ios – 将UIView转换为UIImage时出现透明度问题

我想从UIView创建一个UI Image对象.视图不透明.
因为我正在使用以下代码：

+ (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,而不是透明的背景,我得到一个黑色背景的图像.
我已经尝试将UIGraphicsBeginImageContextWithOptions中的“opaque”参数设置为NO,但没有不同的结果.

任何建议？

(类似的问题CGContext transparency problem没有答案)

解决方法

我确定你创建这样的UIImage时不能定义透明度 – 你可以掩盖使用图像的UIImageView.

类似以下功能的类似操作可以实现：

// 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,colorSpace,kCGImageAlphaPremultipliedFirst);

    if (offscreenContext != NULL) {
        CGContextDrawImage(offscreenContext,CGRectMake(0,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;
}

你可以这样称呼：

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

masked_image.png需要是一个alpha通道的图像,它将剪出你的黑色bg.