Best way to update a bitmap

Hello programmers. I'm trying to create a basic drawing app that follows touches and draws suitable lines. The app itself isn't important -- really it's just a prototype while I try and find out how I can update a bitmap without having to create a copy every time.

Currently, when I want to draw on an image, I do this:

Code:

UIImage* imageByDrawingDotOnImageAtPoint(UIImage* image, int x, int y)
{
	// begin a graphics context
	UIGraphicsBeginImageContext(image.size);
    
	// draw the original image into the context
	[image drawAtPoint:CGPointZero];
    
	// get the context for CoreGraphics
	CGContextRef ctx = UIGraphicsGetCurrentContext();
  
	[[UIColor redColor] setStroke];
        [[UIColor redColor] setFill];
    
	CGRect circleRect = CGRectMake(x, y, 10, 10);
	circleRect = CGRectInset(circleRect, 5, 5);
    
	// draw circle
	CGContextStrokeEllipseInRect(ctx, circleRect);
    
	// make image out of bitmap context
	UIImage *retImage = UIGraphicsGetImageFromCurrentImageContext();
    
	// free the context
	UIGraphicsEndImageContext();
    
	return retImage;
}

As you can see, the first thing I do is create an image context based on the size of the existing image, then I copy the old image in to the new image, then I use the context to do the drawing.

I guess what I want to do is keep hold of the context -- is that possible? Is my variable ctx in the example above always tied to the same image, or does it change based on the context changes that get invoked elsewhere?

I know I can go and try that out (and I will), but I'd like some feedback if there's anybody here who knows better. Certainly creating a copy of the entire image for each frame seems needlessly wasteful.

Thanks for any input.

 

Thanks very much for the pointer, I'll go through the guide.