I vaguely understand that an old CRT monitor/TV's screen is a section of a sphere, and since the section lines of a sphere curved, the edges of the screen are curved as well.

enter image description here

My question is, if you take a 4:3 image and one of these CRTs, is that image being stretched and 'fish-eyed' to fit the curved edges of this monitors, or does it appear square, but with its corners cut off?

In other words, is A or B correct? Note that the blue lines are straight. On A the image is square and orthogonal, and on B the image is warped to conform to the borders of the screen. enter image description here


CRTs were originally round, as can be seen in this 1945 prototype. The cabinet masks off the outer parts of the tube to create the square viewing area. This is called overscan. Later tubes were more rectangular, but still relied on overscan. This was partly because as the tube aged, the scanned area shrank, so the cabinet would hide the blank parts of the screen. The front to back curvature of the tube is not accounted for in the image because every model tube had a different curvature and I don't think it added much distortion.

Curves at the rounded corners of the cabinet were also not accounted for because of the differences in cabinet. But that was part of the motivation for overscanning and for limiting important picture elements to safe areas.

  • I'm not sure I get it yet 100%. I clarified my question with a demonstration of the two things I'm trying to figure out between. The cabinet masks off an area that isn't entirely square, which is what I'm wondering about. Is the image stretched to fit that not-rectangular viewing area, or are the corners cut off more? – Rekov Jan 18 '19 at 5:47
  • The corners are cut off. There is no distortion applied to the image. It would be impossible to do that since every model was different and even within a model, the distortion would change over time. – Michael Liebman Jan 19 '19 at 0:45

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