Editing Interlaced video (section)

===Interline twitter===
Interlace introduces a potential problem called '''interline twitter''', a form of [[moiré]]. This [[aliasing]] effect only shows up under certain circumstances—when the subject contains vertical detail that approaches the horizontal resolution of the video format. For instance, a finely striped jacket on a news anchor may produce a shimmering effect. This is ''twittering''. Television professionals avoid wearing clothing with fine striped patterns for this reason. [[Professional video camera]]s or [[computer-generated imagery]] systems apply a [[low-pass filter]] to the vertical resolution of the signal to prevent interline twitter.

Interline twitter is the primary reason that interlacing is less suited for computer displays. Each scanline on a high-resolution computer monitor typically displays discrete pixels, each of which does not span the scanline above or below. When the overall interlaced framerate is 60 frames per second, a pixel (or more critically for e.g. windowing systems or underlined text, a horizontal line) that spans only one scanline in height is visible for the 1/60 of a second that would be expected of a 60&nbsp;Hz progressive display - but is then followed by 1/60 of a second of darkness (whilst the opposite field is scanned), reducing the per-line/per-pixel refresh rate to 30 frames per second with quite obvious flicker.

To avoid this, standard interlaced television sets typically do not display sharp detail. When computer graphics appear on a standard television set, the screen is either treated as if it were half the resolution of what it actually is (or even lower), or rendered at full resolution and then subjected to a low-pass filter in the vertical direction (e.g. a "motion blur" type with a 1-pixel distance, which blends each line 50% with the next, maintaining a degree of the full positional resolution and preventing the obvious "blockiness" of simple line doubling whilst actually reducing flicker to less than what the simpler approach would achieve). If text is displayed, it is large enough so that any horizontal lines are at least two scanlines high. Most [[fonts]] for television programming have wide, fat strokes, and do not include fine-detail [[serif]]s that would make the twittering more visible; in addition, modern character generators apply a degree of anti-aliasing that has a similar line-spanning effect to the aforementioned full-frame low-pass filter.

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! Interlacing example (Note: high rate of flickering)
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|[[File:Indian Head interlace.gif|center]]
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|''Note – Because the frame rate has been slowed by a factor of 3, one notices additional flicker in simulated interlaced portions of this image.''
This animation demonstrates the interline twitter effect using the [[Indian Head test card]]. On the left are two [[progressive scan]] images. Center are two interlaced images. Right are two images with [[line doubler]]s. Top are original resolution, bottom are with anti-aliasing. The two interlaced images use half the bandwidth of the progressive one. The interlaced scan (center) precisely duplicates the pixels of the progressive image (left), but interlace causes details to twitter. A line doubler operating in "bob" (interpolation) mode would produce the images at far right. Real interlaced video blurs such details to prevent twitter, as seen in the bottom row, but such softening (or anti-aliasing) comes at the cost of image clarity. But even the best line doubler could never restore the bottom center image to the full resolution of the progressive image.

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