Calculated the Mandelbrot Set and Julia Set in blocks

To my knowledge the Mandelbrot Set and Julia Set never have islands of color (pixels not in the set) within the black (pixels in the set). So Jason's Fractal Generator has an optimization where it calculates the outline of blocks (squares except at the edges) and if the whole outline is black then it assumes the inside of the block is black without calculating the pixels in the middle. If not it continues to calculate smaller and smaller squares.