How to create a CGBitmapContext which works for Retina display and not wasting space for regular display?

Question

Is it true that if it is in UIKit, including drawRect, the HD aspect of Retina display is automatically handled? So does that mean in drawRect, th

Answer 1

After doing more research, I found the following solution:

If you have to use CGBitmapContextCreate, then there are two steps that can make the context with a size and coordinate system tailored to a standard display or Retina display:

float scaleFactor = [[UIScreen mainScreen] scale];

CGSize size = CGSizeMake(768, 768);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

CGContextRef context = CGBitmapContextCreate(NULL, 
                           size.width * scaleFactor, size.height * scaleFactor, 
                           8, size.width * scaleFactor * 4, colorSpace, 
                           kCGImageAlphaPremultipliedFirst);

CGContextScaleCTM(context, scaleFactor, scaleFactor);

The sample is to create a 768 x 768 point region, and on The New iPad, it will be 1536 x 1536 pixel. On iPad 2, it is 768 x 768 pixel.

A key factor is that, CGContextScaleCTM(context, scaleFactor, scaleFactor); is used to adjust the coordinate system, so that any drawing by Core Graphics, such as CGContextMoveToPoint, etc, will automatically work, no matter it is standard resolution or the Retina resolution.

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One more note is that UIGraphicsBeginImageContext(CGSizeMake(300, 300)); will create a 300 x 300 pixel on Retina display, while UIGraphicsBeginImageContextWithOptions(CGSizeMake(300, 300), NO, 0.0); will create 600 x 600 pixel on the Retina display. The 0.0 is for the method call to automatically give the proper size for standard display or Retina display.