GstBaseTransform is the class intended for the simple image processing and analysis in GStreamer. Being new to GStreamer, I am looking for some more or less complete plugin that I could use for understanding how to implement mine. Unfortunately, the Plugin Writers Guide does not cover it, and in general I cannot find a complete source code of any simple enough example that uses it. Most of the websites I Google out simply repeat the documentation, some specialized code for Freescale in GitHub and also GLES project are the closest thing I was so far able to find. It is, however, a specialized production code with lots of stuff to remove before trying to understand the rest.
Is it possible to get a complete example/source code of the plugin that uses GstBaseTransform for something very simple, like drawing line over the image?
GStreamer pipeline can be easily launched from the command line like "gst-launch-1.0 videotestsrc ! the_plugin ! videoconvert ! autovideosink".
I recommend a combination of documentation and the element builder. The element builder gives you a template for plugins based on several bases classes including basetransform. The tool can be found at tools/gst-element-maker in the gst-plugins-bad repository.
Then the documentation of the baseclass - e.g. https://gstreamer.freedesktop.org/documentation/base/gstbasetransform.html?gi-language=c
Usually the documentation explains in general how the class is expected to work and does have some documentation on the most important/significant functions.
For basetransform your most important function is going to be transform or transform_ip depending whether your plugin will work in in-place mode or not. There you can do your processing - like writing a line into an image buffer.