Sometimes we'd like a area fill algorithm that replaces all connected pixels of a selected color with a fill color. The flood-fill algorithm does exactly that.

	    public void floodFill(int x, int y, int fill, int old) {
	        if ((x < 0) || (x >= width)) return;
	        if ((y < 0) || (y >= height)) return;
	        if (raster.getPixel(x, y) == old) {
	            raster.setPixel(fill, x, y);
	            floodFill(x+1, y, fill, old);
	            floodFill(x, y+1, fill, old);
	            floodFill(x-1, y, fill, old);
	            floodFill(x, y-1, fill, old);
	        }
	    }
	

Flood fill is a small variant on a boundary fill. It replaces old pixels with the fill color.