Drawing optimized filled circles on Agar surfaces

/*
 * 8-way symmetric circle drawing (filled)
 * uses hline (above)
 * slightly inefficient (rewrites a couple of regions)
 * Public domain
 */
void
circleFilled(AG_Surface *s, int xCenter, int yCenter, int radius, Uint32 c)
{
        int x, y, r2;
 
        r2 = radius * radius;
 
        vline(s, xCenter, yCenter - radius, yCenter + radius, c);
       // hline(s, xCenter - radius, xCenter + radius, yCenter, c);
        y = radius;
        x = 1;
        y = (int) (sqrt(r2 - 1) + 0.5);
        while (x < y) {
            vline(s, xCenter + x, yCenter - y, yCenter + y, c);
            vline(s, xCenter - x, yCenter - y, yCenter + y, c);
            vline(s, xCenter + y, yCenter - x, yCenter + x, c);
            vline(s, xCenter - y, yCenter - x, yCenter + x, c);
            x += 1;
            y = (int) (sqrt(r2 - x*x) + 0.5);
        }
        if (x == y) {
            vline(s, xCenter + x, yCenter - y, yCenter + y, c);
            vline(s, xCenter - x, yCenter - y, yCenter + y, c);
        }
}