gst-element-examples

Trying to creste the most minimal src, transform and sink elements for gstreamer.

Many of the existing examples are skewed towards AV applications and the plugin writers guide has little to cover sources and sinks.

The examples here take a basic memory buffer(src) with 1 byte initialised. This can be passed through to transform (just clears the top nibble of the byte) and outputs to the sink which prints it out (via debug)

The following assumes gst version 1.20.0

Initial template code was generated through the gst-element-maker scripts

./gstreamer/subprojects/gst-plugins-bad/tools/gst-element-maker mysrc basesrc ./gstreamer/subprojects/gst-plugins-bad/tools/gst-element-maker mytransform basetransform ./gstreamer/subprojects/gst-plugins-bad/tools/gst-element-maker mysink basesink

Building

The 3 plugins are built by copying the source directly into the existing plugin framework. TODO move this to a system where this repo can be build standalone against an existing gstreamer-dev install

Install Common Dependencies

sudo apt install flex bison ninja-build libcairo2-dev cmake libgtk-3-dev libbz2-dev libv4l-dev libjpeg-dev libgmp-dev openssl libssl-dev gnutls-dev nasm

Get GStreamer source

git clone https://github.com/GStreamer/gstreamer.git
cd gstreamer
git checkout tags/1.20.0

Build Gstreamer Source

cd gstreamer
meson.py build
ninja -C build
ninja -C build uninstalled

And test the build...

[gst-HEAD] phill@ubuntu:~/src/gstreamer/build$ ./subprojects/gstreamer/tools/gst-launch-1.0 --version
gst-launch-1.0 version 1.20.0
GStreamer 1.20.0
Unknown package origin
[gst-HEAD] phill@ubuntu:~/src/gstreamer/build$ 

Copy the mysrc, mytransform and mysink under gst-element-examples into gstreamer/subprojects/gst-plugins-bad/gst/

Edit gstreamer/subprojects/gst-plugins-bad/meson_options.txt and add the following to options

option('mysrc', type : 'feature', value : 'auto')
option('mytransform', type : 'feature', value : 'auto')
option('mysink', type : 'feature', value : 'auto')

Add

 'mysrc', 'mytransform', 'mysink'

to the component array in gstreamer/subprojects/gst-plugins-bad/gst/meson.build

rebuild and test

Exit the "uninstalled" environment and rebuild

exit
rm -rf build
meson.py build
ninja -C build
ninja -C build uninstalled

Enable gstreamer debug for the components by setting the SHELL env variable

export GST_DEBUG=3,mysrc*:4,mysink*:4,mytransform*:4

and run the graph

./subprojects/gstreamer/tools/gst-launch-1.0 mysrc ! mytransform ! mysink

Sample output will look something like that below. The SRC creates a data byte in this cas 0x2E, the transform removes the top nibble and the sink displays the result (0x0E)

0:00:00.866277137 63477 0x55e3b8ec6b60 INFO                   mysrc gstmysrc.c:570:gst_mysrc_create: buffer[0] = 0x2E
0:00:00.866304951 63477 0x55e3b8ec6b60 INFO             mytransform gstmytransform.c:137:gst_mytransform_chain: Transforming (Strip top nibble) -> 0x2E to 0x0E
0:00:00.866310345 63477 0x55e3b8ec6b60 INFO                  mysink gstmysink.c:517:gst_mysink_render: buffer[0] = 0x0E

You can also connect to fakesink to dump the complete buffer.

For development/testing you can minimise the buffer size and limit the num buffers sent

./subprojects/gstreamer/tools/gst-launch-1.0 mysrc blocksize=1 num-buffers=32 ! mytransform ! mysink

individual elements can be inspected EG:

./subprojects/gstreamer/tools/gst-inspect-1.0 mysink
./subprojects/gstreamer/tools/gst-inspect-1.0 mytransform
./subprojects/gstreamer/tools/gst-inspect-1.0 mysrc

I hope someone finds this a useful starting point.