Can I draw on an existing CGImage directly?

Question

I want to draw on an existing image to achieve the accumulation effect. Currently, the only way I found is:

UIGraphicsBeginImageContext(rect.size);
CGContextRef context = UIGraphicsGetCurrentContext();
...
// draw the existing image
// draw other things
...
UIImage *color = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Can I draw on the the image directly?