// constants
  int Y_AXIS = 1;
  int X_AXIS = 2;
  
  void setup(){
    size(200, 200);
  
    // create some gradients
    // background
    color b1 = color(190, 190, 190);
    color b2 = color(20, 20, 20);
    setGradient(0, 0, width, height, b1, b2, Y_AXIS);
    //center squares
    color c1 = color(255, 120, 0);
    color c2 = color(10, 45, 255);
    color c3 = color(10, 255, 15);
    color c4 = color(125, 2, 140);
    color c5 = color(255, 255, 0);
    color c6 = color(25, 255, 200);
    setGradient(25, 25, 75, 75, c1, c2, Y_AXIS);
    setGradient(100, 25, 75, 75, c3, c4, X_AXIS);
    setGradient(25, 100, 75, 75, c2, c5, X_AXIS);
    setGradient(100, 100, 75, 75, c4, c6, Y_AXIS);
  }
  
  void setGradient(int x, int y, float w, float h, color c1, color c2, int axis ){
    // calculate differences between color components 
    float deltaR = red(c2)-red(c1);
    float deltaG = green(c2)-green(c1);
    float deltaB = blue(c2)-blue(c1);
  
    // choose axis
    if(axis == Y_AXIS){
      /*nested for loops set pixels
       in a basic table structure */
      // column
      for (int i=x; i<=(x+w); i++){
        // row
        for (int j = y; j<=(y+h); j++){
          color c = color(
          (red(c1)+(j-y)*(deltaR/h)),
          (green(c1)+(j-y)*(deltaG/h)),
          (blue(c1)+(j-y)*(deltaB/h)) 
            );
          set(i, j, c);
        }
      }  
    }  
    else if(axis == X_AXIS){
      // column 
      for (int i=y; i<=(y+h); i++){
        // row
        for (int j = x; j<=(x+w); j++){
          color c = color(
          (red(c1)+(j-x)*(deltaR/h)),
          (green(c1)+(j-x)*(deltaG/h)),
          (blue(c1)+(j-x)*(deltaB/h)) 
            );
          set(j, i, c);
        }
      }  
    }
  }