitch_plugin.la \
 	libspatializer_plugin.la \
-	libstereo_widen_plugin.la
+	libstereo_widen_plugin.la \
+	libcenter_plugin.la
 
 # Channel mixers
 libdolby_surround_decoder_plugin_la_SOURCES = \
diff --git a/modules/audio_filter/center.c b/modules/audio_filter/center.c
new file mode 100644
index 0000000000..07df0e18dc
--- /dev/null
+++ b/modules/audio_filter/center.c
@@ -0,0 +1,139 @@
+/*****************************************************************************
+ * center.c : Central channel filter
+ *****************************************************************************
+ * Copyright © 2020 VLC authors and VideoLAN
+ *
+ * Authors: Vedanta Nayak <vedantnayak2 at gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
+ *****************************************************************************/
+#ifdef HAVE_CONFIG_H
+# include "config.h"
+#endif
+
+#include <vlc_common.h>
+#include <vlc_aout.h>
+#include <vlc_filter.h>
+#include <vlc_plugin.h>
+
+
+/*------------ Declare local functions -----------*/
+static int     Open      (vlc_object_t *);
+static block_t *Process  (filter_t *, block_t *);
+static void    Close     (vlc_object_t *);
+typedef struct
+{
+    int i_left;
+    int i_right;
+    int i_center;
+}sys_t;
+/*------------ Module description ----------------*/
+
+vlc_module_begin()
+    set_shortname (N_("Center"))
+    set_description (N_("Create a central channel"))
+    set_category (CAT_AUDIO)
+    set_subcategory (SUBCAT_AUDIO_AFILTER)
+    set_capability ("audio filter",0)
+    set_callbacks (Open,Close)
+vlc_module_end ()
+
+/*------------ Open function ---------------------*/
+
+static int Open (vlc_object_t *in)
+{
+    int i = 0;
+    int i_offset = 0;
+    filter_t *filter = (filter_t *)in;
+    sys_t *p_sys;
+    if (filter->fmt_in.audio.i_channels != 2)
+    {
+        return VLC_EGENERIC;
+    }
+    p_sys = filter->p_sys = malloc (sizeof(sys_t));
+    if (p_sys == NULL)
+        return VLC_ENOMEM;
+    p_sys->i_left = -1;
+    p_sys->i_right = -1;
+
+    while (pi_vlc_chan_order_wg4[i])
+    {
+        if ( filter->fmt_out.audio.i_physical_channels && pi_vlc_chan_order_wg4[i] )
+        {
+            switch( pi_vlc_chan_order_wg4[i] )
+            {
+                case AOUT_CHAN_LEFT:
+                    p_sys->i_left = i_offset;
+                    break;
+                case AOUT_CHAN_RIGHT:
+                    p_sys->i_right = i_offset;
+                    break;
+                case AOUT_CHAN_CENTER:
+                    p_sys->i_center = i_offset;
+            }
+            ++i_offset;
+        }
+        ++i;
+    }
+    filter->fmt_out.audio.i_format = VLC_CODEC_FL32;
+    filter->fmt_in.audio.i_format = VLC_CODEC_FL32;
+    filter->fmt_out.audio.i_physical_channels = AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT | AOUT_CHAN_CENTER;
+    filter->fmt_out.audio.i_rate = filter->fmt_in.audio.i_rate;
+    aout_FormatPrepare(&filter->fmt_in.audio);
+    aout_FormatPrepare(&filter->fmt_out.audio);
+    filter->pf_audio_filter = Process;
+    return VLC_SUCCESS;
+}
+
+static void Close (vlc_object_t *this)
+{
+    filter_t *temp = (filter_t *)this;
+    free(temp->p_sys);
+}
+
+
+/*---------- Process function --------------------*/
+static block_t *Process ( filter_t *filter, block_t *in_buf )
+{
+    float *in = (float*)in_buf->p_buffer;
+    sys_t * p_sys = filter->p_sys;
+    size_t i_nb_samples = in_buf->i_nb_samples;
+    size_t i_nb_channels = aout_FormatNbChannels(&filter->fmt_out.audio);
+    size_t i;
+    block_t *out_buf = block_Alloc(sizeof(float) * i_nb_samples * i_nb_channels );
+    if ( !out_buf )
+    {
+        block_Release(in_buf);
+        return out_buf;
+    }
+    float * p_out = (float*)out_buf->p_buffer;
+    out_buf->i_nb_samples = i_nb_samples;
+    out_buf->i_dts        = in_buf->i_dts;
+    out_buf->i_pts        = in_buf->i_pts;
+    out_buf->i_length     = sizeof(float) * i_nb_samples;
+    memset(out,0,out_buf->i_buffer);
+    const float factor = .70710678;
+    for ( i = 0 ; i < i_nb_samples ; ++i)
+    {
+        float left = in[i*2];
+        float right = in[i*2+1];
+        float center = ( left + right ) / 2;
+        p_out[i * i_nb_channels + p_sys->i_left  ] = left;
+        p_out[i * i_nb_channels + p_sys->i_right ] = right;
+        p_out[i * i_nb_channels + p_sys->i_center] = center * factor;
+    }
+    block_Release(in_buf);
+    return out_buf;
+}
-- 
2.26.0