Simple GStreamer audio filter test

Compile steps

gcc -Wall `pkg-config --cflags --libs gstreamer-audio-1.0` -fPIC -c plugin.c -o build/passthrough-plugin.o
gcc -Wall `pkg-config --cflags --libs gstreamer-audio-1.0` -fPIC -c passthrough.c -o build/passthrough.o
g++ -shared -Wall `pkg-config --cflags --libs gstreamer-audio-1.0` -lssl -lcrypto -o build/libpassthrough.so build/passthrough.o build/passthrough-plugin.o

passthrough.c

#include <stdio.h>
#include <gst/gst.h>
#include <gst/base/gstbasetransform.h>
#include <gst/audio/audio.h>
#include <gst/audio/gstaudiofilter.h>

#include "passthrough.h"

#define GST_CAT_DEFAULT gst_passthrough_debug
GST_DEBUG_CATEGORY_STATIC(GST_CAT_DEFAULT);

enum
{
  PROP_0,
  PROP_PARAM,
};

#define ALLOWED_CAPS                                            \
  "audio/x-raw, "                                               \
  "format=(string) " GST_AUDIO_NE(F32) ", "                     \
                                       "rate=(int)[8000,MAX], " \
                                       "channels=(int)[1,MAX]"

G_DEFINE_TYPE(GstPassthrough, gst_passthrough, GST_TYPE_AUDIO_FILTER);

static void gst_passthrough_finalize(GObject *object);
static void gst_passthrough_set_property(GObject *object, guint prop_id,
                                      const GValue *value, GParamSpec *pspec);
static void gst_passthrough_get_property(GObject *object, guint prop_id,
                                      GValue *value, GParamSpec *pspec);

static gboolean gst_passthrough_setup(GstAudioFilter *filter,
                                   const GstAudioInfo *info);
static GstFlowReturn gst_passthrough_transform_ip(GstBaseTransform *base,
                                               GstBuffer *buf);

static void gst_passthrough_transform_float(GstPassthrough *filter,
                                         gfloat *data, guint num_samples);

static GstElementClass *parent_class = NULL;

/* GObject vmethod implementations */

static void
gst_passthrough_class_init(GstPassthroughClass *klass)
{
  GObjectClass *gobject_class;
  GstElementClass *gstelement_class;
  GstCaps *caps;

  GST_DEBUG_CATEGORY_INIT(gst_passthrough_debug, "passthrough", 0,
                          "passthrough element");

  gobject_class = (GObjectClass *)klass;
  gstelement_class = (GstElementClass *)klass;
  parent_class = g_type_class_peek_parent(klass);

  gobject_class->finalize = gst_passthrough_finalize;
  gobject_class->set_property = gst_passthrough_set_property;
  gobject_class->get_property = gst_passthrough_get_property;

  g_object_class_install_property(gobject_class, PROP_PARAM,
                                  g_param_spec_float("param", "Whatever param",
                                                     "Ignored at the moment", 0.0, 100.0,
                                                     1.0, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS | GST_PARAM_CONTROLLABLE));

  gst_element_class_set_static_metadata(gstelement_class, "Passthrough",
                                        "Filter/Effect/Audio",
                                        "Passthrough",
                                        "Passthrough");

  caps = gst_caps_from_string(ALLOWED_CAPS);
  gst_audio_filter_class_add_pad_templates(GST_AUDIO_FILTER_CLASS(klass),
                                           caps);
  gst_caps_unref(caps);

  GST_BASE_TRANSFORM_CLASS(klass)->transform_ip =
      GST_DEBUG_FUNCPTR(gst_passthrough_transform_ip);
  GST_BASE_TRANSFORM_CLASS(klass)->transform_ip_on_passthrough = FALSE;

  GST_AUDIO_FILTER_CLASS(klass)->setup =
      GST_DEBUG_FUNCPTR(gst_passthrough_setup);
}

static void
gst_passthrough_init(GstPassthrough *filter)
{
  printf("Init plugin\n");
  filter->param = 1.0;
  gst_base_transform_set_in_place(GST_BASE_TRANSFORM(filter), TRUE);
  gst_base_transform_set_gap_aware(GST_BASE_TRANSFORM(filter), TRUE);
}

static void
gst_passthrough_finalize(GObject *object)
{
  printf("Finalize plugin\n");

  G_OBJECT_CLASS(parent_class)->finalize(object);
}

static void
gst_passthrough_set_property(GObject *object, guint prop_id,
                          const GValue *value, GParamSpec *pspec)
{
  GstPassthrough *filter = GST_PASSTHROUGH(object);

  switch (prop_id)
  {
  case PROP_PARAM:
    filter->param = g_value_get_float(value);
    gst_base_transform_set_passthrough(GST_BASE_TRANSFORM(filter), filter->param == 1.0);
    break;
  default:
    G_OBJECT_WARN_INVALID_PROPERTY_ID(object, prop_id, pspec);
    break;
  }
}

static void
gst_passthrough_get_property(GObject *object, guint prop_id,
                          GValue *value, GParamSpec *pspec)
{
  GstPassthrough *filter = GST_PASSTHROUGH(object);

  switch (prop_id)
  {
  case PROP_PARAM:
    g_value_set_float(value, filter->param);
    break;
  default:
    G_OBJECT_WARN_INVALID_PROPERTY_ID(object, prop_id, pspec);
    break;
  }
}

/* GstAudioFilter vmethod implementations */

static gboolean
gst_passthrough_setup(GstAudioFilter *base, const GstAudioInfo *info)
{
  GstPassthrough *filter = GST_PASSTHROUGH(base);
  gboolean ret = TRUE;

  switch (GST_AUDIO_INFO_FORMAT(info))
  {
  case GST_AUDIO_FORMAT_F32:
    filter->process = (GstPassthroughProcessFunc) gst_passthrough_transform_float;
    break;
  default:
    ret = FALSE;
    break;
  }
  return ret;
}

static void
gst_passthrough_transform_float(GstPassthrough *filter,
                             gfloat *data, guint num_samples)
{
  // -> may do something here to implement an audio effect
  // gfloat *copy = malloc(num_samples * sizeof(*copy));
  // memcpy(copy, data, sizeof(*copy) * num_samples);
  // free(copy);
}

/* GstBaseTransform vmethod implementations */
static GstFlowReturn
gst_passthrough_transform_ip(GstBaseTransform *base, GstBuffer *buf)
{
  GstPassthrough *filter = GST_PASSTHROUGH(base);
  guint num_samples;
  GstClockTime timestamp, stream_time;
  GstMapInfo map;

  timestamp = GST_BUFFER_TIMESTAMP(buf);
  stream_time =
      gst_segment_to_stream_time(&base->segment, GST_FORMAT_TIME, timestamp);

  GST_DEBUG_OBJECT(filter, "sync to %" GST_TIME_FORMAT,
                   GST_TIME_ARGS(timestamp));

  if (GST_CLOCK_TIME_IS_VALID(stream_time))
    gst_object_sync_values(GST_OBJECT(filter), stream_time);

  if (G_UNLIKELY(GST_BUFFER_FLAG_IS_SET(buf, GST_BUFFER_FLAG_GAP)))
    return GST_FLOW_OK;

  gst_buffer_map(buf, &map, GST_MAP_READWRITE);
  num_samples = map.size / GST_AUDIO_FILTER_BPS(filter);

  filter->process(filter, map.data, num_samples);

  gst_buffer_unmap(buf, &map);

  return GST_FLOW_OK;
}

passthrough.h

#ifndef __GST_PASSTHROUGH_H__
#define __GST_PASSTHROUGH_H__

#include <gst/gst.h>
#include <gst/base/gstbasetransform.h>
#include <gst/audio/audio.h>
#include <gst/audio/gstaudiofilter.h>

G_BEGIN_DECLS
#define GST_TYPE_PASSTHROUGH (gst_passthrough_get_type())
#define GST_PASSTHROUGH(obj) (G_TYPE_CHECK_INSTANCE_CAST((obj), GST_TYPE_PASSTHROUGH, GstPassthrough))
#define GST_IS_PASSTHROUGH(obj) (G_TYPE_CHECK_INSTANCE_TYPE((obj), GST_TYPE_PASSTHROUGH))
#define GST_PASSTHROUGH_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST((klass), GST_TYPE_PASSTHROUGH, GstPassthroughClass))
#define GST_IS_PASSTHROUGH_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE((klass), GST_TYPE_PASSTHROUGH))
#define GST_PASSTHROUGH_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS((obj), GST_TYPE_PASSTHROUGH, GstPassthroughClass))
typedef struct _GstPassthrough GstPassthrough;
typedef struct _GstPassthroughClass GstPassthroughClass;

typedef void (*GstPassthroughProcessFunc)(GstPassthrough *, guint8 *, guint);

struct _GstPassthrough
{
  GstAudioFilter audiofilter;

  gfloat param;

  /* < private > */
  GstPassthroughProcessFunc process;
};

struct _GstPassthroughClass
{
  GstAudioFilterClass parent;
};

GType gst_passthrough_get_type(void);

G_END_DECLS
#endif /* __GST_PASSTHROUGH_H__ */

plugin.c

#include <gst/gst.h>

#include "passthrough.h"

static gboolean
plugin_init (GstPlugin * plugin)
{
  gboolean ret = FALSE;

  ret |= gst_element_register (plugin, "passthrough", GST_RANK_NONE, GST_TYPE_PASSTHROUGH);

  return ret;
}

#ifndef VERSION
#define VERSION "0.1"
#endif
#ifndef PACKAGE
#define PACKAGE "passthrough"
#endif
#ifndef PACKAGE_NAME
#define PACKAGE_NAME "passthrough"
#endif
#ifndef GST_PACKAGE_ORIGIN
#define GST_PACKAGE_ORIGIN "passthrough"
#endif

GST_PLUGIN_DEFINE (GST_VERSION_MAJOR,
    GST_VERSION_MINOR,
    passthrough,
    "passthrough",
    plugin_init, VERSION, "LGPL", PACKAGE_NAME, GST_PACKAGE_ORIGIN)