for(int h = radius; h >= -radius; --h){ int w = 2 * sqrt( radius*radius - h*h ); int local_x = X - (w / 2); int local_y = Y - h; Rectangle(local_x, local_y, w, 1, color_r, color_g, color_b, color_a);} Rectangle is such a trivial function to code, I''m sure you can handle that 
If you want an unfilled circle, then simply draw a pixel at the start and end points.
