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Raster scan display forms an image with pixel whereas a Random scan display works with Geometric primitives, but even a geometric primitve( a line) is made up of pixels so what is the difference?

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A raster display draws every pixel on the screen in every frame whether there is something to show or not.

A random scan display activates only those pixel which are occupied by an geometric primitive.

So yes, they both use pixels, but the difference is in how they draw the pixels onto the screen.

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  • $\begingroup$ I read that in a raster display a picture is kind-of jaggy but in random we get a smooth line. why and how does this happen when both work on pixel level anyhow? $\endgroup$ – pratik watwani Jan 24 '16 at 14:13
  • $\begingroup$ Both systems produce some aliasing (I think this is what you mean with 'jaggy') on pixel-level since there is only a finite discrete resolution available. I don't know how the amount of aliasing compares, since in the background of a rasterizer geometric primitives also get converted into pixel-grid, and a random scan has to do the same conversion. I can only assume that because a random scan can directly follow the line it is easier to anti-alias. $\endgroup$ – Dragonseel Jan 24 '16 at 14:36
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True random scan displays aren't used anymore. They were based on cathode-ray tubes where the electron beam wasn't driven in a scanline pattern but as arbitrary line segments, with "direct access".

The early devices were similar to oscilloscopes, requiring frequent refreshing of the trace. Later appeared the memory devices, able to retain the image without refreshing, so that they could render arbitrarily complex drawings (with the drawback that modifying a part of the image wasn't possible).

They indeed didn't really suffer from jaggies, as they were analogic devices, anyway because of digital-to-analog limitations, they had a finite resolution in practice (typically 1K x 1K, a wonder at the time). They allowed monochrome line drawings only.

These dinosaurs have been superseded by digital frame buffers and raster displays. But sometimes the concepts of random scan and display list are mixed up.

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