Why does the picture dance?
April 6, 2016 1:28 PM   Subscribe

Can someone please explain this odd tv image effect that I've noticed for several years

This will take awhile...
So, I was watching a bit of the par-3 contest from the Masters this afternoon, and saw a really obvious distortion of the picture that I've also noticed when watching various old movies...

The image was of a player putting on the green. Behind him was the nearby gallery. Behind them was nothing but trees.

Now, imagine the camera shooting the scene was getting buffeted by some strong winds, so the image shakes around a bit, which would make the whole picture at home dance around a bit. With me so far?

Now, imagine that the player and the gallery were moving around, but the background trees were rock steady. No movement. The effect is as if the player and gallery are on a top layer and the trees on a bottom layer. It looks like cut-out animation. The foreground was unglued from the background.

I've also noticed this effect when watching certain older b/w movies. In them, it's often as if the blacks and the whites are separated onto different layers, and they "dance" slightly.

I've seen this on different tv's, in different homes, so it's not something amiss with my own tv. It seems to be more blatant in scenes with a strong contrast in the image. In the Masters example, the player and crowd were in bright sunshine, while the trees were more muted and darker. Ditto, the old movies. It seems to be most apparent when it's black against white, and even areas of the same object (like a desk, for instance) would do the "dance".

Is this effect a result of digital processing? Something else?
posted by Thorzdad to Technology (3 answers total)
 
It's almost surely the result of applying too much compression. The style of compression they use try to figure out places of the image that are barely changing/not visible changes and just doesn't send information about the change. If you can also notice any "blocky"ness or other MPEG/JPEG style artifacts, that lends additional evidence to that.
posted by skynxnex at 1:37 PM on April 6, 2016 [4 favorites]


What skynxnex said. When transmitting pictures you can refresh certain parts of the image. Things that typically don't move are often left out of the refreshing process. So buildings and backgrounds will remain still while the movement will refresh.

When I sold video conferencing equipment, we'd teach people not to move during presentation to prevent issues with refreshing and to keep the picture quality high. More movement = crappier picture quality.
posted by Ruthless Bunny at 2:24 PM on April 6, 2016


skynxnex has it. The effect happens less often and to a lesser extent with OTA broadcasts that you have to pick up with an antenna, because antenna broadcasts generally have higher bandwidth per channel and a different sort of video compression than what you usually get via cable, FIOS or DirecTV/Dish.
posted by infinitewindow at 2:29 PM on April 6, 2016 [2 favorites]


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