[vlc-devel] [PATCH v4] Central Channel Upmixing Filter

[Thomas Guillem]

On Wed, Apr 15, 2020, at 17:54, Vedanta Nayak wrote:
> Hello,
> I understood what I needed to do. I have added a check for stereo audio
> that isn't comprised for left+right.

Yes indeed,

> ---
>  modules/audio_filter/Makefile.am |  5 +-
>  modules/audio_filter/center.c    | 91 ++++++++++++++++++++++++++++++++
>  2 files changed, 95 insertions(+), 1 deletion(-)
>  create mode 100644 modules/audio_filter/center.c
> 
> diff --git a/modules/audio_filter/Makefile.am b/modules/audio_filter/Makefile.am
> index 309074c75b..279be02040 100644
> --- a/modules/audio_filter/Makefile.am
> +++ b/modules/audio_filter/Makefile.am
> @@ -33,6 +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 \
> @@ -46,7 +48,8 @@ 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..80b77f2343
> --- /dev/null
> +++ b/modules/audio_filter/center.c
> @@ -0,0 +1,91 @@
> +/*****************************************************************************
> + * 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>
> +
> +#define I_NB_CHANNELS 3
> +
> +static block_t *Process ( filter_t *filter, block_t *in_buf )
> +{
> +    float *in = (float*)in_buf->p_buffer;
> +    size_t i_nb_samples = in_buf->i_nb_samples;
> +    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;
> +    const float factor = .70710678;
> +    for ( size_t i = 0 ; i < i_nb_samples ; ++i)
> +    {
> +        float left = in[i*2];
> +        float right = in[i*2+1];
> +        float center = ( left + right ) * factor / 2;
> +        p_out[i * I_NB_CHANNELS   ] = left;
> +        p_out[i * I_NB_CHANNELS + 1 ] = right;
> +        p_out[i * I_NB_CHANNELS + 2 ] = center;
> +    }
> +    block_Release(in_buf);
> +    return out_buf;
> +}
> +
> +static int Open (vlc_object_t *in)
> +{
> +    filter_t *filter = (filter_t *)in;
> +    if (filter->fmt_in.audio.i_channels != 2 || 
> +        filter->fmt_in.audio.i_physical_channels != AOUT_CHAN_LEFT 
> +        | AOUT_CHAN_RIGHT)

You could use AOUT_CHANS_STEREO instead.

> +    {
> +        return VLC_EGENERIC;
> +    }
> +    static_assert(AOUT_CHANIDX_CENTER > AOUT_CHANIDX_RIGHT &&
> +        AOUT_CHANIDX_RIGHT > AOUT_CHANIDX_LEFT, "Change in channel 
> order.");
> +    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;
> +}
> +
> +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_callback (Open)
> +vlc_module_end ()
> +
> -- 
> 2.26.0
> 
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