★ CODING ★ LISTINGS ★ INTERLACE DEMO ★ |
| Interlace Demo | Coding Listings |
Just for a change, here's a program especially for green screen owners. (Colour users can run it, but the results are far from perfect.) Interlace doubles the onscreen resolution of your Amstrad in the y axis, that is rather than having 200 vertical pixels, it has 400. This Type-In is just a demo of the effect, but ardent programmers should be able to understand the method used. Interlace has not been readily available before for the CPC. Put simply, the electron beam in the monitor, rather than scanning down the screen once every fiftieth of a second to display a screen, goes down twice per scan. It's set to slightly different increments, so first it does once set of slices down the screen, and then the alternate set. The program works best in green because mono monitors are more persistent than colour ones, that is it takes longer for the image to fade away. This greatly reduces flicker, and in a pure machine code program it would dissappear entirely. Because twice as much screen area is being used, twice as much screen memory is needed to store it. So memory from &4000 is set aside to display half the screen, the other half sitting at &C000 as normal. The Amstrad continually flicks between the two halves while drawing the screen, and carries on when it's finished. (The picture is steadier once it has finished drawing.) In theory, you could use a similar method to display more colours than usual. Just think - true 16 colours in mode 1! Or rather, 16 shades of green... Well done JOHN VALENTINE of Eccles: a really idea provoking program for all those who were getting jaded with the CPC's display. AA
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