Doctor M said:
Anyone with a better method?
Not better, but different...
c = AVISource("SotS.avi") # Assumes c is 1920x1080 already.
c.Subtract(c.ResampleHQ(1280, 720).ResampleHQ(1920, 1080))
Basically: Downscale to 720p, upscale back to 1080p, and then subtract from the original 1080p clip. This should show what's lost rather well.