WriteableBitmap上的异步操作

Question

I'm writing an application in WPF (C#) which does long operations on a collection of Bitmaps. To keep my application responsive, I decided to use another thread to perform the operations on bitmaps and report progress on a progressbar in main UI thread. I thought BackgroundWorker would do anything for me, but looks like it won't be that easy.

I have the following code:

public class ImageProcessor
{
    public Collection<WriteableBitmap> Pictures { get; private set; }
    private BackgroundWorker _worker = new BackgroundWorker();

    public ImageProcessor()
    {
        _worker.DoWork += DoWork;
    }

    public void DoLotsOfOperations()
    {
        _worker.RunWorkerAsync();
    }

    private void DoWork(object sender, DoWorkEventArgs e)
    {
        // operations on Pictures collection
    }
}

At runtime I load images using a standard open file dialog into the Pictures collection and then I invoke DoLotsOfOperations() method. But as soon as I try to access any of the properties of a single bitmap I get InvalidOperationException: "The calling thread cannot access the object because different thread owns it".

It is obviosly true - I loaded bitmaps and populated the collection in the UI thread and I try to read collection elements in another thread. So i tried different approaches:

• I passed the whole collection as a parameter of RunWorkerAsync method and got it back in DoWork method from e.Argument but then when I tried to read properties of a single bitmap I still got the same exception.
• I tried the same thing, this time passing a single bitmap as backgroundworker's argument and still I couldn't get any of the bitmap's properties, let alone bitmap's pixels.

So how can I access bitmap's data inside another thread (and preferably using BackgroundWorker)?

I don't know, maybe my whole approach is wrong. The general idea I want to achieve is:

1. User loads bitmaps which are then displayed in a window.
2. User clicks a button and a long operation on the bitmaps is performed but the UI is responsive (which allows user for example to cancel the opration) and the progress is reported on a progess bar.

Thanks in advance for any help.

Answer 1

1) Proxy class (without thread restriction)

    public class WriteableBitmapProxy
    {
        public IntPtr BackBuffer { get; set; }
        public int BackBufferStride { get; set; }
        public int PixelHeight { get; set; }
        public int PixelWidth { get; set; }
    }

2) Extension methods (unsafe)

    public class RGBColor
    {
        public byte R { get; set; }
        public byte G { get; set; }
        public byte B { get; set; }
        public uint Value
        {
            get
            {
                return (uint)(((uint)R << 16) + ((uint)G << 8) + (B) + ((uint)255 << 24));
            }
        }
    }

   public static RGBColor GetPixel(this WriteableBitmap bmp, uint x, uint y)
    {
        unsafe
        {
            if (y >= bmp.PixelHeight) return default(RGBColor);
            if (x >= bmp.PixelWidth) return default(RGBColor);


            // Get a pointer to the back buffer.
            uint pBackBuffer = (uint)bmp.BackBuffer;

            // Find the address of the pixel to draw.
            pBackBuffer += y * (uint)bmp.BackBufferStride;
            pBackBuffer += x * 4;

            byte* pCol = (byte*)pBackBuffer;
            return new RGBColor() { B = pCol[0], G = pCol[1], R = pCol[2] };
        }
    }

    public static void SetPixel(this WriteableBitmapProxy bmp, uint x, uint y, RGBColor col)
    {
        SetPixel(bmp, x, y, col.Value);
    }

    public static void SetPixel(this WriteableBitmapProxy bmp, uint x, uint y, uint value)
    {
        unsafe
        {
            if (y >= bmp.PixelHeight) return;
            if (x >= bmp.PixelWidth) return;

            // Get a pointer to the back buffer.
            uint pBackBuffer = (uint)bmp.BackBuffer;

            // Find the address of the pixel to draw.
            pBackBuffer += y * (uint)bmp.BackBufferStride;
            pBackBuffer += x * 4;

            // Assign the color data to the pixel.
            *((uint*)pBackBuffer) = value;
        }
    }

3) Procedure to fire long running operation in different thread

      var image = sender as Image;
        var bitmap = image.Source as WriteableBitmap;

        var prx = new WpfImage.MyToolkit.WriteableBitmapProxy()
        {
            BackBuffer = bitmap.BackBuffer,
            BackBufferStride = bitmap.BackBufferStride,
            PixelHeight = bitmap.PixelHeight,
            PixelWidth = bitmap.PixelWidth
        };

        bitmap.Lock();

        Thread loader = new Thread(new ThreadStart(() => 
        {


            Global_Histogramm(prx);

            Dispatcher.BeginInvoke(DispatcherPriority.Background,
                  (SendOrPostCallback)delegate { bitmap.AddDirtyRect(new Int32Rect(0, 0, prx.PixelWidth - 1, prx.PixelHeight - 1)); bitmap.Unlock(); }, null);

        }

        ));
        loader.Priority = ThreadPriority.Lowest;
        loader.Start();

4) Long Running operation implementation

    void Global_Histogramm(WpfImage.MyToolkit.WriteableBitmapProxy src)
    {
        int SrcX = src.PixelWidth;
        int SrcY = src.PixelHeight;

        double[] HR = new double[256];
        double[] HG = new double[256];
        double[] HB = new double[256];
        double[] DR = new double[256];
        double[] DG = new double[256];
        double[] DB = new double[256];
        uint i, x, y;

        //  wyzeruj tablice
        for (i = 0; i < 256; i++) HB[i] = HG[i] = HR[i] = 0;

        //  wypelnij histogramy R G B
        for (y = 0; y < SrcY; y++)
            for (x = 0; x < SrcX; x++)
            {
                var color = src.GetPixel(x, y);
                HB[color.B]++;
                HG[color.G]++;
                HR[color.R]++;
            };

        // oblicz histogramy znormalizowane i przygotuj dystrybuanty
        int ilosc_punktow = SrcX * SrcY;
        double sumaR = 0, sumaG = 0, sumaB = 0;

        for (i = 0; i < 256; i++)
        {
            DB[i] = sumaB + HB[i] / ilosc_punktow;
            DG[i] = sumaG + HG[i] / ilosc_punktow;
            DR[i] = sumaR + HR[i] / ilosc_punktow;
            sumaB = DB[i];
            sumaG = DG[i];
            sumaR = DR[i];
        };

        Dispatcher.BeginInvoke(DispatcherPriority.Background,
              (SendOrPostCallback)delegate { progressBar1.Maximum = SrcY - 1; }, null);



        // aktualizuj bitmape
        for (y = 0; y < SrcY; y++)
        {
            for (x = 0; x < SrcX; x++)
            {

                var stmp = src.GetPixel(x, y);
                var val = new WpfImage.MyToolkit.RGBColor()
                {
                    B = (byte)(DB[stmp.B] * 255),
                    G = (byte)(DG[stmp.G] * 255),
                    R = (byte)(DR[stmp.R] * 255)
                };
                src.SetPixel(x, y, val);                    
            };

            Dispatcher.BeginInvoke(DispatcherPriority.Background,
                  (SendOrPostCallback)delegate { progressBar1.Value = y; }, null);


        }
    }

5) Hope it proves the thing.

Answer 2

WriteableBitmap has explicit support for threading. But you have to follow the protocol, use the Lock() method in the thread to gain access to the BackBuffer.