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.¢
