Ah, overscan... the simple device that hides a lot of sins.
In the olden days of TV, the tubes (cathode ray tubes) had circular faces that also had a spherical curve so that the electron beam always had the same travel length to the phosphor coated surface. That made one focus setting ideal for all positions on the screen. Distorted the pictures like crazy when viewing from any position other than dead center, but it was simple.
Later, rectangular tubes were tried, nastily complicated things. It was hard to mold the envelope, then the deflection power needed to get the beam into the corners wasn't anything like the power for the sides. Extra circuitry was tried to correct this but it wasn't perfect, the resulting picture looked like a pincushion or a pillow, not a rectangle. So the engineers decided to overscan, forcing the irregular edges off the edge of the tube, and hope the viewer didn't notice. It worked.
But, ever since, video producers have to watch where important parts of the picture happen. Too close to the edge and it'll be sliced off by some TV sets. Gradually a sort of standard evolved that TV set makers still follow, anything that must appear on the screen should fall within the '20%' mask, that is the region that's 20% smaller than the video screen size( white are on the image below). Anything that doesn't matter should be within 10% of the screen edge (black area). Anything in that 10% to 20% may show on some TV sets (grey area).
PC screens are different. The CRT's they use are precision made with modern, programable correction circuits that give a near ideal picture. So if your video is for viewing on a PC, you should watch what is near the edges, even to the point of masking off 10% from the edge just to hide artifacts from VCR's and movie film. If your work is for TV, then anything within that 10% can be ignored.
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