Transitions are used in Shotwell for interstitial effects in slideshow mode. They may also be used elsewhere in future releases.
Plugin writers should start by implementing a Descriptor which in turn
Shotwell uses to instantiate an Effect.
Descriptor - A Descriptor offers a factory method for
creating Effect instances.
Effect - An Effect represents an interstitial effect that is
used to transition the display from one image to another.
Visuals - Visuals contains the pertinent drawing information
for the transition that must occur.
Motion - Motion contains all the pertinent information
regarding the animation of the transition.
Direction - Direction indicates what direction (animated
motion) the Effect should simulate the images are moving, if appropriate.