codecalpha: Implement alphacombine element

This element will merge video buffers in order to use the alpha stream
luma plane as the alpha of the video stream. The implementation is zero-copy
and currently only support merging I420 stream with an I420, NV12 or GRAY8
alpha stream.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2199>

4 files changed, 637 insertions, 0 deletions

diff --git a/gst/codecalpha/gstalphacombine.c b/gst/codecalpha/gstalphacombine.c
new file mode 100644
index 000000000..8d417b259
--- /dev/null
+++ b/gst/codecalpha/gstalphacombine.c
@@ -0,0 +1,599 @@
+/* GStreamer
+ * Copyright (C) <2021> Collabora Ltd.
+ *   Author: Nicolas Dufresne <nicolas.dufresne@collabora.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+/**
+ * SECTION:element-alphacombine
+ * @title: Alpha Combiner
+ *
+ * This element can combine a Luma plane from one stream as being the alpha
+ * plane of another stream. This element can only work with planar formats
+ * that have an equivalent format with an alpha plane. This is notably used to
+ * combine VP8/VP9 alpha streams from WebM container.
+ *
+ * ## Example launch line
+ * |[
+ * gst-launch-1.0 -v videotestsrc ! .c videotestsrc pattern=ball ! .c
+ *     alphacombine name=c ! compositor ! autovideosink
+ * ]| This pipeline uses luma of a ball test pattern as alpha, combined with
+ * default test pattern and renders the resulting moving ball on a checker
+ * board.
+ *
+ * Since: 1.20
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <gst/video/video.h>
+
+#include "gstalphacombine.h"
+
+
+#define SUPPORTED_SINK_FORMATS "{ I420 }"
+#define SUPPORTED_ALPHA_FORMATS "{ GRAY8, I420, NV12 }"
+#define SUPPORTED_SRC_FORMATS "{ A420 }"
+
+/* *INDENT-OFF* */
+struct {
+  GstVideoFormat sink;
+  GstVideoFormat alpha;
+  GstVideoFormat src;
+} format_map[] = {
+  {
+    .sink = GST_VIDEO_FORMAT_I420,
+    .alpha = GST_VIDEO_FORMAT_I420,
+    .src = GST_VIDEO_FORMAT_A420
+  },{
+    .sink = GST_VIDEO_FORMAT_I420,
+    .alpha = GST_VIDEO_FORMAT_GRAY8,
+    .src = GST_VIDEO_FORMAT_A420
+ },{
+    .sink = GST_VIDEO_FORMAT_I420,
+    .alpha = GST_VIDEO_FORMAT_NV12,
+    .src = GST_VIDEO_FORMAT_A420
+  }
+};
+/* *INDENT-ON* */
+
+GST_DEBUG_CATEGORY_STATIC (alphacombine_debug);
+#define GST_CAT_DEFAULT (alphacombine_debug)
+
+struct _GstAlphaCombine
+{
+  GstElement parent;
+
+  GstPad *sink_pad;
+  GstPad *alpha_pad;
+  GstPad *src_pad;
+
+  /* protected by sink_pad stream lock */
+  GstBuffer *last_alpha_buffer;
+
+  GMutex buffer_lock;
+  GCond buffer_cond;
+  GstBuffer *alpha_buffer;
+  /* Ref-counted flushing state */
+  guint flushing;
+
+  GstVideoInfo sink_vinfo;
+  GstVideoInfo alpha_vinfo;
+  GstVideoFormat src_format;
+
+  gboolean alpha_is_eos;
+};
+
+#define gst_alpha_combine_parent_class parent_class
+G_DEFINE_TYPE_WITH_CODE (GstAlphaCombine, gst_alpha_combine,
+    GST_TYPE_ELEMENT,
+    GST_DEBUG_CATEGORY_INIT (alphacombine_debug, "alphacombine", 0,
+        "Alpha Combiner"));
+
+GST_ELEMENT_REGISTER_DEFINE (alpha_combine, "alphacombine",
+    GST_RANK_NONE, GST_TYPE_ALPHA_COMBINE);
+
+static GstStaticPadTemplate gst_alpha_combine_sink_template =
+GST_STATIC_PAD_TEMPLATE ("sink",
+    GST_PAD_SINK,
+    GST_PAD_ALWAYS,
+    GST_STATIC_CAPS (GST_VIDEO_CAPS_MAKE (SUPPORTED_SINK_FORMATS))
+    );
+
+static GstStaticPadTemplate gst_alpha_combine_alpha_template =
+GST_STATIC_PAD_TEMPLATE ("alpha",
+    GST_PAD_SINK,
+    GST_PAD_ALWAYS,
+    GST_STATIC_CAPS (GST_VIDEO_CAPS_MAKE (SUPPORTED_ALPHA_FORMATS))
+    );
+
+static GstStaticPadTemplate gst_alpha_combine_src_template =
+GST_STATIC_PAD_TEMPLATE ("src",
+    GST_PAD_SRC,
+    GST_PAD_ALWAYS,
+    GST_STATIC_CAPS (GST_VIDEO_CAPS_MAKE (SUPPORTED_SRC_FORMATS))
+    );
+
+static void
+gst_alpha_combine_unlock (GstAlphaCombine * self)
+{
+  g_mutex_lock (&self->buffer_lock);
+  self->flushing++;
+  g_cond_broadcast (&self->buffer_cond);
+  g_mutex_unlock (&self->buffer_lock);
+}
+
+static void
+gst_alpha_combine_unlock_stop (GstAlphaCombine * self)
+{
+  g_mutex_lock (&self->buffer_lock);
+  g_assert (self->flushing);
+  self->flushing--;
+  g_mutex_unlock (&self->buffer_lock);
+}
+
+static void
+gst_alpha_combine_reset (GstAlphaCombine * self)
+{
+  gst_buffer_replace (&self->alpha_buffer, NULL);
+  gst_buffer_replace (&self->last_alpha_buffer, NULL);
+  self->alpha_is_eos = FALSE;
+}
+
+/*
+ * gst_alpha_combine_negotiate:
+ * @self: #GstAlphaCombine pointer
+ *
+ * Verify that the stream and alpha stream format are compatible and fail
+ * otherwise. There is no effort in helping upstream to dynamically negotiate
+ * a valid combination to keep the complexity low, and because this would be a
+ * very atypical usage.
+ */
+static gboolean
+gst_alpha_combine_negotiate (GstAlphaCombine * self)
+{
+  gint i;
+  GstVideoFormat src_format = GST_VIDEO_FORMAT_UNKNOWN;
+  GstVideoFormat sink_format = GST_VIDEO_INFO_FORMAT (&self->sink_vinfo);
+  GstVideoFormat alpha_format = GST_VIDEO_INFO_FORMAT (&self->alpha_vinfo);
+
+  if (self->src_format != GST_VIDEO_FORMAT_UNKNOWN)
+    return TRUE;
+
+  for (i = 0; i < G_N_ELEMENTS (format_map); i++) {
+    if (format_map[i].sink == sink_format
+        && format_map[i].alpha == alpha_format) {
+      src_format = format_map[i].src;
+      break;
+    }
+  }
+
+  if (src_format == GST_VIDEO_FORMAT_UNKNOWN) {
+    GST_ELEMENT_ERROR (self, STREAM, FORMAT, ("Unsupported formats."),
+        ("Cannot combined '%s' and '%s' into any supported transparent format",
+            gst_video_format_to_string (sink_format),
+            gst_video_format_to_string (alpha_format)));
+    return FALSE;
+  }
+
+  if (GST_VIDEO_INFO_COLORIMETRY (&self->sink_vinfo).range !=
+      GST_VIDEO_INFO_COLORIMETRY (&self->alpha_vinfo).range) {
+    GST_ELEMENT_ERROR (self, STREAM, FORMAT, ("Color range miss-match"),
+        ("We can only combine buffers if they have the same color range."));
+    return FALSE;
+  }
+
+  self->src_format = src_format;
+  return TRUE;
+}
+
+static GstFlowReturn
+gst_alpha_combine_pull_alpha_buffer (GstAlphaCombine * self,
+    GstBuffer ** alpha_buffer)
+{
+  g_mutex_lock (&self->buffer_lock);
+
+  while (!self->alpha_buffer && !self->flushing)
+    g_cond_wait (&self->buffer_cond, &self->buffer_lock);
+
+  if (self->flushing) {
+    g_mutex_unlock (&self->buffer_lock);
+    return GST_FLOW_FLUSHING;
+  }
+
+  /* Now is a good time to validate the formats, as the alpha_vinfo won't be
+   * updated until we signal this alpha_buffer_as being consumed */
+  if (!gst_alpha_combine_negotiate (self)) {
+    g_mutex_unlock (&self->buffer_lock);
+    return GST_FLOW_NOT_NEGOTIATED;
+  }
+
+  *alpha_buffer = self->alpha_buffer;
+  self->alpha_buffer = NULL;
+
+  g_cond_broadcast (&self->buffer_cond);
+  g_mutex_unlock (&self->buffer_lock);
+
+  if (GST_BUFFER_FLAG_IS_SET (*alpha_buffer, GST_BUFFER_FLAG_GAP)) {
+    if (!self->last_alpha_buffer) {
+      GST_ELEMENT_ERROR (self, STREAM, WRONG_TYPE,
+          ("Cannot handle streams without an initial alpha buffer."), (NULL));
+      gst_buffer_replace (alpha_buffer, NULL);
+      return GST_FLOW_ERROR;
+    }
+
+    /* Re-use the last alpha buffer if one is gone missing */
+    gst_buffer_replace (alpha_buffer, self->last_alpha_buffer);
+  }
+
+  return GST_FLOW_OK;
+}
+
+static GstFlowReturn
+gst_alpha_combine_push_alpha_buffer (GstAlphaCombine * self, GstBuffer * buffer)
+{
+  g_mutex_lock (&self->buffer_lock);
+
+  /* We wait for the alpha_buffer to be consumed and store the buffer for the
+   * sink_chain to pick it up */
+  while (self->alpha_buffer && !self->flushing)
+    g_cond_wait (&self->buffer_cond, &self->buffer_lock);
+
+  if (self->flushing) {
+    gst_buffer_unref (buffer);
+    g_mutex_unlock (&self->buffer_lock);
+    return GST_FLOW_FLUSHING;
+  }
+
+  self->alpha_buffer = buffer;
+  GST_DEBUG_OBJECT (self, "Stored pending alpha buffer %p", buffer);
+  g_cond_signal (&self->buffer_cond);
+  g_mutex_unlock (&self->buffer_lock);
+
+  return GST_FLOW_OK;
+}
+
+static GstFlowReturn
+gst_alpha_combine_sink_chain (GstPad * pad, GstObject * object,
+    GstBuffer * src_buffer)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (object);
+  GstFlowReturn ret;
+  GstVideoMeta *vmeta;
+  GstBuffer *alpha_buffer;
+  GstMemory *alpha_mem = NULL;
+  gsize alpha_skip = 0;
+  gint alpha_stride;
+  GstBuffer *buffer;
+
+  ret = gst_alpha_combine_pull_alpha_buffer (self, &alpha_buffer);
+  if (ret != GST_FLOW_OK)
+    return ret;
+
+  GST_DEBUG_OBJECT (self, "Combining buffer %p with alpha buffer %p",
+      src_buffer, alpha_buffer);
+
+  vmeta = gst_buffer_get_video_meta (alpha_buffer);
+  if (vmeta) {
+    guint idx, length;
+    if (gst_buffer_find_memory (alpha_buffer, vmeta->offset[GST_VIDEO_COMP_Y],
+            1, &idx, &length, &alpha_skip)) {
+      alpha_mem = gst_buffer_get_memory (alpha_buffer, idx);
+    }
+
+    alpha_stride = vmeta->stride[GST_VIDEO_COMP_Y];
+  } else {
+    alpha_mem = gst_buffer_get_memory (alpha_buffer, 0);
+    alpha_stride = self->alpha_vinfo.stride[GST_VIDEO_COMP_Y];
+  }
+
+  if (!alpha_mem) {
+    gst_buffer_unref (alpha_buffer);
+    gst_buffer_unref (src_buffer);
+    GST_ELEMENT_ERROR (self, STREAM, WRONG_TYPE,
+        ("Invalid alpha video frame."), ("Could not find the plane"));
+    return GST_FLOW_ERROR;
+  }
+
+  /* FIXME use some GstBuffer cache to reduce run-time allocation */
+  buffer = gst_buffer_copy (src_buffer);
+  vmeta = gst_buffer_get_video_meta (buffer);
+  if (!vmeta)
+    vmeta = gst_buffer_add_video_meta (buffer, 0,
+        GST_VIDEO_INFO_FORMAT (&self->sink_vinfo),
+        GST_VIDEO_INFO_WIDTH (&self->sink_vinfo),
+        GST_VIDEO_INFO_HEIGHT (&self->sink_vinfo));
+
+  alpha_skip += gst_buffer_get_size (buffer);
+  gst_buffer_append_memory (buffer, alpha_mem);
+  vmeta->offset[GST_VIDEO_COMP_A] = alpha_skip;
+  vmeta->stride[GST_VIDEO_COMP_A] = alpha_stride;
+  vmeta->format = GST_VIDEO_FORMAT_A420;
+  vmeta->n_planes = 4;
+
+  /* Keep the origina GstBuffer alive to make this buffer pool friendly */
+  gst_buffer_add_parent_buffer_meta (buffer, src_buffer);
+  gst_buffer_add_parent_buffer_meta (buffer, alpha_buffer);
+
+  gst_buffer_replace (&self->last_alpha_buffer, alpha_buffer);
+  gst_buffer_unref (src_buffer);
+  gst_buffer_unref (alpha_buffer);
+
+  return gst_pad_push (self->src_pad, buffer);
+}
+
+static GstFlowReturn
+gst_alpha_combine_alpha_chain (GstPad * pad, GstObject * object,
+    GstBuffer * buffer)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (object);
+
+  if (self->alpha_is_eos) {
+    gst_buffer_unref (buffer);
+    return GST_FLOW_EOS;
+  }
+
+  return gst_alpha_combine_push_alpha_buffer (self, buffer);
+}
+
+static gboolean
+gst_alpha_combine_set_sink_format (GstAlphaCombine * self, GstCaps * caps)
+{
+  GstEvent *event;
+
+  if (!gst_video_info_from_caps (&self->sink_vinfo, caps)) {
+    GST_ELEMENT_ERROR (self, STREAM, FORMAT, ("Invalid video format"), (NULL));
+    return FALSE;
+  }
+
+  caps = gst_caps_copy (caps);
+
+  gst_caps_set_simple (caps, "format", G_TYPE_STRING, "A420", NULL);
+  event = gst_event_new_caps (caps);
+  gst_caps_unref (caps);
+
+  return gst_pad_push_event (self->src_pad, event);
+}
+
+static gboolean
+gst_alpha_combine_set_alpha_format (GstAlphaCombine * self, GstCaps * caps)
+{
+  /* We wait for the alpha_buffer to be consumed, so that we don't pick the
+   * caps too soon */
+  g_mutex_lock (&self->buffer_lock);
+
+  /* We wait for the alpha_buffer to be consumed and store the buffer for the
+   * sink_chain to pick it up */
+  while (self->alpha_buffer && !self->flushing)
+    g_cond_wait (&self->buffer_cond, &self->buffer_lock);
+
+  if (self->flushing) {
+    g_mutex_unlock (&self->buffer_lock);
+    return GST_FLOW_FLUSHING;
+  }
+
+  if (!gst_video_info_from_caps (&self->alpha_vinfo, caps)) {
+    g_mutex_unlock (&self->buffer_lock);
+    GST_ELEMENT_ERROR (self, STREAM, FORMAT, ("Invalid video format"), (NULL));
+    return FALSE;
+  }
+
+  g_mutex_unlock (&self->buffer_lock);
+
+  return TRUE;
+}
+
+static gboolean
+gst_alpha_combine_sink_event (GstPad * pad, GstObject * object,
+    GstEvent * event)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (object);
+
+  switch (event->type) {
+    case GST_EVENT_FLUSH_START:
+      gst_alpha_combine_unlock (self);
+      break;
+    case GST_EVENT_FLUSH_STOP:
+      gst_alpha_combine_unlock_stop (self);
+      break;
+    case GST_EVENT_CAPS:
+    {
+      GstCaps *caps;
+      gboolean ret;
+
+      gst_event_parse_caps (event, &caps);
+      ret = gst_alpha_combine_set_sink_format (self, caps);
+      gst_event_unref (event);
+
+      return ret;
+    }
+    default:
+      break;
+  }
+
+  return gst_pad_event_default (pad, object, event);
+}
+
+static gboolean
+gst_alpha_combine_alpha_event (GstPad * pad, GstObject * object,
+    GstEvent * event)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (object);
+
+  switch (event->type) {
+    case GST_EVENT_FLUSH_START:
+      gst_alpha_combine_unlock (self);
+      break;
+    case GST_EVENT_FLUSH_STOP:
+      gst_alpha_combine_unlock_stop (self);
+      gst_alpha_combine_reset (self);
+      break;
+    case GST_EVENT_CAPS:
+    {
+      GstCaps *caps;
+      gboolean ret;
+
+      gst_event_parse_caps (event, &caps);
+      ret = gst_alpha_combine_set_alpha_format (self, caps);
+      gst_event_unref (event);
+
+      return ret;
+    }
+    case GST_EVENT_SEGMENT:
+      /* Passthrough the segment from the main stream and ignore this one */
+      gst_event_unref (event);
+      return TRUE;
+    case GST_EVENT_EOS:
+      gst_event_unref (event);
+      self->alpha_is_eos = TRUE;
+      return TRUE;
+    default:
+      break;
+  }
+
+  return gst_pad_event_default (pad, object, event);
+}
+
+static gboolean
+gst_alpha_combine_sink_query (GstPad * pad, GstObject * object,
+    GstQuery * query)
+{
+  switch (query->type) {
+    case GST_QUERY_ALLOCATION:
+    {
+      gst_query_add_allocation_meta (query, GST_VIDEO_META_API_TYPE, NULL);
+      return TRUE;
+      break;
+    }
+    default:
+      break;
+  }
+
+  return gst_pad_query_default (pad, object, query);
+}
+
+static GstStateChangeReturn
+gst_alpha_combine_change_state (GstElement * element, GstStateChange transition)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (element);
+  GstStateChangeReturn ret;
+
+  switch (transition) {
+    case GST_STATE_CHANGE_READY_TO_PAUSED:
+      gst_alpha_combine_unlock_stop (self);
+      break;
+    case GST_STATE_CHANGE_PAUSED_TO_READY:
+      gst_alpha_combine_unlock (self);
+      break;
+    default:
+      break;
+  }
+
+  ret = GST_ELEMENT_CLASS (parent_class)->change_state (element, transition);
+
+  switch (transition) {
+    case GST_STATE_CHANGE_PAUSED_TO_READY:
+      gst_alpha_combine_reset (self);
+      self->src_format = GST_VIDEO_FORMAT_UNKNOWN;
+      gst_video_info_init (&self->sink_vinfo);
+      gst_video_info_init (&self->alpha_vinfo);
+      break;
+    default:
+      break;
+  }
+
+  return ret;
+}
+
+static void
+gst_alpha_combine_dispose (GObject * object)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (object);
+
+  g_clear_object (&self->sink_pad);
+  g_clear_object (&self->alpha_pad);
+  g_clear_object (&self->src_pad);
+
+  G_OBJECT_CLASS (parent_class)->dispose (object);
+}
+
+static void
+gst_alpha_combine_finalize (GObject * object)
+{
+  GstAlphaCombine *self = GST_ALPHA_COMBINE (object);
+
+  g_mutex_clear (&self->buffer_lock);
+  g_cond_clear (&self->buffer_cond);
+
+  G_OBJECT_CLASS (parent_class)->finalize (object);
+}
+
+static void
+gst_alpha_combine_class_init (GstAlphaCombineClass * klass)
+{
+  GstElementClass *element_class = (GstElementClass *) klass;
+  GObjectClass *object_class = (GObjectClass *) klass;
+
+  gst_element_class_set_static_metadata (element_class,
+      "Alpha Combiner", "Codec/Demuxer",
+      "Use luma from an opaque stream as alpha plane on another",
+      "Nicolas Dufresne <nicolas.dufresne@collabora.com>");
+
+  gst_element_class_add_static_pad_template (element_class,
+      &gst_alpha_combine_sink_template);
+  gst_element_class_add_static_pad_template (element_class,
+      &gst_alpha_combine_alpha_template);
+  gst_element_class_add_static_pad_template (element_class,
+      &gst_alpha_combine_src_template);
+
+  element_class->change_state =
+      GST_DEBUG_FUNCPTR (gst_alpha_combine_change_state);
+
+  object_class->dispose = GST_DEBUG_FUNCPTR (gst_alpha_combine_dispose);
+  object_class->finalize = GST_DEBUG_FUNCPTR (gst_alpha_combine_finalize);
+}
+
+static void
+gst_alpha_combine_init (GstAlphaCombine * self)
+{
+  gst_element_create_all_pads (GST_ELEMENT (self));
+  self->sink_pad = gst_element_get_static_pad (GST_ELEMENT (self), "sink");
+  self->alpha_pad = gst_element_get_static_pad (GST_ELEMENT (self), "alpha");
+  self->src_pad = gst_element_get_static_pad (GST_ELEMENT (self), "src");
+  self->flushing = 1;
+
+  g_mutex_init (&self->buffer_lock);
+  g_cond_init (&self->buffer_cond);
+
+  GST_PAD_SET_PROXY_SCHEDULING (self->sink_pad);
+  GST_PAD_SET_PROXY_SCHEDULING (self->alpha_pad);
+  GST_PAD_SET_PROXY_SCHEDULING (self->src_pad);
+
+  GST_PAD_SET_PROXY_ALLOCATION (self->sink_pad);
+  GST_PAD_SET_PROXY_ALLOCATION (self->alpha_pad);
+
+  gst_pad_set_chain_function (self->sink_pad, gst_alpha_combine_sink_chain);
+  gst_pad_set_chain_function (self->alpha_pad, gst_alpha_combine_alpha_chain);
+
+  gst_pad_set_event_function (self->sink_pad, gst_alpha_combine_sink_event);
+  gst_pad_set_event_function (self->alpha_pad, gst_alpha_combine_alpha_event);
+
+  gst_pad_set_query_function (self->sink_pad, gst_alpha_combine_sink_query);
+  gst_pad_set_query_function (self->alpha_pad, gst_alpha_combine_sink_query);
+}
diff --git a/gst/codecalpha/gstalphacombine.h b/gst/codecalpha/gstalphacombine.h
new file mode 100644
index 000000000..423717861
--- /dev/null
+++ b/gst/codecalpha/gstalphacombine.h
@@ -0,0 +1,35 @@
+/* GStreamer
+ * Copyright (C) <2021> Collabora Ltd.
+ *   Author: Nicolas Dufresne <nicolas.dufresne@collabora.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __GST_ALPHA_COMBINE_H__
+#define __GST_ALPHA_COMBINE_H__
+
+#include <gst/gst.h>
+
+G_BEGIN_DECLS
+
+#define GST_TYPE_ALPHA_COMBINE (gst_alpha_combine_get_type())
+G_DECLARE_FINAL_TYPE (GstAlphaCombine,
+    gst_alpha_combine, GST, ALPHA_COMBINE, GstElement);
+
+GST_ELEMENT_REGISTER_DECLARE (alpha_combine);
+
+G_END_DECLS
+#endif
diff --git a/gst/codecalpha/gstplugin.c b/gst/codecalpha/gstplugin.c
index 3a92c6a8c..f886932dc 100644
--- a/gst/codecalpha/gstplugin.c
+++ b/gst/codecalpha/gstplugin.c
@@ -25,6 +25,7 @@
 #include <gst/gst.h>
 
 #include "gstcodecalphademux.h"
+#include "gstalphacombine.h"
 
 static gboolean
 plugin_init (GstPlugin * plugin)
@@ -32,6 +33,7 @@ plugin_init (GstPlugin * plugin)
   gboolean ret = FALSE;
 
   ret |= GST_ELEMENT_REGISTER (codec_alpha_demux, plugin);
+  ret |= GST_ELEMENT_REGISTER (alpha_combine, plugin);
 
   return ret;
 }
diff --git a/gst/codecalpha/meson.build b/gst/codecalpha/meson.build
index 1c9d3b34e..9638f3bfe 100644
--- a/gst/codecalpha/meson.build
+++ b/gst/codecalpha/meson.build
@@ -1,5 +1,6 @@
 codecalpha_sources = [
   'gstplugin.c',
+  'gstalphacombine.c',
   'gstcodecalphademux.c',
 ]