how can I draw the trail of joints with ellipse in kinect?

Question

The program I would like to make is like this:
Track the joint of right hand and on that coordinate, an ellipse is displayed.
I made this ellipse track my hand.

I want to make the trail of ellipse not disappear, so the ellipse can be used like a brush tool.
I added this statement,

canvasPaint.Children.Add(ellipse);

but it doesn't work. Actually, the program stops running because of this statement.
So, how can I draw multiple ellipses like in this video ?

I am developing in c# and xaml.
Do I need openni? is it necessary?

Answer 1

I solved this problem by myself :) The problem occurred because 1. I didn't make a new object and just added an existing ellipse. 2. I should have handled Zindex to print ellipses in front of Kinect video.

So I implemented a method drawEllipse,

void drawEllipse(SolidColorBrush c, double x, double y)
    {
        Ellipse ellipse = new Ellipse()
        {
            Fill = c,
            Height = 30,
            Width = 30,
            Opacity = 0.5
        };

        Canvas.SetLeft(ellipse, x);
        Canvas.SetTop(ellipse, y);
        canvasPaint.Children.Add(ellipse);

    }

and handled the property Zindex.

<Window x:Class="KinectPractice01.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="480" Width="640">
<Grid>
    <Image Height="441" Canvas.Left="10" Canvas.Top="0" Width="618" Name="image1" Panel.ZIndex="0"/>
    <Canvas Margin="0,0,0,0">
        <Canvas Name="canvasPaint" Width="640" Height="480" Margin="90,0,0,0" Panel.ZIndex="1"/>

        <TextBox Height="23" Canvas.Left="32" TextWrapping="Wrap" Text="null" Canvas.Top="10" Width="120" Name="textbox1"/>
        <Ellipse Opacity="0.5" Fill="White" HorizontalAlignment="Left" Height="30"  Stroke="Black" VerticalAlignment="Top" Width="30" Name="ellipse1"/>
        <Ellipse Opacity="0.5" Fill="Cyan" HorizontalAlignment="Left" Height="30"  Stroke="Black" VerticalAlignment="Top" Width="30" Name="ellipse2"/>
    </Canvas>


</Grid>