Posted on 13/10/2016 by Lubosz Sarnecki
Being someone who has already experimented with two transformation box approaches for Pitivi in the past, maintainers thought I might be the right person to do a modern one.
Creating a user interface for a video transformation requires three things:
First of all the implementation of the transformation, which is in our case scaling and translation, is currently done by GES.UriSource, calculated on the CPU. In the first Pitivi transformation box I did in GSoC 2012 this was done by the notorious Frei0r plugins from GStreamer Plugins Bad, which is also a CPU implementation. In the second version this was done on the GPU with the gltransformation element I wrote for GSoC 2014.
In Pitivi’s case, the viewer is a GStreamer sink. In all three versions rendering of the overlay widgets was done by Cairo, but it was done differently for all three implementations, since they all used different sinks.
The first one used a hacky solution where the sink and cairo drew in the same Gtk drawing area, acquired for GStreamer with the Gst Overlay API. Many Gtk and GStreamer devs wondered how this worked at all. This and Pitivi switching to more modern sinks was the reason why the first version of the box didn’t stay upstream for long.
Still using Gst Overlay API, but this time with the glimagesink. The cairo widgets are rendered into OpenGL textures and composed in the glimagesink draw callback in the GStreamer GL context. Worked pretty smooth for me, but didn’t provide a fallback solution for users without GL. Clearly an approach for the future, but how about something solid?
Now we have the almighty GtkSink in Pitivi. It is a Gtk widget and overlays can be added via Gtk Overlay. The sink is also exchangeable with GtkGLSink, which uses a GStreamer GL context to display the video texture and also can use GStreamer GL plugins like gltransformation without needing to download the GPU memory with gldownload. Cairo rendering now can be easily added over GStreamer sinks, yay.
The mapping of the input from the UI to the transformation is clearly dependent on the transformation you are using. In the 2012 version I needed to map the input to frei0r^-1. In 2014 I used an OpenGL Model-View-Projection matrix calculated in Graphene, which could also do rotations and 3D transformations (we have a z-axis, yay).
The 2016 implementation uses the inverse transformation for the GES.UriSource transformation, which is done by the GStreamer elements videomixer and videoscale. Of course things like keeping aspect ratio, maintaining limits and transforming Gtk widget coordinates to the transformation’s coordinates are part of this 3rd ingredient.
The new transformation box fits great with Pitivi by making clips selectable from the viewer, so you can manage multiple overlapping clips quite easily. But the best part of this implementation may be its extensibility. I already made two overlay classes, one for the normal clips which uses a GES.UriSource transformation and one for title clips aka GES.TextSource, which is using different coordinates and different GStreamer plugins. In this fashion other plugins can be written for the Pitivi viewer, for example for 3D transformations with gltransformation. Or you could do crazy stuff like a UI for barrel distortion etc.
Clone the code and contribute!
If you have any questions about this ask me. I’m lubosz on Freenode IRC.
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