Question

我在使用CGBitmapContext操纵像素时遇到了一些非常奇怪的事情。基本上我正在改变PNG的Alpha值。我有一个例子，我可以成功地将PNG像素的Alpha值清零除了我的ImageView的superview是带图像的ImageView时......为什么会这样？我不确定这是一个错误还是上下文中的某些内容设置不正确。

以下是一个例子。它成功地将'alpha值'归零，但是当你给superview的UIImageView一个图像时，它就无法正常工作。应该具有Alpha为零的像素略微可见。但是当你从superview的UIImageView中删除图像时，Alpha值被正确归零，你会看到它背后的白色背景。我尝试过使用几种不同的CGBlendModes，但似乎没有这样做。

以下是展示此奇怪事件的示例项目的Dropbox链接： https://www.dropbox.com/s/e93hzxl5ru5wnss/TestAlpha.zip

以下是代码复制/粘贴：

        // Create a Background ImageView
        UIImageView Background = new UIImageView(this.View.Bounds);
        // Comment out the below line to see the Pixels alpha values change correctly, UnComment the below line to see the pixel's alpha values change incorrectly.  Why is this?  
        // When Commented out, The pixels whose alpha value I set to zero become transparent and I can see the white background through the image.  
        // When Commented in, I SHOULD see the "GrayPattern.png" Image through the transparent pixels.  I can kind of see it, but for some reason I still see the dirt pixels who's alpha is set to zero!  Is this a bug?  Is the CGBitmapContext set up incorrectly?  
        Background.Image = UIImage.FromFile("GrayPattern.png");
        this.View.AddSubview(Background);



        UIImageView MyImageView = new UIImageView(UIScreen.MainScreen.Bounds);
        UIImage sourceImage = UIImage.FromFile("Dirt.png");
        MyImageView.Image = sourceImage;
        Background.AddSubview(MyImageView);


        CGImage image = MyImageView.Image.CGImage;
        Console.WriteLine(image.AlphaInfo);
        int width = image.Width;
        int height = image.Height;
        CGColorSpace colorSpace = CGColorSpace.CreateDeviceRGB();
        int bytesPerRow = image.BytesPerRow;
        int bytesPerPixel = bytesPerRow / width;
        int bitmapByteCount = bytesPerRow * height;
        int bitsPerComponent = image.BitsPerComponent;
        CGImageAlphaInfo alphaInfo = CGImageAlphaInfo.PremultipliedLast;

        // Allocate memory because the BitmapData is unmanaged
        IntPtr BitmapData = Marshal.AllocHGlobal(bitmapByteCount);

        CGBitmapContext context = new CGBitmapContext(BitmapData, width, height, bitsPerComponent, bytesPerRow, colorSpace, alphaInfo);
        context.SetBlendMode(CGBlendMode.Copy);
        context.DrawImage(new RectangleF(0, 0, width, height), image);



        UIImageView MyImageView_brush = new UIImageView(new RectangleF(0, 0, 100, 100));
        UIImage sourceImage_brush = UIImage.FromFile("BB_Brush_Rect_MoreFeather.png");
        MyImageView_brush.Image = sourceImage_brush;
        MyImageView_brush.Hidden = true;
        Background.AddSubview(MyImageView_brush);


        CGImage image_brush = sourceImage_brush.CGImage;
        Console.WriteLine(image_brush.AlphaInfo);
        int width_brush = image_brush.Width;
        int height_brush = image_brush.Height;
        CGColorSpace colorSpace_brush = CGColorSpace.CreateDeviceRGB();
        int bytesPerRow_brush = image_brush.BytesPerRow;
        int bytesPerPixel_brush = bytesPerRow_brush / width_brush;
        int bitmapByteCount_brush = bytesPerRow_brush * height_brush;
        int bitsPerComponent_brush = image_brush.BitsPerComponent;
        CGImageAlphaInfo alphaInfo_brush = CGImageAlphaInfo.PremultipliedLast;

        // Allocate memory because the BitmapData is unmanaged
        IntPtr BitmapData_brush = Marshal.AllocHGlobal(bitmapByteCount_brush);

        CGBitmapContext context_brush = new CGBitmapContext(BitmapData_brush, width_brush, height_brush, bitsPerComponent_brush, bytesPerRow_brush, colorSpace_brush, alphaInfo_brush);
        context_brush.SetBlendMode(CGBlendMode.Copy);
        context_brush.DrawImage(new RectangleF(0, 0, width_brush, height_brush), image_brush);


        for ( int x = 0; x < width_brush; x++ )
        {
            for ( int y = 0; y < height_brush; y++ )
            {

                int byteIndex_brush = (bytesPerRow_brush * y) + x * bytesPerPixel_brush;
                byte alpha_brush = GetByte(byteIndex_brush+3, BitmapData_brush);
                // Console.WriteLine("alpha_brush = " + alpha_brush);

                byte setValue = (byte)(255 - alpha_brush);
                // Console.WriteLine("setValue = " + setValue);

                int byteIndex = (bytesPerRow * y) + x * bytesPerPixel;

                SetByte(byteIndex+3, BitmapData, setValue);

            }
        }

        // Set the MyImageView Image equal to the context image, but I still see all top image, I dont' see any bottom image showing through.  
        MyImageView.Image = UIImage.FromImage (context.ToImage());


        context.Dispose();
        context_brush.Dispose();

        // Free memory used by the BitmapData now that we're finished
        Marshal.FreeHGlobal(BitmapData);
        Marshal.FreeHGlobal(BitmapData_brush);







    public unsafe byte GetByte(int offset, IntPtr buffer)
    {
        byte* bufferAsBytes = (byte*) buffer;
        return bufferAsBytes[offset];
    }


    public unsafe void SetByte(int offset, IntPtr buffer, byte setValue)
    {
        byte* bufferAsBytes = (byte*) buffer;
        bufferAsBytes[offset] = setValue;
    }

更新我找到了一种解决方法（虽然它的性能对于我正在做的事情来说是非常不可接受的）解决方法是简单地从上面的代码中获取结果图像，将其转换为NSData，然后将NSData转换回UIIMage ......奇怪的是，图像看起来是正确的！这种解决方法;但是，我正在做的事情是不可接受的，因为它对我的图像执行此操作需要2秒钟，这对于我的应用程序。几百毫秒会很有趣，但2秒对我来说很长。如果这是一个错误希望它得到解决！

以下是解决方法（性能不佳）代码。只需将此代码添加到上面代码的末尾（或下载我的Dropbox项目并将此代码添加到其末尾并运行）：

        NSData test = new NSData();
        test = MyImageView.Image.AsPNG();
        UIImage workAroundImage = UIImage.LoadFromData(test);
        MyImageView.Image = workAroundImage;

这是图片之前和之后。之前没有解决方法，After包含解决方法。

enter image description here 请注意灰色不会完全显示出来。它覆盖了一些浅棕色，好像alpha值没有完全归零。

enter image description here 请注意，通过解决方法，您可以完美而清晰地看到灰色，根本没有浅棕色。我唯一做的就是将图像转换为NSData，然后再回到UIImage！很奇怪......

CGBitmapContext中的Pixel Manipulation取决于视图的superview？

0 个答案: