"Bresenham's" circle algorithm filling question

I've implemented "Bresenham's" (midpoint) algorithm to plot circles, and everything went well (C++ and OpenGL).

I am wondering now if it is possible to use the same algorithm to fill the circles?

Take the following few circles 开发者_高级运维for example: http://imgur.com/S0Qy6 which are plotted with the following algorithm:

void circle(Point p, int r) {
int x = 0;
int y = r;
int f = 1-r;

// plot vert/horiz points indepedently

while (x<y) {
    x++;

    if (f<0) {
        f += 2*x+1;
    } else {
        y--;
        f += 2*(x-y+1);
    }

    glRecti(p.x+x, p.y+y, p.x+x+1, p.y+y+1);
    // plot other points using 8 way symmetry

    // attempt to fill the circle - didn't go well
    plotLine(Point(p.x, p.y+x), Point(p.x+x, p.y+x));

}

}

I think the best place to put the filling bit would be in my main loop, as I want to essentially fill each quadrant again via 8-way symmetry. So I added my plotLine call, but what I ended up with was a strangely filled circle instead: http://imgur.com/jQW7B

If I then color the circles differently, I end up with horizontal banded stripes rather than actual filled circles - my approach is obviously wrong, but I'm not sure how to fix it.

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I'm not certain, but from squinting at your algorithm, I was expecting the last line to look more like

plotLine(Point(p.x+x, p.y+x), Point(p.x+x, p.y+y));

(that is, start on a northeast 45 degree line from p.x, then go up to the point on the circle that you just drew).

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As I recall the usual thing it to fill using either horizontal or vertical lines.

That is for each iteration you plot from Quadrant I to Quadrant II and from Quadrant III to Quadrant IV (where I chose horizontal lines). Notice that you get a total of four lines out of each iteration.

( p.x+x, p.y+y ) to ( p.x-x, p.y+y )
( p.x+y, p.y+x ) to ( p.x-y, p.y+x )
( p.x+x, p.y-y ) to ( p.x-x, p.y-y )
( p.x+y, p.y-x ) to ( p.x-y, p.y-x )

All of these lines are horizontal, and the start and end on all eight points. When your done the entire circle will be filled with no unnecessary redrawing (except possibly a double drawing of the last two lines when).

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The choice of horizontal or vertical may be completely arbitrary, but if you underlying graphics system draws lines faster in one direction than the other it could make a difference.