¶ 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.
- Python 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(sys.argv)
¶ Step 2: Create the elements
Create the elements by factory.
# Create the elements
## element: videotesetsrc
src = Gst.ElementFactory.make("videotestsrc", "src")
## element: autovideosink
sink = Gst.ElementFactory.make("autovideosink", "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
pipeline_elements = [src, sink]
establish_pipeline(pipeline, pipeline_elements)
The establish_pipeline add and link the elements:
def establish_pipeline(pipeline, pipeline_elements):
## Add elements in pipeline.
for element in pipeline_elements:
pipeline.add(element)
## Link element one by one.
for i in range(len(pipeline_elements) - 1):
pipeline_elements[i].link(pipeline_elements[i + 1])
¶ 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
src.set_property("pattern", 18)
¶ Step 5: Start playing the pipeline
Playing the pipeline we created:
# Start pipeline
pipeline.set_state(Gst.State.PLAYING)
loop = GLib.MainLoop()
# Wait until error or EOS
bus = pipeline.get_bus()
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(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
Go to the folder of this sample source in terminal or cmd:
$ python3 generate-a-basic-pipeline.py
and you will see the moving ball windows displayed:
