I'm trying to generate an image that will ultimately be a heightmap, but I've run into a problem with modifying the pixels array. It seems like the color is looping back around to 0 from 255, for something of a solarized effect that I don't want. I've tried using constrain(foo,0,255) in a few spots, but I either get solid white circles or an all black screen.

Code:
void setup() {
 size(200,200);
 background(0);
 noLoop();
}

void draw() {
 loadPixels();
 
 // 20 hills
 for (int i = 0; i < 20; i++) {
   // hill radius
   int hillr = int(random(0,40));
   // hill location
   int hillx = int(random(hillr*2, width-hillr*2));
   int hilly = int(random(hillr*2, height-hillr*2));
   
   for (int x = 0; x < width; x++) {
     for (int y = 0; y < height; y++) {
       // hill height
       int hillh = hillr*hillr - (x - hillx)*(x - hillx) - (y - hilly)*(y - hilly);
       if (hillh > 0) {
         // brightness by height
         float bright = float(hillh) / 1600 * 255;
         pixels[x+y*width] += color(bright);
       }
     }
   }
 }
 updatePixels();
}