Rosio Pavoris a blog

I’ve written this, so I might as well share it.

In my post on the Mandelbrot set earlier, I mentioned the Julia sets of the quadratic polynomial f_c(z) = z² + c where c is a given (constant) complex number and z are the points of the complex plane. Because I wanted to visualise how those Julia sets changed as c varied, I’ve written a short program to do that for me.
You can find it here. As usual, you’ll need Allegro, and the compilation instruction is on the first line.

What it does is take two complex numbers as parameters, plus the number of steps it should take to go from the first to the second. At each step, it will calculate and display the Julia set of the quadratic polynomial with that complex number as c, and hopefully your computer is fast enough that the successive Julia sets look like an animation.
For example, if you invoke it as:

./julia -0.8 -1 -0.8 1 200

you’ll see the following:

Though probably not at the same speed. I’ve made no effort to maintain a certain frame rate; the whole thing moves as quickly as your CPU can keep up, because I just wanted a visualisation of how Julia sets change, not a screensaver. If it’s moving too slowly for you, you can try reducing the number of steps, or lowering the numbers in the ZOOM or ITERS #defines, though the first one will make the image smaller and the second will make it darker. If you aren’t interested in the window title, you can also remove the snprintf and set_window_title steps for a significant speed-up.
If it’s too fast, you can do the reverse, or you can build in a delay with Allegro’s install_timer and rest, or POSIX’s usleep or nanosleep.

(Once it’s done, it will just pause at the last Julia set. Press any key to close it. If you want to close it before it’s done, you’ll have to kill it manually.)

The interesting points to explore are the ones inside the Mandelbrot set, as anything else will just be Fatou dust (though you’ll probably still be able to see it because of the grey). For those points, the salient area is the one within 2 unit lengths of the origin, which is why the field displayed ranges from (-2, 2) in the top left corner to (2, -2) in the bottom right (or probably (-2, -2) to (2, 2), I don’t remember). If you need a bigger plane, replace all instances of ZOOM * 4 with ZOOM * (bigger number), and all instances of ZOOM * 2 with ZOOM * (half of bigger number) (if you want to keep the origin in the center of the window).
If you actually want to save the animations, man 3alleg save_bitmap and assemble the images yourself in something like the GIMP. I initially started out doing it this way, but animated GIFs get really big really quickly, so I went with this instead.

Enjoy.