How do you do an animated fade?
Question asks it all. How do you do a fade in or out in animation or a transformation from one thing to another? Say I want to do a transformation from Dawn to Dusk, or from a cat to a dog?
You could render a clip of the setting, then render a clip of the cat as a raster object on transparent background (use the holdout node on background if rendering Superfly), then render a clip of the dog.
Load the setting animated frames into a video/GIF editor, then overlay the cat frames, then overlay the dog frames.
Set the transparency of the cat object as 0 at the start, and tween the transparency to 1 at the end. Vice-versa for the dog object.
Or, you could animate the cat's (and dog's) refractivity, and render the frames in one shot. The cat would begin opaque and become a refractive ghost fading out; the dog would fade in.