codec: Introduce GstVideoCodecAlphaMeta

This meta hold one buffer of the same codec data as the parent memory. This
extra frame luma will be used as the alpha values for the final combined
frame. This is notably used to support VP8/VP9 alpha as defined in WebM and
matroska specification.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1128>

gst-libs/gst/video/gstvideocodecalphameta.c

@@ -0,0 +1,155 @@
+/* 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.
+ */
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "gstvideocodecalphameta.h"
+
+/**
+ * SECTION:gstvideocodecalphameta
+ * @title: GstVideoCodecAlphaMeta
+ * @short_description: GstMeta that can carry an extra buffer holding  an
+ * encoded a frame whith luma that can be used as an alpha channel.
+ *
+ * This meta is primarily for internal use in GStreamer elements to support
+ * VP8/VP9 transparent video stored into WebM or Matroska containers, or
+ * transparent static AV1 images. Nothing prevents you from using this meta
+ * for custom purposes, but it generally can't be used to easily to add support
+ * for alpha channels to CODECs or formats that don't support that out of the
+ * box.
+ *
+ * Since: 1.20
+ */
+
+/**
+ * gst_video_codec_alpha_meta_api_get_type:
+ *
+ * Returns: #GType for the #GstVideoCodecAlphaMeta structure.
+ *
+ * Since: 1.20
+ */
+GType
+gst_video_codec_alpha_meta_api_get_type (void)
+{
+  static GType type = 0;
+  static const gchar *tags[] = { GST_META_TAG_VIDEO_STR, NULL };
+
+  if (g_once_init_enter (&type)) {
+    GType _type =
+        gst_meta_api_type_register ("GstVideoCodecAlphaMetaAPI", tags);
+    g_once_init_leave (&type, _type);
+  }
+  return type;
+}
+
+static gboolean
+gst_video_codec_alpha_meta_transform (GstBuffer * dest,
+    GstMeta * meta, GstBuffer * buffer, GQuark type, gpointer data)
+{
+  GstVideoCodecAlphaMeta *dmeta, *smeta;
+
+  smeta = (GstVideoCodecAlphaMeta *) meta;
+
+  if (GST_META_TRANSFORM_IS_COPY (type)) {
+    dmeta =
+        (GstVideoCodecAlphaMeta *) gst_buffer_add_meta (dest,
+        GST_VIDEO_CODEC_ALPHA_META_INFO, NULL);
+
+    if (!dmeta)
+      return FALSE;
+
+    dmeta->buffer = gst_buffer_ref (smeta->buffer);
+  }
+  return TRUE;
+}
+
+static gboolean
+gst_video_codec_alpha_meta_init (GstMeta * meta, gpointer params,
+    GstBuffer * buffer)
+{
+  GstVideoCodecAlphaMeta *ca_meta = (GstVideoCodecAlphaMeta *) meta;
+
+  /* the buffer ownership is transfered to the Meta */
+  ca_meta->buffer = (GstBuffer *) params;
+
+  return TRUE;
+}
+
+static void
+gst_video_codec_alpha_meta_free (GstMeta * meta, GstBuffer * buffer)
+{
+  GstVideoCodecAlphaMeta *ca_meta = (GstVideoCodecAlphaMeta *) meta;
+  gst_clear_buffer (&ca_meta->buffer);
+}
+
+/**
+ * gst_video_codec_alpha_meta_get_info:
+ *
+ * Returns: #GstMetaInfo pointer that describes #GstVideoCodecAlphaMeta.
+ *
+ * Since: 1.20
+ */
+const GstMetaInfo *
+gst_video_codec_alpha_meta_get_info (void)
+{
+  static const GstMetaInfo *info = NULL;
+
+  if (g_once_init_enter ((GstMetaInfo **) & info)) {
+    const GstMetaInfo *meta =
+        gst_meta_register (GST_VIDEO_CODEC_ALPHA_META_API_TYPE,
+        "GstVideoCodecAlphaMeta",
+        sizeof (GstVideoCodecAlphaMeta),
+        gst_video_codec_alpha_meta_init,
+        gst_video_codec_alpha_meta_free,
+        gst_video_codec_alpha_meta_transform);
+    g_once_init_leave ((GstMetaInfo **) & info, (GstMetaInfo *) meta);
+  }
+
+  return info;
+}
+
+/**
+ * gst_buffer_add_video_codec_alpha_meta:
+ * @buffer: (transfer none): a #GstBuffer
+ * @alpha_buffer: (transfer full): a #GstBuffer
+ *
+ * Attaches a #GstVideoCodecAlphaMeta metadata to @buffer with
+ * the given alpha buffer.
+ *
+ * Returns: (transfer none): the #GstVideoCodecAlphaMeta on @buffer.
+ *
+ * Since: 1.20
+ */
+GstVideoCodecAlphaMeta *
+gst_buffer_add_video_codec_alpha_meta (GstBuffer * buffer,
+    GstBuffer * alpha_buffer)
+{
+  GstVideoCodecAlphaMeta *meta;
+
+  g_return_val_if_fail (buffer != NULL, NULL);
+  g_return_val_if_fail (alpha_buffer != NULL, NULL);
+
+  meta =
+      (GstVideoCodecAlphaMeta *) gst_buffer_add_meta (buffer,
+      GST_VIDEO_CODEC_ALPHA_META_INFO, alpha_buffer);
+
+  return meta;
+}

gst-libs/gst/video/gstvideocodecalphameta.h

@@ -0,0 +1,88 @@
+/* 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_VIDEO_CODEC_ALPHA_META_H__
+#define __GST_VIDEO_CODEC_ALPHA_META_H__
+
+#include <gst/gst.h>
+#include <gst/video/video.h>
+
+G_BEGIN_DECLS
+
+/**
+ * GST_VIDEO_CODEC_ALPHA_META_API_TYPE:
+ *
+ * Since: 1.20
+ */
+#define GST_VIDEO_CODEC_ALPHA_META_API_TYPE (gst_video_codec_alpha_meta_api_get_type())
+
+/**
+ * GST_VIDEO_CODEC_ALPHA_META_INFO:
+ *
+ * Since: 1.20
+ */
+#define GST_VIDEO_CODEC_ALPHA_META_INFO  (gst_video_codec_alpha_meta_get_info())
+
+typedef struct _GstVideoCodecAlphaMeta GstVideoCodecAlphaMeta;
+
+/**
+ * GstVideoCodecAlphaMeta:
+ * @meta: parent #GstMeta
+ * @buffer: the encoded alpha frame
+ *
+ * Encapsulate an extra frame containing the encoded alpha channel for the
+ * currently negotiated CODEC. The streams must be of the same dimention as
+ * the original one.
+ *
+ * Since: 1.20
+ */
+struct _GstVideoCodecAlphaMeta
+{
+  GstMeta meta;
+
+  GstBuffer *buffer;
+};
+
+GST_VIDEO_API
+GType gst_video_codec_alpha_meta_api_get_type          (void);
+
+GST_VIDEO_API
+const GstMetaInfo *gst_video_codec_alpha_meta_get_info (void);
+
+/**
+ * gst_buffer_get_video_codec_alpha_meta:
+ * @b: A #GstBuffer pointer, must be writable.
+ *
+ * Helper macro to get #GstVideoCodecAlphaMeta from an existing #GstBuffer.
+ *
+ * Returns: (nullable): the #GstVideoCodecAlphaMeta pointer, or %NULL if none.
+ *
+ * Since: 1.20
+ */
+#define gst_buffer_get_video_codec_alpha_meta(b) \
+    ((GstVideoCodecAlphaMeta *)gst_buffer_get_meta((b),GST_VIDEO_CODEC_ALPHA_META_API_TYPE))
+
+GST_VIDEO_API
+GstVideoCodecAlphaMeta *gst_buffer_add_video_codec_alpha_meta (GstBuffer * buffer,
+                                                               GstBuffer * alpha_buffer);
+
+G_END_DECLS
+
+#endif /* __GST_VIDEO_CODEC_ALPHA_META_H__ */

gst-libs/gst/video/meson.build

@@ -3,6 +3,7 @@ video_sources = [
   'colorbalancechannel.c',
   'convertframe.c',
   'gstvideoaffinetransformationmeta.c',
+  'gstvideocodecalphameta.c',
   'gstvideoaggregator.c',
   'gstvideodecoder.c',
   'gstvideoencoder.c',
@@ -40,6 +41,7 @@ video_headers = [
   'colorbalance.h',
   'colorbalancechannel.h',
   'gstvideoaffinetransformationmeta.h',
+  'gstvideocodecalphameta.h',
   'gstvideoaggregator.h',
   'gstvideodecoder.h',
   'gstvideoencoder.h',

gst-libs/gst/video/video.h

@@ -167,6 +167,7 @@ G_END_DECLS
 #include <gst/video/colorbalance.h>
 #include <gst/video/gstvideoaffinetransformationmeta.h>
 #include <gst/video/gstvideoaggregator.h>
+#include <gst/video/gstvideocodecalphameta.h>
 #include <gst/video/gstvideodecoder.h>
 #include <gst/video/gstvideoencoder.h>
 #include <gst/video/gstvideofilter.h>