__________________________________››  COMMENTS ON THE "SNOW.ACT" PROGRAM›  __________________________________›››   I was very happy to read your›programming challenge, because the›snow problem was one I solved a few›months ago! In a couple of hours, my›ACTION! cartridge and I managed to›produce an illusion that was good›enough to fool everyone: people who›didn't know about the program thought›my TV was on the fritz, and the others›thought I was pulling their leg!››   The basic idea is simplicity ›itself. I created an oversize display›to eliminate the black border; each›line is in GR.8 mode with the LMS›option set; this way, I can instantly›change all 320 pixels on a line by›pointing its address somewhere else.›I created a 304-byte array, filled›with values taken from the Atari's›random number generator at 53770.›To make a totally different display,›I simply have to modify the random›array and reload each line with a new›address, pointing somewhere inside›that array. This is easy enough for›ACTION! to do once each 1/60th of a›second, so that the end result is a›constantly-changing display. Compare›to the normal method of putting random›bytes at random locations onscreen:›that method would update the display›only partially before the TV scan›process caught up with the processor,›giving a much less dynamic display.›(Reason: only part of each screen›would be different from the previous›one, and your eye wouldn't be fooled)››   Now that I've explained what goes›on behind the scenes, the program›itself is quite easy to read, even›if you haven't seen ACTION! code›before. I dimension three arrays and›set their location in memory so as to›be sure they don't get split by a 4K›boundary (a no-no with the ANTIC chip).›Same thing goes for other variables›like 'vcount' and 'dlist'; referencing›such variables makes a program much›more readable than using straight›addresses.››   The PROCedure MAKE_FLAKES stuffs›random values in the 'flakes' array.›Notice I loop from 0 to 151, storing›two values in each pass: this is›faster than going 304 times through›a single store. When you want to›update the screen every frame, you've›got to watch those machine cycles!››   MAKE_DISPLAY builds a custom›display list from scratch. Each line›will point to the address (flakes+›random). Since 'random' returns a›number between 0 and 255, each line›will then seem to have its own set of›random pixels. Nothing will prevent›two consecutive lines from pointing to›the same address, but since we rebuild›our display each 1/60 of a second the›effect will go by unnoticed.››   Once the display list is done, we›tell ANTIC where it's located and›change the colors to have a nice,›high-contrast black-and-white display.›Setting ANTIC to "wide playfield" mode,›along with having 228 lines of data,›effectively removes the border from the›TV picture. (The effect is much better›on a TV screen than on a monitor)›The main program sets everything up,›turns on a white noise-type sound, and›starts changing the screen every TV›frame. Notice how easy it was to do›the vertical roll bar: if a line is›inside the bar, I simply point to 48›bytes of blanks instead of 48 bytes of›random data. This adds almost no›processing time, yet greatly increases›the realism of the simulation.››   There you have it folks, yet another›shining example of the Atari supremacy›in the graphics field! We managed to›have our faithful, unexpanded 8-bit do›tricks that only a $999+ computer (ST›or Amiga) can! Not bad for a 7-year old›machine, eh?›                                    \›                      Charles Blaquiere››(can you typeset the accent in my name?›I'd be sooo grateful!)›LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL