Digital Ink in WPF

Originally available only as part of the Tablet PC SDK, digital ink is now integrated into Microsoft's Windows Presentation Foundation.  With the exception of handwriting recognition, Windows programmers can now leverage digital ink in their applications without additional SDKs or runtime libraries.

Enabling Ink

To enable the collection and display of ink input, developers need only include the InkCanvas element in their XAML document.

<Window x:Class="WindowsApplication1.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="WindowsApplication1" Width="550" Height="400">

<Grid>

                    <InkCanvas />

</Grid>

</Window>

When the above code is compiled, a small black square replaces the mouse cursor when the user hovers over the application window. When the user left-clicks and drags the mouse in the content area, "ink" is invoked and the user is able to draw. 

Like other tags in XAML, InkCanvas can be used in conjunction with other layout elements and controls. As such, it may be positioned or layered over other elements. The following example displays an image within an InkCanvas.

<Window x:Class="WindowsApplication1.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="WindowsApplication1" Width="550" Height="400">

          <Grid>

                   <InkCanvas>

                             <Image Source="http://www.google.com/intl/en/images/logo.gif" />

                   </InkCanvas>

          </Grid>

</Window> 

When compiled, the user is able to draw on the image because the graphic is a child of the InkCanvas element.

Controlling Stroke Appearance

Using just a bit of C#, it is also possible to control a number of ink properties including stroke color and thickness.  These properties are controlled using the DrawingAttributes class.

Using this class, you can either control the appearance of individual strokes using Stroke.DrawingAttributes or the default appearance of any new stroke collected by the InkCanvas using InkCanvas.DefaultDrawingAttributes.

Below is a list of properties supported by the DrawingAttributes class:

   Member names and descriptions taken from Microsoft's WinFX SDK

Using a few of the above properties, we can create a simple method for setting the default ink color to blue with a stroke thickness of 2px (2px by 2px).

private void setInkBlue(object sender, RoutedEventArgs e)

  {

      // set the DefaultDrawingAttributes for a blue pen.

      myInkCanvas.DefaultDrawingAttributes.Color = Colors.Blue;

      myInkCanvas.DefaultDrawingAttributes.Height = 2;

      myInkCanvas.DefaultDrawingAttributes.Width = 2;

  }

Editing with InkCanvas

InkCanvas also comes with a few "freebies".  One of these features is a built-in editing mode, allowing users to select, drag, resize, and erase ink strokes. 

There are two main properties available to control editing: InkCanvas.EditingMode and InkCanvas.EditingModeInverted.  The first property controls the tip of the stylus.  The second controls the "eraser".  Both of these properties use InkCanvasEditingMode to specify how stylus input is handled.

By default, the EditingMode of InkCanvas is set to Ink while EditingModeInverted is set to EraseByStroke.

However, InkCanvasEditingMode supports a number of settings: 

 Specifying either Select or EraseByStroke will override this setting providing alternate functionality. The following method sets the editing mode to select:

// selection

private void Select(object sender, RoutedEventArgs e)

{

   // set the EditingMode to Select

   myInkCanvas.EditingMode = InkCanvasEditingMode.Select;

}

 When this method is called, the user can select and drag a stroke or group of strokes by either directly clicking on the stroke or
"lassoing" a group of strokes.

A similar method can be created to set the editing mode to erase:

// erase

private void Erase(object sender, RoutedEventArgs e)

{

    // set the EditingMode to EraseByStroke

    myInkCanvas.EditingMode = InkCanvasEditingMode.EraseByStroke;

}

When this method is called, the user can erase ink on the canvas by holding down the left mouse button and moving the cursor (an eraser in this mode) over any stroke.