sprat r,m;
void setup(){
  size(400,400);
  String[] a = {"lam.gif","lin.gif"};
  m = new sprat(220,230,a);
  r = new sprat(229,200,"lin.gif");
}

int c;

void draw(){
  m.x = mouseX;
  m.y = mouseY;

  c++;
  if(c % 50 == 0){
         m.changeframe(m.currentframe==0?1:0); 
  }

  if(m.overlap(r)){
    background(255,0,0); 
  } 
  else {
    background(128);    
  }
  r.draw();
  m.draw();
}

class sprat{
  int x,y,w,h;
  int framecount;
  int currentframe;
  PImage img[];
    sprat(int px,int py, String imgname){
      String s[] = new String[1];
      s[0] = imgname;
      init(px,py,s);
    } 
  sprat(int px,int py, String[] imgname){
    init(px,py,imgname);
  } 

  void init (int px,int py, String[] imgname){
    x = px; 
    y = py; 
    img = new PImage[imgname.length];
    for(int i = 0; i < imgname.length; i++){
    img[i] = loadImage(imgname[i]);      
    }
    changeframe(0);
  } 
  void changeframe(int whatframe){
      currentframe = whatframe;
      w = img[currentframe].width;
      h = img[currentframe].height;
  }

  void draw(){
    image(img[currentframe],x,y);
  }

  //so we have a physical x and y on screen
  //find the pixel for this thing
  //WARNING no bounds checking
  color pixelAtPhysicalLocation(int px, int py){
    int rx = px - x;
    int ry = py - y;
    return img[currentframe].pixels[rx + (ry * w)];
  }

  boolean overlap(sprat other){
    intRange vOverlap = spratOverlapVert(other);
    intRange hOverlap = spratOverlapHoriz(other);
    if(vOverlap == null || hOverlap == null){
      return false;
    }
    //hrrm, why couldn't this be <= ????
    for(int a = hOverlap.min; a < hOverlap.max; a++){
          for(int b = vOverlap.min; b < vOverlap.max; b++){
            if(alpha(this.pixelAtPhysicalLocation(a,b)) > 128 && alpha(other.pixelAtPhysicalLocation(a,b)) > 128    ){
               return true; 
            }
          }
    }
    
    return false;
  }

// to see if things overlap on one dimension
// we sort the 4 points. if both points of 
// one thing are lesser than both points of the other,
// they can't be overlapping...
intRange spratOverlapHoriz(sprat b){
  sprat a = this;
  intWithRef vals[] = new intWithRef[4];
  vals[0] = new intWithRef((int)a.x,a);
  vals[1] = new intWithRef((int)(a.x+a.w),a);
  vals[2] = new intWithRef((int)b.x,b);
  vals[3] = new intWithRef((int)(b.x+b.w),b);
  Arrays.sort(vals);
  if (vals[0].src == vals[1].src){
    return null; 
  }
  return new intRange(vals[1].val,vals[2].val);
}

intRange spratOverlapVert(sprat b){
  sprat a = this;
  intWithRef vals[] = new intWithRef[4];
  vals[0] = new intWithRef((int)a.y,a);
  vals[1] = new intWithRef((int)(a.y+a.h),a);
  vals[2] = new intWithRef((int)b.y,b);
  vals[3] = new intWithRef((int)(b.y+b.h),b);
  Arrays.sort(vals);
  if (vals[0].src == vals[1].src){
    return null; 
  }
  return new intRange(vals[1].val,vals[2].val);

}

}

class intRange{
  int min, max;
  intRange(int p1, int p2){
    min = p1<p2?p1:p2; 
    max = p1>p2?p1:p2; 
  }
}

class intWithRef implements Comparable{
  int val; 
  Object src;
  intWithRef(int pval,Object psrc){
    val = pval; 
    src = psrc;
  } 
  public int compareTo(Object o){
    return val - ((intWithRef)o).val;
  }

}