Creating Bitmap Context for Retina and regular iOS devices

When it is a Retina display iOS device, the resolution is 4 times as a regular display.  How is a bitmap context created in this case that can make use of the higher resolution, while for a regular display, a regular bitmap context is used so that memory isn't wasted?

The answer is using the [[UIScreen mainScreen] scale], and create the bitmap context accordingly.  But will any drawing routine also take special care to draw on this bitmap context because now the pixels on x and y-axis have both doubled?

The solution is that we can just do a transform, and everything will be taken care of.  By doing this, any drawing routine will not need to tailor to any particular size.  Moving to a point at (300, 300) will be actually moving to pixel (600, 600), but the drawing can just use (300, 300) for both a regular and Retina device.  The solution is:

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);

note that the last line, the CGContextScaleCTM is important.  It does the work of making (300, 300) to be the actually pixel (600, 600) on a Retina device.  The line that does the CGSizeMake(768, 768) is how big you'd like the bitmap context to be.  It works on a regular display and is automatically scaled up for a Retina display in the code above.