/*
 * GStreamer
 * Copyright (C) 2022 Matthew Waters <matthew@centricular.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-qml6gloverlay
 *
 * `qml6gloverlay` provides a way to render an almost-arbitrary QML scene within
 * GStreamer pipeline using the same OpenGL context that GStreamer uses
 * internally.  This avoids attempting to share multiple OpenGL contexts
 * avoiding increased synchronisation points and attempting to share an OpenGL
 * context at runtime which some drivers do not like.  The Intel driver on
 * Windows is a notable example of the last point.
 *
 * `qml6gloverlay` will attempt to retrieve the windowing system display connection
 * that Qt is using (#GstGLDisplay).  This may be different to any already
 * existing window system display connection already in use in the pipeline for
 * a number of reasons.  A couple of examples of this are:
 *
 * 1. Adding `qml6gloverlay` to an already running pipeline
 * 2. Not having any `qml6gloverlay` (or `qml6glsink`, or `qml6glmixer`) element
 *    start up before any other OpenGL-based element in the pipeline.
 *
 * If one of these scenarios occurs, then there will be multiple OpenGL contexts
 * in use in the pipeline.  This means that either the pipeline will fail to
 * start up correctly, a downstream element may reject buffers, or a complete
 * GPU->System memory->GPU transfer is performed for every buffer.
 *
 * The requirement to avoid this is that all elements share the same
 * #GstGLDisplay object and as Qt cannot currently share an existing window
 * system display connection, GStreamer must use the window system display
 * connection provided by Qt.  This window system display connection can be
 * retrieved by either a `qml6glsink` element, a `qml6gloverlay`, or a
 * `qml6glmixer` element. The recommended usage is to have either element
 * (`qml6glsink` or `qml6gloverlay` or `qml6glmixer`) be the first to propagate
 * the #GstGLDisplay for the entire pipeline to use by setting either element
 * to the READY element state before any other OpenGL element in the pipeline.
 *
 * In the dynamically adding `qml6gloverlay` (or `qml6glsink`, or `qml6glmixer`)
 * to a pipeline case, there are some considerations for ensuring that the
 * window system display and OpenGL contexts are compatible with Qt.  When the
 * `qml6gloverlay` (or `qml6glsink`, or `qml6glmixer`) element is added and
 * brought up to READY, it will propagate it's own #GstGLDisplay using the
 * #GstContext mechanism regardless of any existing #GstGLDisplay used by the
 * pipeline previously.  In order for the new #GstGLDisplay to be used, the
 * application must then set the provided #GstGLDisplay containing #GstContext
 * on the pipeline.  This may effectively cause each OpenGL element to replace
 * the window system display and also the OpenGL context it is using.  As such
 * this process may take a significant amount of time and resources as objects
 * are recreated in the new OpenGL context.
 *
 * All instances of `qml6gloverlay`, `qml6glsink`, and `qml6glmixer` will
 * return the exact same #GstGLDisplay object while the pipeline is running
 * regardless of whether any `qml6gloverlay` or `qml6glsink` elements are
 * added or removed from the pipeline.
 *
 * The Qml scene will run at the pace of incoming buffers.  One input buffer
 * will cause a render of one output buffer.  The timestamps on the input
 * buffers are used to drive the animation time.
 *
 * Since: 1.24
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "gstqt6elements.h"
#include "gstqml6gloverlay.h"
#include "qt6glrenderer.h"
#include "gstqt6glutility.h"

#include <QtGui/QGuiApplication>

#include <gst/gl/gstglfuncs.h>

#define GST_CAT_DEFAULT gst_debug_qml6_gl_overlay
GST_DEBUG_CATEGORY (GST_CAT_DEFAULT);

/* *INDENT-OFF* */
static GstStaticPadTemplate qml6_overlay_src_pad_template =
GST_STATIC_PAD_TEMPLATE ("src",
    GST_PAD_SRC,
    GST_PAD_ALWAYS,
    GST_STATIC_CAPS ("video/x-raw(" GST_CAPS_FEATURE_MEMORY_GL_MEMORY "), "
      "format = (string) RGBA, "
      "width = " GST_VIDEO_SIZE_RANGE ", "
      "height = " GST_VIDEO_SIZE_RANGE ", "
      "framerate = " GST_VIDEO_FPS_RANGE ","
      "texture-target = (string) 2D"
    ));

static GstStaticPadTemplate qml6_overlay_sink_pad_template =
GST_STATIC_PAD_TEMPLATE ("sink",
    GST_PAD_SINK,
    GST_PAD_ALWAYS,
    GST_STATIC_CAPS ("video/x-raw(ANY), "
      "format = (string) { RGBA, BGRA, YV12 }, "
      "width = " GST_VIDEO_SIZE_RANGE ", "
      "height = " GST_VIDEO_SIZE_RANGE ", "
      "framerate = " GST_VIDEO_FPS_RANGE ","
      "texture-target = (string) 2D"
    ));
/* *INDENT-ON* */

static void gst_qml6_gl_overlay_finalize (GObject * object);
static void gst_qml6_gl_overlay_set_property (GObject * object, guint prop_id,
    const GValue * value, GParamSpec * param_spec);
static void gst_qml6_gl_overlay_get_property (GObject * object, guint prop_id,
    GValue * value, GParamSpec * param_spec);

static GstCaps * gst_qml6_overlay_transform_internal_caps (GstGLFilter * filter,
    GstPadDirection direction, GstCaps * caps, GstCaps * filter_caps);

static gboolean gst_qml6_gl_overlay_gl_start (GstGLBaseFilter * bfilter);
static void gst_qml6_gl_overlay_gl_stop (GstGLBaseFilter * bfilter);
static gboolean gst_qml6_gl_overlay_gl_set_caps (GstGLBaseFilter * bfilter,
    GstCaps * in_caps, GstCaps * out_caps);

static GstFlowReturn gst_qml6_gl_overlay_prepare_output_buffer (GstBaseTransform * btrans,
    GstBuffer * buffer, GstBuffer ** outbuf);
static GstFlowReturn gst_qml6_gl_overlay_transform (GstBaseTransform * btrans,
    GstBuffer * inbuf, GstBuffer * outbuf);

static GstStateChangeReturn gst_qml6_gl_overlay_change_state (GstElement * element,
    GstStateChange transition);

enum
{
  PROP_0,
  PROP_WIDGET,
  PROP_QML_SCENE,
  PROP_ROOT_ITEM,
};

enum
{
  SIGNAL_0,
  SIGNAL_QML_SCENE_INITIALIZED,
  SIGNAL_QML_SCENE_DESTROYED,
  LAST_SIGNAL
};

static guint gst_qml6_gl_overlay_signals[LAST_SIGNAL] = { 0 };

#define gst_qml6_gl_overlay_parent_class parent_class
G_DEFINE_TYPE_WITH_CODE (GstQml6GLOverlay, gst_qml6_gl_overlay,
    GST_TYPE_GL_FILTER, GST_DEBUG_CATEGORY_INIT (GST_CAT_DEFAULT,
        "qml6gloverlay", 0, "Qt6 Video Overlay"));
GST_ELEMENT_REGISTER_DEFINE_WITH_CODE (qml6gloverlay, "qml6gloverlay",
    GST_RANK_NONE, GST_TYPE_QML6_GL_OVERLAY, qt6_element_init (plugin));

static void
gst_qml6_gl_overlay_class_init (GstQml6GLOverlayClass * klass)
{
  GObjectClass *gobject_class;
  GstElementClass *gstelement_class;
  GstBaseTransformClass *btrans_class;
  GstGLBaseFilterClass *glbasefilter_class;
  GstGLFilterClass *glfilter_class;
  GstElementClass *element_class;

  gobject_class = (GObjectClass *) klass;
  gstelement_class = (GstElementClass *) klass;
  glbasefilter_class = (GstGLBaseFilterClass *) klass;
  glfilter_class = (GstGLFilterClass *) klass;
  btrans_class = (GstBaseTransformClass *) klass;
  element_class = (GstElementClass *) klass;

  gobject_class->set_property = gst_qml6_gl_overlay_set_property;
  gobject_class->get_property = gst_qml6_gl_overlay_get_property;
  gobject_class->finalize = gst_qml6_gl_overlay_finalize;

  gst_element_class_set_static_metadata (gstelement_class, "Qt Video Overlay",
      "Filter/QML/Overlay", "A filter that renders a QML scene onto a video stream",
      "Matthew Waters <matthew@centricular.com>");

  g_object_class_install_property (gobject_class, PROP_QML_SCENE,
      g_param_spec_string ("qml-scene", "QML Scene",
          "The contents of the QML scene", NULL,
          (GParamFlags) (G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS)));

  g_object_class_install_property (gobject_class, PROP_WIDGET,
      g_param_spec_pointer ("widget", "QQuickItem",
          "The QQuickItem to place the input video in the object hierarchy",
          (GParamFlags) (G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS)));

  g_object_class_install_property (gobject_class, PROP_ROOT_ITEM,
      g_param_spec_pointer ("root-item", "QQuickItem",
          "On set, provides the QML scene to render. If not set (!= NULL), "
          "then the qml-scene property is used to construct the QML scene. "
          "In both cases, the returned value is the root QQuickItem used for rendering.",
          (GParamFlags) (G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS)));

  /**
   * GstQmlGLOverlay::qml-scene-initialized
   * @element: the #GstQmlGLOverlay
   * @user_data: user provided data
   */
  gst_qml6_gl_overlay_signals[SIGNAL_QML_SCENE_INITIALIZED] =
      g_signal_new ("qml-scene-initialized", G_TYPE_FROM_CLASS (klass),
      G_SIGNAL_RUN_LAST, 0, NULL, NULL, NULL, G_TYPE_NONE, 0);

  /**
   * GstQmlGLOverlay::qml-scene-destroyed
   * @element: the #GstQmlGLOverlay
   * @user_data: user provided data
   */
  gst_qml6_gl_overlay_signals[SIGNAL_QML_SCENE_DESTROYED] =
      g_signal_new ("qml-scene-destroyed", G_TYPE_FROM_CLASS (klass),
      G_SIGNAL_RUN_LAST, 0, NULL, NULL, NULL, G_TYPE_NONE, 0);

  gst_element_class_add_static_pad_template (element_class,
      &qml6_overlay_src_pad_template);
  gst_element_class_add_static_pad_template (element_class,
      &qml6_overlay_sink_pad_template);

  btrans_class->prepare_output_buffer = gst_qml6_gl_overlay_prepare_output_buffer;
  btrans_class->transform = gst_qml6_gl_overlay_transform;

  glfilter_class->transform_internal_caps = gst_qml6_overlay_transform_internal_caps;

  glbasefilter_class->gl_start = gst_qml6_gl_overlay_gl_start;
  glbasefilter_class->gl_stop = gst_qml6_gl_overlay_gl_stop;
  glbasefilter_class->gl_set_caps = gst_qml6_gl_overlay_gl_set_caps;

  element_class->change_state = gst_qml6_gl_overlay_change_state;
}

static void
gst_qml6_gl_overlay_init (GstQml6GLOverlay * qml6_gl_overlay)
{
  qml6_gl_overlay->widget = QSharedPointer<Qt6GLVideoItemInterface>();
  qml6_gl_overlay->qml_scene = NULL;
  qml6_gl_overlay->renderer = new GstQt6QuickRenderer;
}

static void
gst_qml6_gl_overlay_finalize (GObject * object)
{
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (object);

  g_free (qml6_gl_overlay->qml_scene);
  qml6_gl_overlay->qml_scene = NULL;

  qml6_gl_overlay->widget.clear();

  delete qml6_gl_overlay->renderer;
  qml6_gl_overlay->renderer = NULL;

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

static void
gst_qml6_gl_overlay_set_property (GObject * object, guint prop_id,
    const GValue * value, GParamSpec * pspec)
{
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (object);

  switch (prop_id) {
    case PROP_WIDGET: {
      Qt6GLVideoItem *qt_item = static_cast<Qt6GLVideoItem *> (g_value_get_pointer (value));
      if (qt_item)
        qml6_gl_overlay->widget = qt_item->getInterface();
      else
        qml6_gl_overlay->widget.clear();
      break;
    }
    case PROP_QML_SCENE:
      g_free (qml6_gl_overlay->qml_scene);
      qml6_gl_overlay->qml_scene = g_value_dup_string (value);
      break;
    case PROP_ROOT_ITEM: {
      GST_OBJECT_LOCK (qml6_gl_overlay);
      qml6_gl_overlay->root_item = (QQuickItem *) g_value_get_pointer (value);
      GST_OBJECT_UNLOCK (qml6_gl_overlay);
      break;
    }
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
  }
}

static void
gst_qml6_gl_overlay_get_property (GObject * object, guint prop_id,
    GValue * value, GParamSpec * pspec)
{
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (object);

  switch (prop_id) {
    case PROP_WIDGET:
      /* This is not really safe - the app needs to be
       * sure the widget is going to be kept alive or
       * this can crash */
      if (qml6_gl_overlay->widget)
        g_value_set_pointer (value, qml6_gl_overlay->widget->videoItem());
      else
        g_value_set_pointer (value, NULL);
      break;
    case PROP_QML_SCENE:
      g_value_set_string (value, qml6_gl_overlay->qml_scene);
      break;
    case PROP_ROOT_ITEM: {
      QQuickItem *root;
      GST_OBJECT_LOCK (qml6_gl_overlay);
      root = qml6_gl_overlay->renderer->rootItem();
      if (root)
        g_value_set_pointer (value, root);
      else if (qml6_gl_overlay->root_item)
        g_value_set_pointer (value, qml6_gl_overlay->root_item);
      else
        g_value_set_pointer (value, NULL);
      GST_OBJECT_UNLOCK (qml6_gl_overlay);
      break;
    }
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
  }
}

static gboolean
gst_qml6_gl_overlay_gl_start (GstGLBaseFilter * bfilter)
{
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (bfilter);
  QQuickItem *root;
  GError *error = NULL;
  gboolean have_scene = FALSE;

  GST_TRACE_OBJECT (bfilter, "using scene:\n%s", qml6_gl_overlay->qml_scene);

  have_scene = qml6_gl_overlay->qml_scene && g_strcmp0 (qml6_gl_overlay->qml_scene, "") != 0;
  if (!have_scene && qml6_gl_overlay->root_item == NULL) {
    GST_ELEMENT_ERROR (bfilter, RESOURCE, NOT_FOUND, ("root-item and qml-scene properties not set"), (NULL));
    return FALSE;
  }

  if (!GST_GL_BASE_FILTER_CLASS (parent_class)->gl_start (bfilter))
    return FALSE;

  GST_OBJECT_LOCK (bfilter);
  if (!qml6_gl_overlay->renderer->init (bfilter->context, &error)) {
    GST_ELEMENT_ERROR (GST_ELEMENT (bfilter), RESOURCE, NOT_FOUND,
        ("%s", error->message), (NULL));
    delete qml6_gl_overlay->renderer;
    qml6_gl_overlay->renderer = new GstQt6QuickRenderer;
    GST_OBJECT_UNLOCK (bfilter);
    return FALSE;
  }

  /* FIXME: Qml may do async loading and we need to propagate qml errors in that case as well */
  if (have_scene) {
    if (!qml6_gl_overlay->renderer->setQmlScene (qml6_gl_overlay->qml_scene, &error)) {
      GST_ELEMENT_ERROR (GST_ELEMENT (bfilter), RESOURCE, NOT_FOUND,
          ("%s", error->message), (NULL));
      goto fail_renderer;
    }

    root = qml6_gl_overlay->renderer->rootItem();
    if (!root) {
      GST_ELEMENT_ERROR (GST_ELEMENT (bfilter), RESOURCE, NOT_FOUND,
          ("Qml scene does not have a root item"), (NULL));
      goto fail_renderer;
    }
    GST_OBJECT_UNLOCK (bfilter);

    g_object_notify (G_OBJECT (qml6_gl_overlay), "root-item");
  } else {
    root = qml6_gl_overlay->root_item;
    if (!root) {
      GST_ELEMENT_ERROR (GST_ELEMENT (bfilter), RESOURCE, NOT_FOUND,
          ("No root item provided"), (NULL));
      goto fail_renderer;
    }
    qml6_gl_overlay->renderer->setRootItem(root);
    GST_OBJECT_UNLOCK (bfilter);
  }

  g_signal_emit (qml6_gl_overlay, gst_qml6_gl_overlay_signals[SIGNAL_QML_SCENE_INITIALIZED], 0);

  GST_OBJECT_LOCK (bfilter);
  if (!qml6_gl_overlay->widget) {
    Qt6GLVideoItem *qt_item = static_cast<Qt6GLVideoItem *>(root->findChild<Qt6GLVideoItem *> ());
    if (qt_item)
      qml6_gl_overlay->widget = qt_item->getInterface();
  }
  GST_OBJECT_UNLOCK (bfilter);

  return TRUE;

fail_renderer:
  {
    qml6_gl_overlay->renderer->cleanup();
    delete qml6_gl_overlay->renderer;
    qml6_gl_overlay->renderer = new GstQt6QuickRenderer;
    GST_OBJECT_UNLOCK (bfilter);
    return FALSE;
  }
}

static void
gst_qml6_gl_overlay_gl_stop (GstGLBaseFilter * bfilter)
{
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (bfilter);
  GstQt6QuickRenderer *renderer = NULL;

  /* notify before actually destroying anything */
  GST_OBJECT_LOCK (qml6_gl_overlay);
  if (qml6_gl_overlay->renderer) {
    renderer = qml6_gl_overlay->renderer;
    qml6_gl_overlay->renderer = new GstQt6QuickRenderer;
  }
  GST_OBJECT_UNLOCK (qml6_gl_overlay);

  g_signal_emit (qml6_gl_overlay, gst_qml6_gl_overlay_signals[SIGNAL_QML_SCENE_DESTROYED], 0);
  g_object_notify (G_OBJECT (qml6_gl_overlay), "root-item");

  if (qml6_gl_overlay->widget)
    qml6_gl_overlay->widget->setBuffer (NULL);

  if (renderer) {
    renderer->cleanup();
    delete renderer;
  }

  GST_GL_BASE_FILTER_CLASS (parent_class)->gl_stop (bfilter);
}

static gboolean
gst_qml6_gl_overlay_gl_set_caps (GstGLBaseFilter * bfilter, GstCaps * in_caps,
    GstCaps * out_caps)
{
  GstGLFilter *filter = GST_GL_FILTER (bfilter);
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (bfilter);

  if (!GST_GL_BASE_FILTER_CLASS (parent_class)->gl_set_caps (bfilter, in_caps, out_caps))
    return FALSE;

  qml6_gl_overlay->renderer->setSize (GST_VIDEO_INFO_WIDTH (&filter->out_info),
      GST_VIDEO_INFO_HEIGHT (&filter->out_info));

  return TRUE;
}

static GstCaps *
gst_qml6_overlay_transform_internal_caps (GstGLFilter * filter,
    GstPadDirection direction, GstCaps * caps, GstCaps * filter_caps)
{
  GstCaps *tmp = GST_GL_FILTER_CLASS (parent_class)->transform_internal_caps (filter, direction, caps, filter_caps);
  int i, n;

  n = gst_caps_get_size (tmp);
  for (i = 0; i < n; i++) {
    GstStructure *s = gst_caps_get_structure (tmp, i);

    gst_structure_remove_fields (s, "format", "colorimetry", "chroma-site",
        "texture-target", NULL);
  }

  return tmp;
}

static GstFlowReturn
gst_qml6_gl_overlay_prepare_output_buffer (GstBaseTransform * btrans,
    GstBuffer * buffer, GstBuffer ** outbuf)
{
  GstBaseTransformClass *bclass = GST_BASE_TRANSFORM_GET_CLASS (btrans);
  GstGLBaseFilter *bfilter = GST_GL_BASE_FILTER (btrans);
  GstGLFilter *filter = GST_GL_FILTER (btrans);
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (btrans);
  GstGLMemory *out_mem;
  GstGLSyncMeta *sync_meta;

  if (gst_buffer_n_memory (buffer) <= 0) {
    GST_ELEMENT_ERROR (btrans, RESOURCE, NOT_FOUND,
        (NULL), ("Buffer must have a memory object"));
    return GST_FLOW_ERROR;
  }

  if (qml6_gl_overlay->widget) {
    GstMemory *mem;
    GstGLMemory *gl_mem;

    qml6_gl_overlay->widget->setCaps (bfilter->in_caps);

    mem = gst_buffer_peek_memory (buffer, 0);
    if (!gst_is_gl_memory (mem)) {
      GST_ELEMENT_ERROR (btrans, RESOURCE, NOT_FOUND,
          (NULL), ("Input memory must be a GstGLMemory"));
      return GST_FLOW_ERROR;
    }
    gl_mem = (GstGLMemory *) mem;
    if (!gst_gl_context_can_share (gl_mem->mem.context, bfilter->context)) {
      GST_WARNING_OBJECT (bfilter, "Cannot use the current input texture "
          "(input buffer GL context %" GST_PTR_FORMAT " cannot share "
          "resources with the configured OpenGL context %" GST_PTR_FORMAT ")",
          gl_mem->mem.context, bfilter->context);
    } else {
      qml6_gl_overlay->widget->setBuffer (buffer);
    }
  }

  /* XXX: is this the correct ts to drive the animation */
  out_mem = qml6_gl_overlay->renderer->generateOutput (GST_BUFFER_PTS (buffer));
  if (!out_mem) {
    GST_ERROR_OBJECT (qml6_gl_overlay, "Failed to generate output");
    return GST_FLOW_ERROR;
  }

  *outbuf = gst_buffer_new ();
  gst_buffer_append_memory (*outbuf, (GstMemory *) out_mem);
  gst_buffer_add_video_meta (*outbuf, (GstVideoFrameFlags) 0,
      GST_VIDEO_INFO_FORMAT (&filter->out_info),
      GST_VIDEO_INFO_WIDTH (&filter->in_info),
      GST_VIDEO_INFO_HEIGHT (&filter->out_info));

  sync_meta = gst_buffer_add_gl_sync_meta (bfilter->context, *outbuf);
  gst_gl_sync_meta_set_sync_point (sync_meta, bfilter->context);

  bclass->copy_metadata (btrans, buffer, *outbuf);

  return GST_FLOW_OK;
}

static GstFlowReturn
gst_qml6_gl_overlay_transform (GstBaseTransform * btrans, GstBuffer * inbuf,
    GstBuffer * outbuf)
{
  return GST_FLOW_OK;
}

static GstStateChangeReturn
gst_qml6_gl_overlay_change_state (GstElement * element,
    GstStateChange transition)
{
  GstQml6GLOverlay *qml6_gl_overlay = GST_QML6_GL_OVERLAY (element);
  GstGLBaseFilter *filter = GST_GL_BASE_FILTER (element);
  GstStateChangeReturn ret = GST_STATE_CHANGE_SUCCESS;

  GST_DEBUG_OBJECT (filter, "changing state: %s => %s",
      gst_state_get_name (GST_STATE_TRANSITION_CURRENT (transition)),
      gst_state_get_name (GST_STATE_TRANSITION_NEXT (transition)));

  switch (transition) {
    case GST_STATE_CHANGE_NULL_TO_READY: {
      QGuiApplication *app;
      GstGLDisplay *display = NULL;

      app = static_cast<QGuiApplication *> (QCoreApplication::instance ());
      if (!app) {
        GST_ELEMENT_ERROR (element, RESOURCE, NOT_FOUND,
            ("%s", "Failed to connect to Qt"),
            ("%s", "Could not retrieve QGuiApplication instance"));
        return GST_STATE_CHANGE_FAILURE;
      }

      display = gst_qml6_get_gl_display (FALSE);

      if (display != filter->display)
        /* always propagate. The application may need to choose between window
         * system display connections */
        gst_gl_element_propagate_display_context (GST_ELEMENT (qml6_gl_overlay), display);
      gst_object_unref (display);
      break;
    }
    default:
      break;
  }

  ret = GST_ELEMENT_CLASS (parent_class)->change_state (element, transition);
  if (ret == GST_STATE_CHANGE_FAILURE)
    return ret;

  switch (transition) {
    default:
      break;
  }

  return ret;

}
