Synfig
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Synfig is an open-source 2D vector graphics and animation program created by Robert Quattlebaum.
The goal of the developers is to create a program that is capable of producing "feature-film quality animation with fewer people and resources."[1] The program offers an alternative to manual tweening so that the animator doesn't have to draw each and every frame.
The software is capable of simulating soft-shading using curved gradients within an area so that the animator doesn't have to draw shading into every single frame.
Currently, it is available for Mac OS X, Windows, and Linux.