int _width = 500; // stagewidth
int _height = 500; // stageheight
int cellSize = 10; // resolution
int _w = _width/cellSize;
int _h = _height/cellSize;
int MAX = int(sq(_w)); // calculate number of cells
int near = 30; // affected area from mousepointer
float as = 15; // ageing speed
Cell [] cells = new Cell[MAX];
void setup() {
  size(500,500);  
  background(255);
  noStroke();
  for (int i = 0; i < MAX; i++)
    cells[i] = new Cell((i%_w)*cellSize, (i/_w)*cellSize, cellSize);
  rectMode(CENTER);
}
void draw() {
  background(255);
  for (int i = 0; i < MAX; i++) { 
    cells[i].draw();
  }
  burn(int(mouseX/cellSize),int(mouseY/cellSize),near/cellSize);
}
void burn(int x, int y, int radius){
  //cast square radius
  int topLeftX = x - radius;
  int topLeftY = y - radius;
  int bottomRightX = x + radius;
  int bottomRightY = y + radius;
  if(topLeftX < 0) topLeftX = 0;
  if(topLeftY < 0) topLeftY = 0;
  if(bottomRightX >= _w) bottomRightX = _w - 1;
  if(bottomRightY >= _h) bottomRightY = _h - 1;
  int wide = bottomRightX - topLeftX;
  int high = bottomRightY - topLeftY;
  int cellsInside = wide * high;
  int [] burnCells = new int [cellsInside];
  int i = 0;
  for(int row = topLeftX + topLeftY * _w; row < topLeftX + (topLeftY + high) * _w; row += _w){
    for(int col = 0; col < wide; col++){
	burnCells[i++] = row + col;
    }
  }
  for(i = 0; i < burnCells.length; i++){
    cells[burnCells[i]].stoke();
  }
}
class Cell {
  float xpos;
  float ypos;
  int xsize;
  int ysize;
  float r = 255;
  float g = 255;
  float b = 255;
  Cell(float xp, float yp, int _size) {
    xpos = xp;
    ypos = yp;
    xsize = _size;
    ysize = xsize;
  }
  void draw () {
    fill(r,g,b);
    rect(xpos,ypos,xsize,ysize);
  }  
  void stoke(){  
    if ((dist(xpos, ypos, mouseX, mouseY) < near) && (b >= 0)) {
	r-=as;
	b-=as;  
    } 
    if ((dist(xpos, ypos, mouseX, mouseY) < near) && (b < 0)) {
	r+=as;
    } 
    if ((dist(xpos, ypos, mouseX, mouseY) < near) && (b < 0) && (r >= 255) ) {
	g-=as;
    }
  }
}