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 Flickering (or Unsteady Shinning) of Horizontal Lines on Interlaced Video What's Flickering
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Video Editing

Flickering (or Unsteady Shinning) of Horizontal Lines on Interlaced Video

A   TV signal is interlaced, meaning that each full “screen” of information is actually composed of two separate “fields” – the odd lines and the even lines. First the odd lines are painted on the screen. Then, before the odd lines have had time to completely fade, the even lines are “painted” in between the odd lines. This happens so quickly that it is difficult for the human eye to perceive, especially when viewing moving images. However, the interlaced format is inadequate for clearly displaying certain non-moving objects, particularly those with thin horizontal lines. The appearance and disappearance of the odd and even picture fields causes noticeable flickering of thin, horizontal lines, causing them shining unsteadily like on the picture below:

Odd field
Even field
Odd Fields Even Fields Interlaced & Progressive

Read more about interlaced video on Wikipedia

The flickering affects mostly sharp still pictures but not video because:

i) A human eye detect movement first, then details
ii) Sharp stills often contains single horizontal lines or gradients
iii) In contrast, video footage is usually blurred enough due to both nature of video recording and special filters adopted in camcoders

The common technique to eliminate (or reduce, at least) flickering is applying a blur filter in vertical direction. One sample is so-called three-tap (1/4,1/2,1/4) filter that derives a new value for a pixel by mixing upper(25%) + old(50%) + lower(25%) pixels. This reduces flickering in most cases but also blurs the original picture significantly. The idea of the AntiFlicker filter is to apply special type of vertical blur to the problem areas only, not distorting all the image.

Pictures below demonstrate different flicker removal techniques applied to the typical potentially flickering image (a fragment of original 720x576 picture is shown).

3-tap blur
Original 3-tap blur AntiFlicker

Note, that 3-tap filter blurs the image a lot but it does not guarantee efficient flicker suppression. To see the result, those images should be converted to the interlaced video clip. 3-tap filter produces video that still makes flicker on the TV screen while AntiFlicker makes shinning virtually unnoticeable (by my eyes, at least)

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