This is an extension of coverage projection and arises when you have a ‘hero’ projection that is projecting an image on to some geometry but, at some point along the motion path of the shot camera, the projected image begins to fail, either through a gap in coverage, stretching or smearing of the image or the doubling of pixels from another part of the projection. I will explain these terms shortly.
To execute a successful patch projection, we would, looking through the shot camera, locate the frame where the degradation begins to occur and generate a new projection camera from this. We would then render a still frame of the matte painting from this position, take this into a painting application (i.e. Photoshop) and replace the offending pixels. This new image would then be projected back onto the geometry from our ‘patch’ projector, and sitting on top of the original matte painting.
I have produced a short screencast demonstrating my experimentation and application of the technique using Nuke
Peter Holmes PhD Journal 