LIGHTSPEED C FRACTAL GRAPHICS BY MATT RATCLIFF Here's everything you need to grow your own Julia fractal curves using Ralph Walden's Lightspeed C and your Atari 8-bit computer: You'll need MANDEL.COM, MANVIEW.COM and RUNTIME.OBJ from this month's disk. RUNTIME.OBJ is from the LIGHTSPEED C support library. It is automatically loaded whenever you run MANDEL.COM or MANVIEW.COM. (MANDEL.COM generates the fractal pictures and MANVIEW.COM displays them.) Although the Lightspeed C compiler is fast, MANDEL.COM isn't. Each image will take at least four hours, and maybe more than 24, depending on the colors and number of iterations chosen, but the results are worth the wait. GETTING STARTED From the DOS menu, choose option 'L' to Load D:MANDEL.COM. When the program begins, you'll be asked whether you want to make two-color or four-color fractal patterns. The two-color pictures are done in GRAPHICS 8 and the four-color pictures use GRAPHICS 7.5 (XL and XE only). The two-color pictures take longer to generate, since twice as many points are calculated and displayed. ALARM MODE Next, the program will ask if you want to use its "alarm" mode. Since the fractal images take so long to grow, you are not likely to sit in front of the monitor until it is finished. The alarm mode makes it easy to disconnect your disk drive from your spare Atari (that's growing your fractal curves) and attach it to your primary computer. Once the fractal curves have been drawn, an alarm will sound. Now, reconnect the disk drive and press the joystick button to shut off the alarm. Finally, follow the menu prompts to save the image to disk. IMPORTANT: Be sure your disk drive is turned OFF whenever you connect and disconnect it from your Atari! Next, the program will prompt you for a value for "pmax", the maximum real number axis value. This should be between -2.05 and 0.55. You are then prompted for "pmin," which should be less than pmax, of course. The following prompt is for "qmax", the maximum imaginary value to plot along the vertical axis. (Values above 1.25 are not very interesting.) The qmin is automatically caculated, so the height of the plot is the same as the width. Once the plot limits have been entered, you are asked for the maximum number of iterations. Values between 30 and 50 result in the best displays, in the shortest time. Due to the way the program is written, the iteration count should always be a multiple of the total number of colors (an even number for two color plots, for example), otherwise the resulting colors might not be what you expect. Next, you are prompted for a disk filename for your fractal image. Just type the name; the default drive number and extender will be taken care of. (You can enter a different drive number if you want, however.) Finally, all your data is redisplayed and you are given the opportunity to press [ESCAPE] to abort or [RETURN] to begin the plot. The graphics display is enabled and filled with white or blue, (depending on the number of colors selected), and the screen will suddenly go blank. MANDEL.COM uses Lightspeed's "Fast" command, which turns off the ANTIC chip, thereby turning off your screen. This speeds up the calculations by 2% to 15%. If you are curious to see your image-in-progress, just press the joystick button to see the display. When the button is released, the screen is blanked again. If you chose a plot area that is pretty much a dead zone -- all black -- then you can press [CONTROL] [A] to abort the plot. The program saves the file anyway, just in case. Since black points take longer to calculate than colored ones, MANDEL.COM fills the screen before plotting, making it easier to see the program's progress. MANVIEW Load MANVIEW.COM from DOS to take a look at your creations. You are first asked whether you'll view two- or four-color pictures, and then you're prompted for a file specification for a directory of picture files. If you just press [RETURN], a default of D1:*.PI2 or D1:*.PI4 will be used. Next, type in the filename of the picture and MANVIEW will display it. Press [ESCAPE] to return to DOS, or [RETURN] to view another picture. Should you return to the main menu, the parameters used for the previous plot are displayed. Pmax and pmin values should be chosen from the horizontal axis, labeled along the bottom of the graphic. Qmax can be selected from the vertical axis, labeled to the left of the page.
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