Global Shutter vs. Rolling Shutter “The Jello Effect”

July 11, 2014

The “Jello-effect” occurs with rolling shutter CMOS cameras. It’s a disturbing effect that occurs most obviously when the camera pans or moves horizontal. This happens because the bottom and top  of your picture are recorded at different times. The picture is essentially skewed, like pulling a plate of jello. The top of the block of jello stays in position as the bottom moves first. Blur is most obvious and the picture looks jumpy. This is because the imaging chip in your camera steps off your individual frames one line of pixels at a time. Each line is progressively dropped down the chip and sent off to a buffer. The chip continuously captures light so when the lines of pixels vacate the top row the chip is already capturing the next frame. Now it moves quite quickly and works remarkably well with a static camera. Since the background of your subject is static and only the subject is moving there is little issue. However when you move the camera running at 30 frames per second there is a slight misalignment of the image where the first line starts and the last finishes. In other words the top line records earlier in the move (bottom of your jello) and  your last line records the image later in the move (the top of your jello).

With Global Shutter all the lines are extracted from the imaging chip simultaneously – a complete frame. There are several techniques for doing this but it requires a much more sophisticated imaging chip and much better electronics. And as expected, much higher cost.

There are several great video cameras with global shutters. We really like working with the Sony PMW F55.