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Creating an Ink Input Control

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You can create a custom control that dynamically and statically renders ink. That is, render ink as a user draws a stroke, causing the ink to appear to "flow" from the tablet pen, and display ink after it is added to the control, either via the tablet pen, pasted from the Clipboard, or loaded from a file. To dynamically render ink, your control must use a DynamicRenderer. To statically render ink, you must override the stylus event methods (OnStylusDown, OnStylusMove, and OnStylusUp) to collect StylusPoint data, create strokes, and add them to an InkPresenter (which renders the ink on the control).

This topic contains the following subsections:

How to: Collect Stylus Point Data and Create Ink Strokes

To create a control that collects and manages ink strokes do the following:

  1. Derive a class from Control or one of the classes derived from Control, such as Label.

    using System;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Input.StylusPlugIns;
using System.Windows.Controls;
using System.Windows;

    class InkControl : Label
{

    }

  2. Add an InkPresenter to the class and set the Content property to the new InkPresenter.

    InkPresenter ip;

public InkControl()
{
    // Add an InkPresenter for drawing.
    ip = new InkPresenter();
    this.Content = ip;
}

  3. Attach the RootVisual of the DynamicRenderer to the InkPresenter by calling the AttachVisuals method, and add the DynamicRenderer to the StylusPlugIns collection. This allows the InkPresenter to display the ink as the stylus point data is collected by your control.

    public InkControl()
{

        // Add a dynamic renderer that
    // draws ink as it "flows" from the stylus.
    dr = new DynamicRenderer();
    ip.AttachVisuals(dr.RootVisual, dr.DrawingAttributes);
    this.StylusPlugIns.Add(dr);
}

  4. Override the OnStylusDown method. In this method, capture the stylus with a call to Capture. By capturing the stylus, your control will to continue to receive StylusMove and StylusUp events even if the stylus leaves the control's boundaries. This is not strictly mandatory, but almost always desired for a good user experience. Create a new StylusPointCollection to gather StylusPoint data. Finally, add the initial set of StylusPoint data to the StylusPointCollection.

    protected override void OnStylusDown(StylusDownEventArgs e)
{
    // Capture the stylus so all stylus input is routed to this control.
    Stylus.Capture(this);

    // Allocate memory for the StylusPointsCollection and
    // add the StylusPoints that have come in so far.
    stylusPoints = new StylusPointCollection();
    StylusPointCollection eventPoints =
        e.GetStylusPoints(this, stylusPoints.Description);

    stylusPoints.Add(eventPoints);
}

  5. Override the OnStylusMove method and add the StylusPoint data to the StylusPointCollection object that you created earlier.

    protected override void OnStylusMove(StylusEventArgs e)
{
    if (stylusPoints == null)
    {
        return;
    }

    // Add the StylusPoints that have come in since the
    // last call to OnStylusMove.
    StylusPointCollection newStylusPoints =
        e.GetStylusPoints(this, stylusPoints.Description);
    stylusPoints.Add(newStylusPoints);
}

  6. Override the OnStylusUp method and create a new Stroke with the StylusPointCollection data. Add the new Stroke you created to the Strokes collection of the InkPresenter and release stylus capture.

    protected override void OnStylusUp(StylusEventArgs e)
{
    if (stylusPoints == null)
    {
        return;
    }

    // Add the StylusPoints that have come in since the
    // last call to OnStylusMove.
    StylusPointCollection newStylusPoints =
        e.GetStylusPoints(this, stylusPoints.Description);
    stylusPoints.Add(newStylusPoints);

    // Create a new stroke from all the StylusPoints since OnStylusDown.
    Stroke stroke = new Stroke(stylusPoints);

    // Add the new stroke to the Strokes collection of the InkPresenter.
    ip.Strokes.Add(stroke);

    // Clear the StylusPointsCollection.
    stylusPoints = null;

    // Release stylus capture.
    Stylus.Capture(null);
}

How to: Enable Your Control to Accept Input from the Mouse

If you add the preceding control to your application, run it, and use the mouse as an input device, you will notice that the strokes are not persisted. To persist the strokes when the mouse is used as the input device do the following:

  1. Override the OnMouseLeftButtonDown and create a new StylusPointCollection Get the position of the mouse when the event occurred and create a StylusPoint using the point data and add the StylusPoint to the StylusPointCollection.

    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
{

    base.OnMouseLeftButtonDown(e);

    // If a stylus generated this event, return.
    if (e.StylusDevice != null)
    {
        return;
    }

    // Start collecting the points.
    stylusPoints = new StylusPointCollection();
    Point pt = e.GetPosition(this);
    stylusPoints.Add(new StylusPoint(pt.X, pt.Y));
}

  2. Override the OnMouseMove method. Get the position of the mouse when the event occurred and create a StylusPoint using the point data. Add the StylusPoint to the StylusPointCollection object that you created earlier.

    protected override void OnMouseMove(MouseEventArgs e)
{

    base.OnMouseMove(e);

    // If a stylus generated this event, return.
    if (e.StylusDevice != null)
    {
        return;
    }

    // Don't collect points unless the left mouse button
    // is down.
    if (e.LeftButton == MouseButtonState.Released ||
        stylusPoints == null)
    {
        return;
    }

    Point pt = e.GetPosition(this);
    stylusPoints.Add(new StylusPoint(pt.X, pt.Y));
}

  3. Override the OnMouseLeftButtonUp method. Create a new Stroke with the StylusPointCollection data, and add the new Stroke you created to the Strokes collection of the InkPresenter.

    protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
{

    base.OnMouseLeftButtonUp(e);

    // If a stylus generated this event, return.
    if (e.StylusDevice != null)
    {
        return;
    }

    if (stylusPoints == null)
    {
        return;
    }

    Point pt = e.GetPosition(this);
    stylusPoints.Add(new StylusPoint(pt.X, pt.Y));

    // Create a stroke and add it to the InkPresenter.
    Stroke stroke = new Stroke(stylusPoints);
    stroke.DrawingAttributes = dr.DrawingAttributes;
    ip.Strokes.Add(stroke);

    stylusPoints = null;
}

Putting it together

The following example is a custom control that collects ink when the user uses either the mouse or the pen.

using System;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Input.StylusPlugIns;
using System.Windows.Controls;
using System.Windows;

// A control for managing ink input
class InkControl : Label
{
    InkPresenter ip;
    DynamicRenderer dr;

    // The StylusPointsCollection that gathers points
    // before Stroke from is created.
    StylusPointCollection stylusPoints = null;

    public InkControl()
    {
        // Add an InkPresenter for drawing.
        ip = new InkPresenter();
        this.Content = ip;

        // Add a dynamic renderer that
        // draws ink as it "flows" from the stylus.
        dr = new DynamicRenderer();
        ip.AttachVisuals(dr.RootVisual, dr.DrawingAttributes);
        this.StylusPlugIns.Add(dr);
    }

    static InkControl()
    {
        // Allow ink to be drawn only within the bounds of the control.
        Type owner = typeof(InkControl);
        ClipToBoundsProperty.OverrideMetadata(owner,
            new FrameworkPropertyMetadata(true));
    }

    protected override void OnStylusDown(StylusDownEventArgs e)
    {
        // Capture the stylus so all stylus input is routed to this control.
        Stylus.Capture(this);

        // Allocate memory for the StylusPointsCollection and
        // add the StylusPoints that have come in so far.
        stylusPoints = new StylusPointCollection();
        StylusPointCollection eventPoints =
            e.GetStylusPoints(this, stylusPoints.Description);

        stylusPoints.Add(eventPoints);
    }

    protected override void OnStylusMove(StylusEventArgs e)
    {
        if (stylusPoints == null)
        {
            return;
        }

        // Add the StylusPoints that have come in since the
        // last call to OnStylusMove.
        StylusPointCollection newStylusPoints =
            e.GetStylusPoints(this, stylusPoints.Description);
        stylusPoints.Add(newStylusPoints);
    }

    protected override void OnStylusUp(StylusEventArgs e)
    {
        if (stylusPoints == null)
        {
            return;
        }

        // Add the StylusPoints that have come in since the
        // last call to OnStylusMove.
        StylusPointCollection newStylusPoints =
            e.GetStylusPoints(this, stylusPoints.Description);
        stylusPoints.Add(newStylusPoints);

        // Create a new stroke from all the StylusPoints since OnStylusDown.
        Stroke stroke = new Stroke(stylusPoints);

        // Add the new stroke to the Strokes collection of the InkPresenter.
        ip.Strokes.Add(stroke);

        // Clear the StylusPointsCollection.
        stylusPoints = null;

        // Release stylus capture.
        Stylus.Capture(null);
    }

    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
    {

        base.OnMouseLeftButtonDown(e);

        // If a stylus generated this event, return.
        if (e.StylusDevice != null)
        {
            return;
        }

        // Start collecting the points.
        stylusPoints = new StylusPointCollection();
        Point pt = e.GetPosition(this);
        stylusPoints.Add(new StylusPoint(pt.X, pt.Y));
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {

        base.OnMouseMove(e);

        // If a stylus generated this event, return.
        if (e.StylusDevice != null)
        {
            return;
        }

        // Don't collect points unless the left mouse button
        // is down.
        if (e.LeftButton == MouseButtonState.Released ||
            stylusPoints == null)
        {
            return;
        }

        Point pt = e.GetPosition(this);
        stylusPoints.Add(new StylusPoint(pt.X, pt.Y));
    }

    protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
    {

        base.OnMouseLeftButtonUp(e);

        // If a stylus generated this event, return.
        if (e.StylusDevice != null)
        {
            return;
        }

        if (stylusPoints == null)
        {
            return;
        }

        Point pt = e.GetPosition(this);
        stylusPoints.Add(new StylusPoint(pt.X, pt.Y));

        // Create a stroke and add it to the InkPresenter.
        Stroke stroke = new Stroke(stylusPoints);
        stroke.DrawingAttributes = dr.DrawingAttributes;
        ip.Strokes.Add(stroke);

        stylusPoints = null;
    }
}

Using Additional Plug-ins and DynamicRenderers

Like the InkCanvas, your custom control can have custom StylusPlugIn and additional DynamicRenderer objects. Add these to the StylusPlugIns collection. The order of the StylusPlugIn objects in the StylusPlugInCollection affects the appearance of the ink when it is rendered. Suppose you have a DynamicRenderer called dynamicRenderer and a custom StylusPlugIn called translatePlugin that offsets the ink from the tablet pen. If translatePlugin is the first StylusPlugIn in the StylusPlugInCollection, and dynamicRenderer is the second, the ink that "flows" will be offset as the user moves the pen. If dynamicRenderer is first, and translatePlugin is second, the ink will not be offset until the user lifts the pen.

Conclusion

You can create a control that collects and renders ink by overriding the stylus event methods. By creating your own control, deriving your own StylusPlugIn classes, and inserting them the into StylusPlugInCollection, you can implement virtually any behavior imaginable with digital ink. You have access to the StylusPoint data as it is generated, giving you the opportunity to customize Stylus input and render it on the screen as appropriate for your application. Because you have such low-level access to the StylusPoint data, you can implement ink collection and render it with optimal performance for your application.

See also