wpf inkcanvas 多点触控 解决部分屏幕画笔笔迹在绘制时会多出一段的问题

添加一个支持多点触摸的画布

/// <summary>
    /// 支持多点触摸的InkCanvas
    /// </summary
    public class MultiTouchCanvas : FrameworkElement
    {
        private InkCanvasProxy _inkCanvas = new InkCanvasProxy();
        private Grid transparentOverlay = new Grid();
        private StrokeType _strokeType = StrokeType.Stroke;
        private Dictionary<object, StrokeCollection> _strokes = new Dictionary<object, StrokeCollection>();
        private Dictionary<object, Stroke> _currentStroke = new Dictionary<object, Stroke>();

        private Color _penColor = Colors.Green;
        public Color PenColor{
            get{
                return this._inkCanvas.DefaultDrawingAttributes.Color;
            }
            set{
                this._penColor = value;
                this._inkCanvas.DefaultDrawingAttributes.Color = value;
                if (this.PenType == StrokeType.HighlighterStroke)
                {
                    value.ScA = System.Convert.ToSingle(0.5);
                    this._inkCanvas.DefaultDrawingAttributes.Color = value;
                }
            }
        }

        private double _penWidth = 4.0;
        public double PenWidth
        {
            get { return this._penWidth; }
            set
            {
                this._penWidth = value;
                this._inkCanvas.DefaultDrawingAttributes.Width = value;
                this._inkCanvas.DefaultDrawingAttributes.Height = value;
            }
        }

        private double _eraseWidth = 30.0;
        public double EraseWidth
        {
            get
            {
                return this._eraseWidth;
            }
            set
            {
                this._eraseWidth = value;
                this._inkCanvas.DefaultDrawingAttributes.Height = value;
                this._inkCanvas.DefaultDrawingAttributes.Width = value;
            }
        }

        public StrokeType PenType
        {
            get { return this._strokeType; }
            set
            {
                this._strokeType = value;
                if (this._strokeType == StrokeType.Stroke || this._strokeType == StrokeType.HighlighterStroke)
                {
                    this._inkCanvas.EditingMode = InkCanvasEditingMode.Ink;
                    this._inkCanvas.DefaultDrawingAttributes.Width = this.PenWidth;
                    this._inkCanvas.DefaultDrawingAttributes.Height = this.PenWidth;
                    this._inkCanvas.DefaultDrawingAttributes.Color = this.PenColor;
                }

                if (this._strokeType == StrokeType.HighlighterStroke)
                {
                    Color dColor = this.PenColor;
                    dColor.ScA = System.Convert.ToSingle(0.5);
                    this._inkCanvas.DefaultDrawingAttributes.Color = dColor;
                }

                if (this._strokeType == StrokeType.EraseByPoint)
                {
                    this._inkCanvas.EditingMode = InkCanvasEditingMode.EraseByPoint;
                    this._inkCanvas.DefaultDrawingAttributes.Width = this.EraseWidth;
                    this._inkCanvas.DefaultDrawingAttributes.Height = this.EraseWidth;
                }
            }
        }

        public Brush Background
        {
            get { return this._inkCanvas.Background; }
            set
            {
                this._inkCanvas.Background = value;
            }
        }

        protected override int VisualChildrenCount
        {
            get
            {
                return 2; //grid + inkcanvas
            }
        }
            
        public MultiTouchCanvas(){
            base.IsManipulationEnabled = true;
            this.transparentOverlay.Background = Brushes.Transparent;
            base.IsEnabled =true;

            this.InitInkCanvasPropertys();
            this._inkCanvas.DefaultDrawingAttributes = new DrawingAttributes();
            this._inkCanvas.DefaultDrawingAttributes.Color = Colors.Green;
            this._inkCanvas.DefaultDrawingAttributes.Width = 4;
            this._inkCanvas.DefaultDrawingAttributes.Height = 4;

            this.PenType = StrokeType.Stroke;
        }

        public void ClearStrokes(object device)
        {
            if (this._strokes.ContainsKey(device) && this._inkCanvas.Strokes != null && this._inkCanvas.Strokes.Count > 0)
            {
                StrokeCollection sc = this._strokes[device];
                this._inkCanvas.Strokes.Remove(sc);
                this._strokes.Remove(device);
            }
        }

        public StrokeCollection GetStrokes(object device)
        {
            return this._strokes.ContainsKey(device) ? this._strokes[device] : null;
        }

        #region Event handle
        protected override void OnPreviewTouchDown(TouchEventArgs e)
        {
            TouchPoint tp = e.GetTouchPoint(this);
            if (this._inkCanvas.EditingMode == InkCanvasEditingMode.Ink)
            {
                this._startStroke(e.Device, tp.Position);
            }
            else
            {
                if (this._inkCanvas.EditingMode == InkCanvasEditingMode.EraseByPoint)
                {
                    this._removeStroke(e.Device, tp.Position);
                }
            }

            e.Handled = true;
            base.Focusable = true;
            base.Focus();
            base.Focusable = false;
            e.TouchDevice.Capture(this);
        }

        protected override void OnPreviewTouchMove(TouchEventArgs e)
        {
            _handleTouchMove(e);
        }

        protected override void OnTouchUp(TouchEventArgs e)
        {
            e.TouchDevice.Capture(null); //
        }

        protected override void onm ouseDown(MouseButtonEventArgs e)
        {
            if (base.Visibility == System.Windows.Visibility.Visible)
            {
                if (this._inkCanvas.EditingMode == InkCanvasEditingMode.Ink)
                {
                    this._startStroke(e.Device, e.GetPosition(this));
                }
                else
                {
                    if (this._inkCanvas.EditingMode == InkCanvasEditingMode.EraseByPoint)
                    {
                        this._removeStroke(e.Device, e.GetPosition(this));
                    }
                }

                e.MouseDevice.Capture(this);
            }
        }

        protected override void onm ouseMove(MouseEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                if (this._inkCanvas.EditingMode == InkCanvasEditingMode.EraseByPoint)
                {
                    this._removeStroke(e.Device, e.GetPosition(this));
                    return;
                }
                if (this._strokes.ContainsKey(e.Device) && this._currentStroke.ContainsKey(e.Device))
                {
                    this._addPointToStroke(e.Device, e.GetPosition(this));
                }
                else
                {
                    this._startStroke(e.Device, e.GetPosition(this));
                }
            }
        }

        protected override void onm ouseUp(MouseButtonEventArgs e)
        {
            e.MouseDevice.Capture(null);
        }
        #endregion

        protected override Visual GetVisualChild(int index)
        {
            switch (index)
            {
                case 0: return this._inkCanvas;
                case 1: return this.transparentOverlay;
                default:
                    throw new ArgumentOutOfRangeException("index");
            }
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            this._inkCanvas.Measure(availableSize);
            this.transparentOverlay.Measure(availableSize);
            return this._inkCanvas.DesiredSize;
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            this._inkCanvas.Arrange(new Rect(finalSize));
            this.transparentOverlay.Arrange(new Rect(finalSize));
            return base.ArrangeOverride(finalSize);

        }

        protected override void OnInitialized(EventArgs e)
        {
            base.OnInitialized(e);
            base.AddVisualChild(this._inkCanvas);
            base.AddVisualChild(this.transparentOverlay);
        }
        private  void _handleTouchMove(TouchEventArgs e)
        {
            if(this._inkCanvas.EditingMode == InkCanvasEditingMode.EraseByPoint){
                this._removeStroke(e.Device , e.GetTouchPoint(this).Position);
                e.Handled = true;
                return ;
            }

            if(this._strokes.ContainsKey(e.Device) && this._currentStroke.ContainsKey(e.Device))
            {
                Stroke stroke = this._currentStroke[e.Device];
                StylusPointCollection sps = stroke.StylusPoints;
                if(sps != null){
                    TouchPoint tp = e.GetTouchPoint(this);
                    Point p = tp.Position;
                    this._addPointToStroke( e.Device , p);
                }
            }
            else
            {
                TouchPoint tp = e.GetTouchPoint(this);
                this._startStroke( e.Device ,tp.Position);
            }
            e.Handled = true;
        }

        private void _addPointToStroke(object device, Point position)
        {
            Stroke stroke = this._currentStroke[device];
            if (stroke != null)
            {
                StylusPointCollection spc = stroke.StylusPoints;
                if (spc != null)
                {
                    spc.Add(new StylusPoint(position.X, position.Y, 0.5f));
                }
            }
        }
            
        private void _removeStroke(object device, Point position)
        {
            for (int i = 0; i < this._inkCanvas.Strokes.Count; i++)
            {
                Stroke stroke = this._inkCanvas.Strokes[i];
                StrokeCollection sc = stroke.GetEraseResult(new Rect(position.X, position.Y, this._inkCanvas.DefaultDrawingAttributes.Width, this._inkCanvas.DefaultDrawingAttributes.Height));
                this._inkCanvas.Strokes.Replace(stroke, sc);
            }
        }

        private void _startStroke(object device, Point inputPosition)
        {
            StylusPointCollection stylusPointCollection = new StylusPointCollection();
            stylusPointCollection.Add(new StylusPoint(inputPosition.X, inputPosition.Y, 0.5f));

            if (stylusPointCollection.Count > 0)
            {
                Stroke stroke = new Stroke(stylusPointCollection);
                stroke.DrawingAttributes.Width = this._inkCanvas.DefaultDrawingAttributes.Width;
                stroke.DrawingAttributes.Height = this._inkCanvas.DefaultDrawingAttributes.Height;
                stroke.DrawingAttributes.Color = this._inkCanvas.DefaultDrawingAttributes.Color;
                this._inkCanvas.Strokes.Add(stroke);//添加到canvas
                if (this._currentStroke.ContainsKey(device))
                {
                    this._currentStroke.Remove(device);
                }
                this._currentStroke.Add(device, stroke);

                if (this._strokes.ContainsKey(device))
                {
                    this._strokes[device].Add(this._currentStroke[device]);
                    return;
                }

                this._strokes.Add(device, new StrokeCollection { this._currentStroke[device] });
            }
        }

        private void InitInkCanvasPropertys(){
            this._inkCanvas.Focusable = base.Focusable;
            this._inkCanvas.Background = Brushes.Transparent;
            this._inkCanvas.EditingMode = InkCanvasEditingMode.Ink;
            this._inkCanvas.UseCustomCursor = true;
        }
    }

添加辅助的代理类型

public enum StrokeType
    {
        Stroke,
        HighlighterStroke,
        EraseByPoint
    }

    public class InkCanvasProxy : InkCanvas
    {
        public InkCanvasProxy()
            : base()
        {
           // base.IsHitTestVisible = false;
          //  base.StylusPlugIns.Remove(base.DynamicRenderer);
        }
    }

测试

<Window x:Class="Metro.G.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="600" Width="600"
        xmlns:ll="clr-namespace:Metro.G">
    <Canvas Name="container" >
        <ll:MultiTouchCanvas Width="600" Height="600" x:Name="_canvas"></ll:MultiTouchCanvas>
   </Canvas>
</Window>

[参考]
多点触摸画板(MultiTouchCanvas)