¶ What can you learn from this sample code?
Through this sample code, you can learn:
- Quick launch a command pipeline into an application without handle connections between plugins/elements.
- 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.
- Python programming.
¶ About this sample code(Summary of this sample code)
This sample shows how to generate a simple pipeline equivalent to:
gst-launch-1.0 videotestsrc pattern=18 ! autovideosink
Gst.parse_launch is fast and useful when you don’t want to handle connections between plugins/elements manually and just want to launch some existing pipeline. Creating pipeline with Gst.parse_launch is very simple and flexible solution.
¶ Description(Detail description of this sample code)
There are seven main steps to form the application of generate-a-basic-pipeline.c:
• Step 1: Initialize GStreamer
• Step 2: Define gstreamer command pipeline
• Step 3: Create pipeline via parse_launch
• Step 4: Allow bus to emit messages to main thread
• Step 5: Start pipeline
• Step 6: Message handling
• Step 7: Free resources
Here, we will take generate-a-pipeline-with-parse-launch.c for explanation.
¶ Step 1: Initialize GStreamer
In this step, it is required and must to initialize the GStreamer by init for the application.
Gst.init(sys.argv)
¶ Step 2: Define gstreamer command pipeline
Next, define the correct pipeline command. Make sure the command can successfulll execute in terminal.
pipeline_command = "videotestsrc pattern=18 ! autovideosink"
¶ Step 3: Create pipeline via parse_launch
Call Gst.parse_launch with the command just defined:
pipeline = Gst.parse_launch(pipeline_command)
¶ Step 4: Allow bus to emit messages to main thread
Get the bus from pipeline instance and allow bus to emit message to main thread:
bus = pipeline.get_bus()
bus.add_signal_watch()
¶ Step 5: Start pipeline
Playing the pipeline we created:
pipeline.set_state(Gst.State.PLAYING)
loop = GObject.MainLoop()
# Add handler to specific signal
bus.connect("message", on_message, loop)
try:
print("Start to run the pipeline.\n")
loop.run()
except Exception:
print("in exception")
traceback.print_exc()
loop.quit()
¶ 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.
def on_message(bus: Gst.Bus, message: Gst.Message, loop: GObject.MainLoop):
mtype = message.type
if mtype == Gst.MessageType.EOS:
print("End of stream")
loop.quit()
elif mtype == Gst.MessageType.ERROR:
err, debug = message.parse_error()
print("Gst.MessageType.ERROR catched in on_message:")
print(err, debug)
loop.quit()
elif mtype == Gst.MessageType.ANY:
err, debug = message.parse_warning()
print(err, debug)
return True
¶ Step 7: Free resources
Finally, release all used GStreamer instances:
# Stop Pipeline
pipeline.set_state(Gst.State.NULL)
del pipeline
¶ Run this sample code(How to run and what is the results or displayed)
Go to the folder of this sample source in terminal or cmd:
$ python3 generate-a-pipeline-with-parse-launch.py
and you will see the moving ball windows displayed:
