Determine if a CGImage is fully opaque

gistfile1.m

BOOL CGImageIsOpaque(CGImageRef image)
{
    size_t width = CGImageGetWidth(image);
    size_t height = CGImageGetHeight(image);
    unsigned char pixelData[width * height];
    CGContextRef context = CGBitmapContextCreate( pixelData, width, height, 8, width, NULL, (kCGBitmapAlphaInfoMask & kCGImageAlphaOnly) );
    CGContextDrawImage( context, CGRectMake(0, 0, width, height), image );
    CGContextRelease( context );
    
    for (NSInteger i = 0; i < width * height; ++i) {
        if (pixelData[i] < 255) return NO;
    }
    
    return YES;
}