Flicker Removal

A flickering image can be the result of many possibilities. Most common would be the visual update frequency of monitors and some light bulbs. Since there are a variety of sources for flicker, the fixes for them vary, some more difficult or even impossible to apply.

Keep in mind that the best way to work around flicker is to not record it in the first place. Nearly every method of "fixing" flicker will introduce some kind of image degredation or undesirable quality. If you're aware of the possibility, take steps to avoid it before it happens.

So why does flicker happen to begin with?

One cause is computer and television monitors. The image drawn on a video screen, especially in old CRT monitors, may not happen all at once. A monitor updates a still image many times a second to simulate animation. Most monitors will just draw part of the screen in one update, and then the rest of the screen the next update. If you take a picture of a monitor you may capture a time when it has drawn part of the sreen, but when the other half was not drawn and beginning to fade out.

Another cause of flicker are lights themselves. Some light sources emight light in pulses instead of a constant "flow". Since video is captured in "frames" we sometimes get situations where the update frequency of a light doesn't match with the fram rate of a camera, meaning that the light will actually be at different brighntess levels in different frames.

(This is not to be confused with the frequency wavelength that gives light a particular color.)

In the real world there are likely to be pulses between each frame as well (since the light frequency will be higher than the shutter speed), but what matters is that the pulse intensity is the same during each shutter exposure.

So how do we fix these problems?

Here we see an ideal situation under pulse emitting lights. The rate at which the camera shutter stays open (in red) matches the rate the light pulses (in blue).
In the second image the pulses of light happen at a different rate than the image capture. Frames that record during a high point in the pulse will be brighter than frames that record during the low intensities of a pulse, causing flicker.

Monitor flicker

These days footage containing video monitors, tvs, and computer displays almost always have their content nserted during post production over a blank screen (sometimes the screen may be showing a recording of still motion tracking points as helpers). But if you're shooting live or are getting flicker off any source you are not be able to control, electric billboards for instance, you may have to resort to moving masks or professional plug-ins to correct the iamge.

Ambient light flicker

Flicker caused by certain kinds of lights, mostly fluorescents, is easier to correcty because the light emitted already conforms to objects in your shot unlike the partial surface flicker of a tv or computer screen. It may become especailly in recordings with higher frame rates.

The basic process to fix this kind of flicker is to duplicate the iamge layer, offset the top layer, and lower the opacity of it.

Step by step:

  1. Identify the number of frames between each "flicker".
  2. Make as many layers as there are frames between each flicker.
  3. Offset each layer by one frame.
  4. Divide the opacity accordingly. So if there are two layers, the top is at 50%, the bottom 100%. If you've made four layers, the top are around 25%, the bottom sill 100%.

The obvious drawback to this approach is that by putting semi-transparent layers above one another, you introduce artificial motion blur that will lower image sharpness.

For this reason, this approach works best with footage that contains minimal movement, in both the frame and the subject.

Here we have two video clips. The first is raw footage recorded as a high frame rate which did not sync with the frequency of the ambient lights.
The second shows the footage after the video layer is duplicated once, offset by one frame, and the top layer set to 50% opacity.