修改图片的透明度

- (UIImage *)imageByApplyingAlpha:(CGFloat)alpha image:(UIImage*)image
{
    UIGraphicsBeginImageContextWithOptions(image.size, NO, 0.0f);
    
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGRect area = CGRectMake(0, 0, image.size.width, image.size.height);
    
    CGContextScaleCTM(ctx, 1, -1);
    CGContextTranslateCTM(ctx, 0, -area.size.height);
    
    CGContextSetBlendMode(ctx, kCGBlendModeMultiply);
    
    CGContextSetAlpha(ctx, alpha);
    
    CGContextDrawImage(ctx, area, image.CGImage);
    
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    UIGraphicsEndImageContext();
    
    return newImage;
}