velocity[x,y] += (height[x-1,y] +
                  height[x+1,y] +
                  height[x,y-1] +
                  height[x,y+1])/4.0 - height[x,y];

velocity[x,y]  =  velocity[x,y] * DAMPING_FACTOR;

height[x,y]   +=  velocity[x,y];