Retrieving a pixel alpha value for a UIImage

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If all you want is the alpha value of a single point, all you need is an alpha-only single-point buffer. I believe this should suffice:

// assume im is a UIImage, point is the CGPoint to test
CGImageRef cgim = im.CGImage;
unsigned char pixel[1] = {0};
CGContextRef context = CGBitmapContextCreate(pixel, 
                                         1, 1, 8, 1, NULL,
                                         kCGImageAlphaOnly);
CGContextDrawImage(context, CGRectMake(-point.x, 
                                   -point.y, 
                                   CGImageGetWidth(cgim), 
                                   CGImageGetHeight(cgim)), 
               cgim);
CGContextRelease(context);
CGFloat alpha = pixel[0]/255.0;
BOOL transparent = alpha < 0.01;

If the UIImage doesn’t have to be recreated every time, this is very efficient.

EDIT December 8 2011:

A commenter points out that under certain circumstances the image may be flipped. I’ve been thinking about this, and I’m a little sorry that I didn’t write the code using the UIImage directly, like this (I think the reason is that at the time I didn’t understand about UIGraphicsPushContext):

// assume im is a UIImage, point is the CGPoint to test
unsigned char pixel[1] = {0};
CGContextRef context = CGBitmapContextCreate(pixel, 
                                             1, 1, 8, 1, NULL,
                                             kCGImageAlphaOnly);
UIGraphicsPushContext(context);
[im drawAtPoint:CGPointMake(-point.x, -point.y)];
UIGraphicsPopContext();
CGContextRelease(context);
CGFloat alpha = pixel[0]/255.0;
BOOL transparent = alpha < 0.01;

I think that would have solved the flipping issue.

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