Images captured by videocards, especially when fast movement is
recorded, may look blurred and stripped, with split objects. This is
due to how cameras work. They don't record 25 images per second, but 50,
with half vertical resolution. There are two interlaced images in one
frame. First line of first image is followed by first line of second
image followed by second line of first image... etc. So, if there have
been an important move between the two images, objects will appear
split, shifted, stripped.
The Deinterlace filter keeps only one of both images and replaces
missing lines by a gradient between previous and following lines. The
resulting image, or selection, will be somewhat blurred, but can be
improved by enhance filters
You can find interlaced images at
Wikipedia.
