¶ What can you learn from this sample code?
Through this sample code, you can learn:
- Establish a basic gstreamer pipeline in an application program.
- Including initialization pipeline, declaration relative instances, linkage of the element and releasing the resources.
- Set the property of the element in the pipeline.
- Handle the pipeline messages if required.
¶ Essential knowledge
- Ever execute the GSTREAMER pipeline command in terminal(Linux) or cmd(Windows).
- The difference between application code and plugin/element code.
- Basic C/C++ programming.
¶ About this sample code
This sample shows how to generate a simple pipeline equivalent to:
gst-launch-1.0 videotestsrc pattern=18 ! autovideosink
Besides the establishment of the pipeline, this example also illustrate how to handle the 2 return messages: GST_MESSAGE_ERROR and GST_MESSAGE_EOS. More messages return could be found HERE. See the GstMessageType of Enumerations section. Based on the requirement to add the message if needed in advance.
¶ Description
There are seven main steps to form the application of generate-a-basic-pipeline.c:
• Step 1: Initialize GStreamer
• Step 2: Create the elements
• Step 3: Create the empty pipeline and then build it
• Step 4: Modify the source's properties
• Step 5: Start playing the pipeline
• Step 6: Message handling
• Step 7: Free resources
Here, we will take generate-a-basic-pipeline.c for explanation.
¶ Step 1: Initialize GStreamer
In this step, it is required and must to initialize the GStreamer by init for the application.
/* Initialize GStreamer */
gst_init (&argc, &argv);
¶ Step 2: Create the elements
Create the elements by factory.
/* Create the elements */
source = gst_element_factory_make ("videotestsrc", "source");
sink = gst_element_factory_make ("autovideoconvert", "sink");
¶ Step 3: Create the empty pipeline and then build it
Generate a pipe line in order to put all element in it and then playing it later.
/* Create the empty pipeline */
pipeline = gst_pipeline_new ("test-pipeline");
/* Build the pipeline */
gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL);
¶ Step 4: Modify the source's properties
This step is optional if you are going to set some properties to each element. In ex_application.c, we required to set properties, pattern, for switch the pattern images we are going to use:
/* Modify the source's properties */
g_object_set (source, "pattern", 18, NULL);
¶ Step 5: Start playing the pipeline
Playing the pipeline we created:
/* Start playing */
ret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
¶ Step 6: Message handling
This step makes sure the message be thrown by the pipeline we created through the GstBus. This message handling could customize based on different event thrown.
/* Wait until error or EOS */
bus = gst_element_get_bus (pipeline);
msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS);
/* Parse message */
if (msg != NULL)
{
GError *err;
gchar *debug_info;
switch (GST_MESSAGE_TYPE (msg))
{
case GST_MESSAGE_ERROR:
gst_message_parse_error (msg, &err, &debug_info);
g_printerr ("Error received from element %s: %s\n", GST_OBJECT_NAME (msg->src), err->message);
g_printerr ("Debugging information: %s\n", debug_info ? debug_info : "none");
g_clear_error (&err);
g_free (debug_info);
break;
case GST_MESSAGE_EOS:
g_print ("End-Of-Stream reached.\n");
break;
default:
/* Handle other messages, otherwise the default case will called. */
g_printerr ("Unexpected message received.\n");
break;
}
gst_message_unref (msg);
}
¶ Step 7: Free resources
Finally, release all used GStreamer instances:
/* Free resources */
gst_object_unref (bus);
gst_element_set_state (pipeline, GST_STATE_NULL);
gst_object_unref (pipeline);
¶ Run this sample code
Go to the folder of the binary and run binary in terminal or cmd:
$ ./generate-a-basic-pipeline
and you will see the moving ball windows displayed:
