Lecture 8 --- 6.837 Fall '98

Boundary Fills

Boundary fills start from a point known to be inside of a region
and fill the region until a boundry is found.

A simple recursive algorithm can be used:


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

Lecture 8

Slide 5

6.837 Fall '98

A second important class of area-filling algorithms start at a point know to be inside a figure and start filling in the figure outward from the point. Using these algorithms a graphics artist may sketch the outline of a figure and then select a color or patten from a menu with which to fill it. The actual filling process begins when a point inside of the figure is slected. These routines are like the paint-can function seen in common interactive paint packages.

The first such method that we will discuss is called the boundary-fill algorithm. The boundary-fill method requires the coordinate of a starting point, a fill color, and a background color as arguments.

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

Note: that this is a recusive routine. Each invocation of boundaryFill( ) may call itself four more times.

The logic of this routine is very simple. If we are not either on a boundary or already filled we first fill our point, and then tell our neighbors to fill themselves.