Watching my nifty new High Definition-capable LCD TV I've noticed an unusual visual effect on HD TV shows that perhaps one of you can explain. While watching a scene on TV consisting of an object of different visual texture than it's surroundings, I sometimes notice that that object moves relative to the surroundings, and the effect is usually aggravated under certain conditions of image motion. For example, this evening I was watching a nature program where the camera was mounted on a small boat traveling down a river, and it was focussed on a cliff that lined the river bank. The whole scene was bouncing around from the motion of the boat, and the object of interest in the center of the screen was a mine shaft opening cut into the side of the cliff. The mine shaft seemed to move around relative to the cliff, almost as if it's image was superimposed on the cliff, and a poor job was being done integrating it into the scene.

At other times I see faces in close-ups suddenly move jerkily relative to the distant background, almost as if they were being blue-screened onto a separate background and, again, poorly superimposed. Of course, it is clear from the context of the photography that this is not the case, it is just a talking head filmed against a slightly out of focus landscape. I only notice this on HD channels, and (I'm not as sure about this) on shows photographed in HD. It's almost as if the HD recording process processes different objects on the screen separately, and there is a time-lag involved. Is this an artifact of the HD process? I believe so because I've never noticed this on non-HD broadcasts.

I have a background in digital image processing of still images, and if I had to speculate, I would guess that some sort of on-the-fly post-processing, like a contrast stretch or edge enhancement, has been done on parts of the image of special interest that may be under different illumination or focus than the rest of the scene. A time lag results causing a slight delay or pixel shift in that part of the image, and the human visual system is extremely good at picking up on this sort of anomaly. But I don't really know for sure.

I'm sure this is not a perceptual artifact, because I never see this effect on non-HD programming viewed on the same TV.