An exploration with the modulo operation and changing patterns on the X and Y axes to generate a circular texture.

void setup() {
  size(600, 400);
}

void draw() {
  background(255);
  stroke(0);
  strokeWeight(2);

  int step = 20;

  for (int i = step; i < width; i += step) {
    for (int j = step; j < height; j += step) {
      float diameter = (i + j) * 0.1;
      fill(i*7%255,j*7%255, 255);
      ellipse(i, j, mouseX, mouseY);
    }
  }
}