Question

我在iOS5和iOS6上开发了一个应用程序。在我升级到XCode 5和iOS7后，我有一些新的bug。

主要的一个是colorMasking不再有效。完全相同的代码仍可编译并适用于iOS6手机。在iOS7上，蒙面颜色仍然存在。我试图在谷歌上找到答案，但还没找到答案。这是iOS7的错误，还是有人知道更好的做彩色掩模的方法吗？

以下是代码：

- (UIImage*) processImage :(UIImage*) image
{
    UIImage *inputImage = [UIImage imageWithData:UIImageJPEGRepresentation(image, 1.0)];
    const float colorMasking[6]={100.0, 255.0, 0.0, 100.0, 100.0, 255.0};
    CGImageRef imageRef = CGImageCreateWithMaskingColors(inputImage.CGImage, colorMasking);
    UIImage* finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    return finalImage;
}

以下是我发现的几个StackOverflow帖子，帮助我在iOS6中首先使用它： Transparency iOS iOS color to transparent in UIImage

Answer 1

我偶然发现了CGImageCreateWithMaskingColors与UIImagePNGRepresentation一起的一些奇怪行为。这可能与您的问题有关，也可能与您无关。我发现如果：

如果使用CGImageCreateWithMaskingColors并立即将该图片添加到图片视图，我可以看到透明度似乎已正确应用;
但是在iOS 7中，如果我那么：
- 从CGImageCreateWithMaskingColors获取此图片，并使用NSData创建UIImagePNGRepresentation;以及
- 如果使用NSData从imageWithData重新加载图片，则生成的图片将不再具有透明度。
要确认这一点，如果我writeToFile为此NSData并在Photoshop等工具中检查保存的图像，我可以确认该文件没有应用任何透明度。

这只能在iOS 7中体现出来。在iOS 6中它很好。
但如果我在步骤1中拍摄图像并通过drawInRect往返，则保存图像并随后加载图像的过程相同。

以下代码说明了问题：

- (UIImage*) processImage :(UIImage*) inputImage
{
    const float colorMasking[6] = {255.0, 255.0, 255.0, 255.0, 255.0, 255.0};
    CGImageRef imageRef = CGImageCreateWithMaskingColors(inputImage.CGImage, colorMasking);
    UIImage* finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);

    // If I put this image in an image view, I see the transparency fine.

    self.imageView.image = finalImage;                           // this works

    // But if I save it to disk and the file does _not_ have any transparency

    NSString *documentsPath           = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
    NSString *pathWithoutTransparency = [documentsPath stringByAppendingPathComponent:@"image-but-no-transparency.png"];
    NSData   *data                    = UIImagePNGRepresentation(finalImage);
    [data writeToFile:pathWithoutTransparency atomically:YES];   // save it so I can check out the file in Photoshop

    // In iOS 7, the following imageview does not honor the transparency

    self.imageView2.image = [UIImage imageWithData:data];        // this does not work in iOS 7

    // but, if I round-trip the original image through `drawInRect` one final time,
    // the transparency works

    UIGraphicsBeginImageContextWithOptions(finalImage.size, NO, 1.0);
    [finalImage drawInRect:CGRectMake(0, 0, finalImage.size.width, finalImage.size.height)];
    UIImage *anotherRendition = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    data = UIImagePNGRepresentation(anotherRendition);
    NSString *pathWithTransparency = [documentsPath stringByAppendingPathComponent:@"image-with-transparancy.png"];
    [data writeToFile:pathWithTransparency atomically:YES];

    // But this image is fine

    self.imageView3.image = [UIImage imageWithContentsOfFile:pathWithTransparency];   // this does work

    return anotherRendition;
}

Answer 2

我正在加载JPEG，由于某种原因加载了alpha通道，这在遮罩时不起作用，所以在这里我重新创建忽略alpha通道的CGImage。可能有一种更好的方法可以做到这一点，但这有效！

- (UIImage *)imageWithChromaKeyMasking {
    const CGFloat colorMasking[6]={255.0,255.0,255.0,255.0,255.0,255.0};
    CGImageRef oldImage = self.CGImage;
    CGBitmapInfo oldInfo = CGImageGetBitmapInfo(oldImage);
    CGBitmapInfo newInfo = (oldInfo & (UINT32_MAX ^ kCGBitmapAlphaInfoMask)) | kCGImageAlphaNoneSkipLast;
    CGDataProviderRef provider = CGImageGetDataProvider(oldImage);
    CGImageRef newImage = CGImageCreate(self.size.width, self.size.height, CGImageGetBitsPerComponent(oldImage), CGImageGetBitsPerPixel(oldImage), CGImageGetBytesPerRow(oldImage), CGImageGetColorSpace(oldImage), newInfo, provider, NULL, false, kCGRenderingIntentDefault);
    CGDataProviderRelease(provider); provider = NULL;
    CGImageRef im = CGImageCreateWithMaskingColors(newImage, colorMasking);
    UIImage *ret = [UIImage imageWithCGImage:im];
    CGImageRelease(im);
    return ret;
}

CGImageCreateWithMaskingColors不适用于iOS7

2 个答案: