MAMEWorld Forums - EmuChat - Re: I was playing around with the new MAME screen settings.

I feel obligated, as the pretentious CRT guy, to present some possibly useless information.

CRT screens come in three basic shapes: spherical, cylindrical, and flat. This determines how the picture from the CRT gets distorted. The thing is, there's no guarantee that a CRT outputs a flat image. In fact most don't.

You see, manufacturers were quite aware that their screens were not, in fact, flat, but were instead curved, and so they adjusted the CRT so that it would project a distorted image. This image would be stretched inwards, as if you took a finger and pressed the center of it. When the image hits the screen, the screen would distort it outwards (as you'd expect) and thus the output image would look pretty flat (except at the very edges), so long as you were in front of the screen.

You might be familiar with controls such as 'trapezoid', 'rotation', and 'pincushion'. These, and other controls, can be used to distort the CRT image, with the goal to make the image look as flat as possible on the curved screen. You could even make it look close to what the screenshot you posted looks like if you wanted.

As CRTs age, they start to distort more and more, in often unpredictable ways. What is common is for the screen to 'droop' or 'sag', so that the top corners of the screen start to sink downward. The corners and edges are prone to blowing inward or outward, making for some bizarre shapes (esp. combined with blooming). Basically, the weird geometry problems associated with CRTs has more to do with what the gun is doing rather than the screen curvature.

So really MAME should ideally have two parameters, one to set the screen curvature, and another that affects the CRT geometry. The CRT geometry would be a lot more complex. TVs have loads of geometry controls, like setting the pincushion of the top-left corner. (These would be in the service menu, a special menu for manufacturers and repairmen.) I guess to simplify things you could just have MAME do any arbitrary transformation on the screen, and then put it through the screen curvature.