Question

如何设置一个UIImage的不透明度/alpha

Answer 1

1. 我只需要做到这一点，但认为史蒂芬的解决方案将是缓慢的。这应该图形硬件。创建UIImage的类别：- (UIImage *)imageByApplyingAlpha:(CGFloat) alpha {
UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0f);

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGRect area = CGRectMake(0, 0, self.size.width, self.size.height);

CGContextScaleCTM(ctx, 1, -1);
CGContextTranslateCTM(ctx, 0, -area.size.height);

CGContextSetBlendMode(ctx, kCGBlendModeMultiply);

CGContextSetAlpha(ctx, alpha);

CGContextDrawImage(ctx, area, self.CGImage);

UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

return newImage;
}