将某些像素的alpha设置为零

Question

假设我有2张图片，一张在另一张图片之上。我想将顶部图像的像素的alpha值的一部分设置为零，所以我得到如下所示的效果：

棕色是顶部图像，蓝色水是底部图像。最终我要做的是让顶部图像像素的alpha通道随着用户触摸iPad屏幕而变为零，这样他们就可以用指尖绘制并显示蓝色水像。

我有一个函数可以检查图像计算其alpha等于零的像素。我还有一个函数，将像素的alpha设置为零，但我认为它不起作用。 我可以读取alpha值，并设置alpha值。然后我可以重新读取alpha值以确认它已设置。但是当我设置新图像时，我看不出有什么区别！

以下是我示例项目的保管箱链接：

https://www.dropbox.com/s/e93hzxl5ru5wnss/TestAlpha.zip

以下是来源：

        Water = new UIImageView(this.Bounds);
        Water_OriginalImage = UIImage.FromFile("Media/Images/Backgrounds/WaterTexture.png");
        Water.Image = Water_OriginalImage;
        this.AddSubview(Water);

        Dirt = new UIImageView(this.Bounds);
        Dirt_ModifiableImage = UIImage.FromFile("Media/Images/Backgrounds/DirtBackground.png");
        Dirt.Image = null;
        this.AddSubview(Dirt);


        CGImage image = Dirt_ModifiableImage.CGImage;
        int width = image.Width;
        int height = image.Height;
        CGColorSpace colorSpace = image.ColorSpace;
        int bytesPerRow = image.BytesPerRow;
        int bitmapByteCount = bytesPerRow * height;
        int bitsPerComponent = image.BitsPerComponent;
        CGImageAlphaInfo alphaInfo = image.AlphaInfo;

        // 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);


        // Console output from this function call says "alpha count = 0"
        CountZeroedAlphas(BitmapData, Dirt_ModifiableImage);

        RemoveSomeAlpha(BitmapData, Dirt_ModifiableImage);

        // Console output from this function call says "alpha count = 2000".  So it seems that I have set the alpha of 2000 pixels to zero...
        CountZeroedAlphas(BitmapData, Dirt_ModifiableImage);


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


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




    public void CountZeroedAlphas( IntPtr bitmapData, UIImage Image )
    {
        int widthIndex = 0;
        int heightIndex = 0;
        int count = 0;

        while ( widthIndex < Image.Size.Width )
        {
            while ( heightIndex < Image.Size.Height )
            {
                PointF point = new PointF(widthIndex, heightIndex);
                var startByte = (int) ((point.Y * Image.Size.Width + point.X) * 4);
                byte alpha = GetByte(startByte, bitmapData);

                if ( alpha == 0 )
                    count++;

                heightIndex++;
            }


            widthIndex++;
        }

        Console.WriteLine("alpha count = " + count);

    }


    public void RemoveSomeAlpha( IntPtr bitmapData, UIImage Image )
    {
        int widthIndex = 0;
        int heightIndex = 0;

        while ( widthIndex < Image.Size.Width )
        {
            while ( heightIndex < Image.Size.Height )
            {
                PointF point = new PointF(widthIndex, heightIndex);
                var startByte = (int) ((point.Y * Image.Size.Width + point.X) * 4);
                ZeroByte(startByte, bitmapData);

                heightIndex++;
            }

            widthIndex++;
        }

    }


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


    public unsafe void ZeroByte(int offset, IntPtr buffer)
    {
        byte* bufferAsBytes = (byte*) buffer;
        bufferAsBytes[offset] = 0;
    }

Answer 1

搞定了！我不确定问题究竟是什么（假设我在事故或其他事情上访问了错误的字节），但我有工作代码。

This非常有用：

        // Create layers of matter on the battlefield.  Example:  Grass, dirt, water
        Water = new UIImageView(UIScreen.MainScreen.Bounds);
        Water_OriginalImage = UIImage.FromFile("WaterTexture.png");
        Water.Image = Water_OriginalImage;
        this.View.AddSubview(Water);

        Dirt = new UIImageView(UIScreen.MainScreen.Bounds);
        Dirt_ModifiableImage = UIImage.FromFile("DirtBackground.png");
        Dirt.Image = null;
        this.View.AddSubview(Dirt);



        CGImage image = Dirt_ModifiableImage.CGImage;
        int width = image.Width;
        int height = image.Height;
        CGColorSpace colorSpace = image.ColorSpace;
        int bytesPerRow = image.BytesPerRow;
        // int bytesPerPixel = 4;
        int bytesPerPixel = bytesPerRow / width;
        int bitmapByteCount = bytesPerRow * height;
        int bitsPerComponent = image.BitsPerComponent;
        CGImageAlphaInfo alphaInfo = image.AlphaInfo;

        // 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);

        int tempX = 0;

        while ( tempX < Dirt_ModifiableImage.Size.Width )
        {
            int tempY = 0;
            while ( tempY < Dirt_ModifiableImage.Size.Height )
            {
                int byteIndex = (bytesPerRow * tempY) + tempX * bytesPerPixel;


                ZeroByte(byteIndex+3, BitmapData);

                byte red = GetByte(byteIndex, BitmapData);
                byte green = GetByte(byteIndex+1, BitmapData);
                byte blue = GetByte(byteIndex+2, BitmapData);
                byte alpha = GetByte(byteIndex+3, BitmapData);


                //Console.WriteLine("red = " + red + " green = " + green + " blue = " + blue + " alpha = " + alpha);

                tempY++;
            }
            tempX++;
        }


            NSData newImageData = NSData.FromBytes(BitmapData, (uint)(bitmapByteCount));
            Dirt.Image = UIImage.LoadFromData(newImageData);