We've just discovered this. Here's what they say about themselves
CROSS PLATFORM & OPEN-SOURCE COMPOSITOR
FOR VISUAL EFFECTS AND MOTION GRAPHICS INDUSTRY
Natron is a powerful Digital Compositor that can handle all of your 2D/2.5D needs. Its robust OIIO file formats and OpenFX architecture is what make Natron the most flexible open source compositor for the visual effects community. Its interface and functionally are the same across all platforms such as macOS, Linux and Windows. Natron has a powerful keying, roto/rotopaint, 2D tracking tools that are staple for all current film production project that requires visual effects.
Natron was a proud recipient of Inria funding from 2013 to 2018, and is now actively developed by its community.