This idea just happened to pop into my head a while ago, so I thought maybe I'd share it. I'm not a coder, so I don't know where this idea is any good or not. Maybe it might would be usefull for detecting those interlaced mouths in anime or something... Anyways, couldn't it be possible to detect interlaced frames by first blending the fields, then comparing this to the original frame? The non-interlaced parts of the image would stay mostly the same, while the interlaced areas would change a good bit. Depending on the ammount of change it detects, perhaps it could find interlaced areas?