Question

我想从UIImage创建一个UIView对象。视图不是不透明的。因为我使用以下代码：

+ (UIImage *) imageWithView:(UIView *)view 
{     
    UIGraphicsBeginImageContextWithOptions(view.bounds.size, view.opaque, [[UIScreen mainScreen] scale]);     
    [view.layer renderInContext:UIGraphicsGetCurrentContext()];     
    UIImage * img = UIGraphicsGetImageFromCurrentImageContext();  
    UIGraphicsEndImageContext();     
    return img; 
}

但由于某些原因而不是透明背景我得到的图像是黑色背景。我已尝试将UIGraphicsBeginImageContextWithOptions中的“opaque”参数设置为NO，但没有不同的结果。

有什么建议吗？

（类似问题CGContext transparency problem没有答案）

Answer 1

除非我遗漏了某些东西，否则我相信你可以通过将opaque设置为NO来获得所需的结果

UIGraphicsBeginImageContextWithOptions( CGSize size, BOOL opaque, CGFloat scale );


+ (UIImage *) imageWithView:(UIView *)view 
{     
    UIGraphicsBeginImageContextWithOptions(view.bounds.size, NO, [[UIScreen mainScreen] scale]);     
    [view.layer renderInContext:UIGraphicsGetCurrentContext()];     
    UIImage * img = UIGraphicsGetImageFromCurrentImageContext();     UIGraphicsEndImageContext();     
    return img; 
}

Answer 2

我很确定在创建像这样的UIImage时你无法定义透明度 - 你可以屏蔽使用图像的UIImageView。

类似以下功能的东西允许这种类型的操作：

// handles img masking operation
CGImageRef CopyImageAndAddAlphaChannel(CGImageRef sourceImage) {
    CGImageRef retVal = NULL;

    size_t width = CGImageGetWidth(sourceImage);
    size_t height = CGImageGetHeight(sourceImage);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef offscreenContext = CGBitmapContextCreate(NULL, width, height, 
                                                          8, 0, colorSpace, kCGImageAlphaPremultipliedFirst);

    if (offscreenContext != NULL) {
        CGContextDrawImage(offscreenContext, CGRectMake(0, 0, width, height), sourceImage);

        retVal = CGBitmapContextCreateImage(offscreenContext);
        CGContextRelease(offscreenContext);
    }

    CGColorSpaceRelease(colorSpace);

    return retVal;
}
    // png masks can mask png, gif or jpg...
- (UIImage*)maskImage:(UIImage *)image withMask:(UIImage *)maskImage 
{
    CGImageRef maskRef = maskImage.CGImage;
    CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                        CGImageGetHeight(maskRef),
                                        CGImageGetBitsPerComponent(maskRef),
                                        CGImageGetBitsPerPixel(maskRef),
                                        CGImageGetBytesPerRow(maskRef),
                                        CGImageGetDataProvider(maskRef), NULL, false);

    CGImageRef sourceImage = [image CGImage];
    CGImageRef imageWithAlpha = sourceImage;
    //add alpha channel for images that don't have one (ie GIF, JPEG, etc...)
    //this however has a computational cost
    if ((CGImageGetAlphaInfo(sourceImage) == kCGImageAlphaNone) || (CGImageGetAlphaInfo(sourceImage) == kCGImageAlphaNoneSkipFirst)) 
    { 
        imageWithAlpha = CopyImageAndAddAlphaChannel(sourceImage);
    }

    CGImageRef masked = CGImageCreateWithMask(imageWithAlpha, mask);
    CGImageRelease(mask);

    //release imageWithAlpha if it was created by CopyImageAndAddAlphaChannel
    if (sourceImage != imageWithAlpha) 
    {
        CGImageRelease(imageWithAlpha);
    }

    UIImage* retImage = [UIImage imageWithCGImage:masked];
    CGImageRelease(masked);

    return retImage;
}

您可以这样称呼它：

UIImage *imgMasked = [self maskImage:[UIImage imageNamed:@"full_image.png"] withMask:[UIImage imageNamed:@"masked_image.png"]];

masked_image.png需要是一个alpha通道图像，它会切掉你的黑色bg。

Answer 3

我刚遇到同样的问题。对我有用的是确保我正在使用PNG而不是JPG，因为JPG不支持alpha通道。希望这对你有用。

透明度问题 - 将UIView转换为UIImage时

3 个答案: