Editing Interlaced video (section)

==Interlacing problems==
{{Unreferenced section|date=May 2023}}
[[Image:Interlaced video frame (car wheel).jpg|thumb|250px|When someone watches interlaced video on a progressive monitor with poor (or no) deinterlacing, they can see "combing" in movement between two fields of one frame.]]
[[File:Interlaced video frame (car wheel)Xcorr.png|thumb|258px|Picture of a moving car tire, interlace combing reduced by realigning the even and odd field on the X axis. The other field has been moved 16 pixels right, reducing the combing on the bumper and the tire outline, but the hub cap that has turned between the fields has notable combing.]]
Interlaced video is designed to be captured, stored, transmitted, and displayed in the same interlaced format. Because each interlaced video frame is two fields captured at different moments in time, interlaced video frames can exhibit motion artifacts known as ''interlacing effects'', or ''combing'', if recorded objects move fast enough to be in different positions when each individual field is captured. These artifacts may be more visible when interlaced video is displayed at a slower speed than it was captured, or in still frames.

While there are simple methods to produce somewhat satisfactory progressive frames from the interlaced image, for example by doubling the lines of one field and omitting the other (halving vertical resolution), or [[Spatial anti-aliasing|anti-aliasing the image]] in the vertical axis to hide some of the combing, there are sometimes methods of producing results far superior to these. If there is only sideways (X axis) motion between the two fields and this motion is even throughout the full frame, it is possible to align the scanlines and crop the left and right ends that exceed the frame area to produce a visually satisfactory image. Minor Y&nbsp;axis motion can be corrected similarly by aligning the scanlines in a different sequence and cropping the excess at the top and bottom. Often the middle of the picture is the most necessary area to put into check, and whether there is only X or Y&nbsp;axis alignment correction, or both are applied, most artifacts will occur towards the edges of the picture. However, even these simple procedures require motion tracking between the fields, and a rotating or tilting object, or one that moves in the Z&nbsp;axis (away from or towards the camera) will still produce combing, possibly even looking worse than if the fields were joined in a simpler method. 
Some [[deinterlacing]] processes can analyze each frame individually and decide the best method. The best and only perfect conversion in these cases is to treat each frame as a separate image, but that may not always be possible. For framerate conversions and zooming it would mostly be ideal to line-double each field to produce a double rate of progressive frames, resample the frames to the desired resolution and then re-scan the stream at the desired rate, either in progressive or interlaced mode.

===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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