Tuesday 28 April 2009

CMOS: wobble - jello effect -

In my first experiences with wobbling of footage after stabilization process,I thought it was caused by cheap lenses distorting the images, but the main factor is the CMOS sensor inside many photo cameras, video cámeras and most of 'one-piece helmetcams' that you can buy.
It is the result of operation by 'rolling shutter', opposite to 'global shutter' usual in CCD sensors.

Why?... CCD versus CMOS

Technology of CCD sensors scan all picture in same time (all together). A CMOS sensor scans the image line by line, from top to bottom. The time used for a complete image could be up to 1/30 second and during this time the subject may have moved.
In next video, a good quality CMOS is unable to record entire coin at same fraction of time:

It is not a 'problem' about quality of the camcorder but cameras equiped with CMOS are not appropriated to record fast movements, simply. Watch this VIDEO comparative between two identical cameras with only difference of image sensors.

Effect recording on MTB
Helmetcam videos are full of shaking and sudden movements. Many raw videos seem completly trembling, so is difficult to appreciate the jello effect. But a stabilization process, (e.g. SteadyHand or Mercalli) reveals the effect. The software process is not capable to stretch or to contract the still (only can move it)
Next image shows two near stills (only separated by 2/30 seconds) from my latests video:

In first the cam is moving up, and the second the camera is moving down. The result is that image seems to contract and to stretch alternatively. In the VIDEO, the jello effect is visible.

Other sample to understand the effect:

Only a few CMOS cameras are ready to resolve the effect of rolling shutter (e.g POV 1 uses CMOS with "electronic global shutter"). In most of CMOS photo cameras and most of one-piece helmet cameras the 'wobble' can be a problem, specially if you are intending to obtain steady footage.

More information HERE. With some ilustrative graphics.

1 comment:

Anonymous said...

Hi, great article, I recently purchased an action cam on ebay for a very low price, and it too suffered from jello effect, its a great shame though as I have it mounted on my motorcycle and was looking forward to some footage.