How to fill gap between concentric circle using two different colors?

Question

I have created UIView subclass which contains two concentric circles. I have fill the gap between them with some color. My code looks like following:

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextAddEllipseInRect(ctx, rect);
CGContextAddEllipseInRect(ctx, CGRectMake(rect.origin.x + self.thick,
                                          rect.origin.y + self.thick,
                                          rect.size.width - 2 * self.thick,
                                          rect.size.height - 2 * self.thick));
[self.fillColor set]; // Fill color is color value
CGContextEOFillPath(ctx);

This does fill the gap with one color, I was wondering if I can fill the gap using two different colors ? For example half of gap is filled with white color and other with gray.

Answer 1

The quick way would be to fill the circle's twice, clipping the region the second time.

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextAddEllipseInRect(ctx, rect);
CGContextAddEllipseInRect(ctx, CGRectMake(rect.origin.x + self.thick,
                                          rect.origin.y + self.thick,
                                          rect.size.width - 2 * self.thick,
                                          rect.size.height - 2 * self.thick));
[self.fillColor set]; // Fill color is color value
CGContextEOFillPath(ctx);

CGContextAddRect(ctx,tophalfRect);
CGContextClip();
CGContextAddEllipseInRect(ctx, rect);
CGContextAddEllipseInRect(ctx, CGRectMake(rect.origin.x + self.thick,
                                          rect.origin.y + self.thick,
                                          rect.size.width - 2 * self.thick,
                                          rect.size.height - 2 * self.thick));
[self.fillColor2 set]; // Fill color is color value
CGContextEOFillPath(ctx);

You could do this with fewer draw calls if you are worried about performance.

Answer 2

/*program for concentric circles with color fill*/
#include<stdio.h>
#include<conio.h>
#include<graphics.h>
void main()
{
clrscr();
int gd,gm;
gd=DETECT;
initgraph(&gd,&gm,"c:\\turboc3\\bgi");
int rad,i=1;
for(rad=100;rad>=10;rad++)
{
setcolor(i);
circle(340,220,rad);
setfillstyle(SOLID_FILL,i);
floodfill(340,220,i);
rad=rad-10;
i++;
}

getch();
closegraph();
}