Apple tech note QA1037 says "To create an alpha-only bitmap context simply pass NULL for the colorspace parameter."
However, I am not sure what to pass for the bitmapInfo
parameter.
I was trying something like this, with kCGImageAlphaOnly:
CGContextRef ctx = CGBitmapContextCreate(NULL, scaledSize.width, scaledSize.height, 8, scaledSize.width, NULL, kCGImageAlphaOnly);
But this gives me an warning about the enum being wrong. What should I put there instead?
The docs clearly state you can use values from the CGImageAlphaInfo
even though the parameter is defined as needed a value from the CGBitmapInfo
enum.
One solution would be to cast the value:
(CGBitmapInfo)kCGImageAlphaOnly
That will make the compiler happy.
来源:https://stackoverflow.com/questions/17245787/how-do-i-create-an-alpha-only-bitmap-context