Cairo overlay demo

Taken from
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-good/tests/examples/cairo/cairo_overlay.c?ref_type=heads

.gitignore

@@ -0,0 +1,2 @@
+cairo
+compile_commands.json

Makefile

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+CC = gcc
+CFLAGS = -g -Wall -Wshadow `pkg-config --cflags gstreamer-1.0 gstreamer-video-1.0 cairo`
+LDFLAGS = `(pkg-config --libs gstreamer-1.0 gstreamer-video-1.0 cairo)`
+
+SRCS = $(wildcard *.c)
+
+all:
+	make clean
+	make build
+	./cairo
+
+build:
+	$(CC) $(CFLAGS) $(LDFLAGS) $(SRCS) -o cairo
+
+clean:
+	$(RM) cairo

cairo.c

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+/* GStreamer
+ * Copyright (C) 2011 Jon Nordby <jononor@gmail.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.
+ */
+
+/*
+ * Example showing usage of the cairooverlay element
+ */
+
+#include <gst/gst.h>
+#include <gst/video/video.h>
+
+#include <cairo-gobject.h>
+#include <cairo.h>
+
+#include <glib.h>
+
+static gboolean on_message(GstBus *bus, GstMessage *message,
+                           gpointer user_data) {
+  GMainLoop *loop = (GMainLoop *)user_data;
+
+  switch (GST_MESSAGE_TYPE(message)) {
+  case GST_MESSAGE_ERROR: {
+    GError *err = NULL;
+    gchar *debug;
+
+    gst_message_parse_error(message, &err, &debug);
+    g_critical("Got ERROR: %s (%s)", err->message, GST_STR_NULL(debug));
+    g_main_loop_quit(loop);
+    break;
+  }
+  case GST_MESSAGE_WARNING: {
+    GError *err = NULL;
+    gchar *debug;
+
+    gst_message_parse_warning(message, &err, &debug);
+    g_warning("Got WARNING: %s (%s)", err->message, GST_STR_NULL(debug));
+    g_main_loop_quit(loop);
+    break;
+  }
+  case GST_MESSAGE_EOS:
+    g_main_loop_quit(loop);
+    break;
+  default:
+    break;
+  }
+
+  return TRUE;
+}
+
+/* Datastructure to share the state we are interested in between
+ * prepare and render function. */
+typedef struct {
+  gboolean valid;
+  GstVideoInfo vinfo;
+} CairoOverlayState;
+
+/* Store the information from the caps that we are interested in. */
+static void prepare_overlay(GstElement *overlay, GstCaps *caps,
+                            gpointer user_data) {
+  CairoOverlayState *state = (CairoOverlayState *)user_data;
+
+  state->valid = gst_video_info_from_caps(&state->vinfo, caps);
+}
+
+/* Draw the overlay.
+ * This function draws a cute "beating" heart. */
+static void draw_overlay(GstElement *overlay, cairo_t *cr, guint64 timestamp,
+                         guint64 duration, gpointer user_data) {
+  CairoOverlayState *s = (CairoOverlayState *)user_data;
+  double scale;
+  int width, height;
+
+  if (!s->valid)
+    return;
+
+  width = GST_VIDEO_INFO_WIDTH(&s->vinfo);
+  height = GST_VIDEO_INFO_HEIGHT(&s->vinfo);
+
+  scale = 2 * (((timestamp / (int)1e7) % 70) + 30) / 100.0;
+  cairo_translate(cr, width / 2, (height / 2) - 30);
+
+  /* FIXME: this assumes a pixel-aspect-ratio of 1/1 */
+  cairo_scale(cr, scale, scale);
+
+  cairo_move_to(cr, 0, 0);
+  cairo_curve_to(cr, 0, -30, -50, -30, -50, 0);
+  cairo_curve_to(cr, -50, 30, 0, 35, 0, 60);
+  cairo_curve_to(cr, 0, 35, 50, 30, 50, 0);
+  cairo_curve_to(cr, 50, -30, 0, -30, 0, 0);
+  cairo_set_source_rgba(cr, 0.9, 0.0, 0.1, 0.7);
+  cairo_fill(cr);
+}
+
+static GstElement *setup_gst_pipeline(CairoOverlayState *overlay_state) {
+  GstElement *pipeline;
+  GstElement *cairo_overlay;
+  GstElement *source, *adaptor1, *adaptor2, *sink;
+
+  pipeline = gst_pipeline_new("cairo-overlay-example");
+
+  /* Adaptors needed because cairooverlay only supports ARGB data */
+  source = gst_element_factory_make("videotestsrc", "source");
+  adaptor1 = gst_element_factory_make("videoconvert", "adaptor1");
+  cairo_overlay = gst_element_factory_make("cairooverlay", "overlay");
+  adaptor2 = gst_element_factory_make("videoconvert", "adaptor2");
+  sink = gst_element_factory_make("ximagesink", "sink");
+  if (sink == NULL)
+    sink = gst_element_factory_make("autovideosink", "sink");
+
+  /* If failing, the element could not be created */
+  g_assert(cairo_overlay);
+
+  /* Hook up the necessary signals for cairooverlay */
+  g_signal_connect(cairo_overlay, "draw", G_CALLBACK(draw_overlay),
+                   overlay_state);
+  g_signal_connect(cairo_overlay, "caps-changed", G_CALLBACK(prepare_overlay),
+                   overlay_state);
+
+  gst_bin_add_many(GST_BIN(pipeline), source, adaptor1, cairo_overlay, adaptor2,
+                   sink, NULL);
+
+  if (!gst_element_link_many(source, adaptor1, cairo_overlay, adaptor2, sink,
+                             NULL)) {
+    g_warning("Failed to link elements!");
+  }
+
+  return pipeline;
+}
+
+int main(int argc, char **argv) {
+  GMainLoop *loop;
+  GstElement *pipeline;
+  GstBus *bus;
+  CairoOverlayState *overlay_state;
+
+  gst_init(&argc, &argv);
+  loop = g_main_loop_new(NULL, FALSE);
+
+  /* allocate on heap for pedagogical reasons, makes code easier to transfer */
+  overlay_state = g_new0(CairoOverlayState, 1);
+
+  pipeline = setup_gst_pipeline(overlay_state);
+
+  bus = gst_pipeline_get_bus(GST_PIPELINE(pipeline));
+  gst_bus_add_signal_watch(bus);
+  g_signal_connect(G_OBJECT(bus), "message", G_CALLBACK(on_message), loop);
+  gst_object_unref(GST_OBJECT(bus));
+
+  gst_element_set_state(pipeline, GST_STATE_PLAYING);
+  g_main_loop_run(loop);
+
+  gst_element_set_state(pipeline, GST_STATE_NULL);
+  gst_object_unref(pipeline);
+
+  g_free(overlay_state);
+  return 0;
+}