Mouse-over effect altering the circle size and color of circles processing

Hi, Viz-man. The trick here is to figure out some quick method for collision detection between two circles. When I have questions like this, the first thing I do is draw a picture, just like in good old high-school geometry. For example:



Looking at the picture, it seems pretty clear that the two circles intersect if the sum of their radii is less than or equal to the distance between their centers. You can use the dist() function to find the distance b/t centers, so you're pretty much half-way home at this point.

So, in your original example, you probably have the index of the circle the mouse is already touching. Let's say that is stored in a variable called targetIdx. What you need to do now is loop through the array of circles, do this distance test, and then return a flag (or set a value in another array, or update a field in a class, or whatever).

Here's a snippet that will show you how to do this if you're using a class

Class circle {

   int x, y, r;
   boolean is_touching_flag = false;

  ...
}

...

Circle[] c = new Circle[3];  //Suppose this is the array of circles, and that they get initialized somewhere

...

int targetIdx = 0;   //index of the circle we're hovering over
for (int j=0; j < c.length; j++) {
   float d = dist(c[targetIdx].x,c[targetIdx].y, c[j].x, c[j].y);
   if ( d < (c[targetIdx].r + c[j].r) {
      c[j].is_touching_flag = true;
   } else {
      c[j].is_touching_flag = false;  //be sure to reset this each time, or it will be wonky!
   }
}

Hope this helps!

Also, as a plug, O'Reilly has a great book called Getting Started with Processing that you should check out. It's by the authors of the language, and it's a fun, quick read.