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Creating Compass in Qt and Windows Phone

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This article demonstrates how to create a Compass in Qt and Windows Phone.

WP Metro Icon WP8.png
SignpostIcon WP7 70px.png
Article Metadata
Code ExampleTested with
SDK: Windows Phone SDK 8.0, Windows Phone SDK 7.1, Qt SDK v1.2
Devices(s): N8 (Nokia Belle), Nokia Lumia 820, Lumia 920
Compatibility
Platform(s): Windows Phone 8.0/ 7.1, Symbian^3
Windows Phone 8
Windows Phone 7.5
Device(s): All with compass sensors in platform
Platform Security
Signing Required: Self-Signed
Capabilities: sensors
Article
Keywords: Compass, Sensors
Created: somnathbanik (16 Apr 2012)
Last edited: hamishwillee (26 Jun 2013)

Contents

Introduction

In this article we will create a Compass in QML using QML Compass Element which is a part of QtMobility 1.x and for Windows Phone we will use Compass Class. This is a magnetic compass used to determine the direction of the earth's magnetic north pole.

Compass using QML

Implementation

Let’s create an empty project for both Qt and Windows Phone. First we will work on Qt project and then will move on to Windows Phone project.

Qt Project (MainPage.qml)

Let’s first create a circle using Rectangle Element as shown in the above figure. This circle will determine the compass outline. In QML Rectangle can be used to draw a circle with 1:1 dimensions and a radius.

Rectangle
{
id:circle
width: 250
height: 250
color: "transparent"
border.color: "green"
border.width: 3
radius: width*0.5
anchors.centerIn: parent
}

We add an Image to create the compass needle and rotate the Image with its change in degrees (angle).

 Image {
id: arrowImage
anchors.bottom: circle.verticalCenter
anchors.horizontalCenter: circle.horizontalCenter
height: circle.paintedHeight / 2
fillMode: Image.PreserveAspectFit
source: "upload.png"
rotation: -needleRotation
transformOrigin: Item.Bottom
}

QML Compass Element is used to get the change in magnetic north reading in the Compass.

 Compass {
id: compass
active: true
onReadingChanged: {
timerCompassCheck.running= false; //disable timer because Compass is working
mainPage.needleRotation = reading.azimuth;
}
}

We also set a Timer which expires after 7 seconds to check whether the compass is working properly (for example, it may not be supported on the device).

Timer {
id: timerCompassCheck
running: true
repeat: false
interval: 7000
onTriggered: {
compassTextError.visible=true;
}
}

As soon as the application launches the Timer starts running and expires after 7 seconds, the Timer doesn't repeat as the repeat property is set to false. When the onReadingChanged signal of the Compass Element is called it first stops the Timer and gets the compass reading and set it to needleRotation property which then used to rotate the compass needle angle. If within 7 seconds the signal onReadingChanged of the Compass Element is not called, then the Timer expires and displays a message that the device doesn't support compass. A Text element is being added to display the magnetic north value of the compass change on the device screen.

Text {
id:needleRotationText
text: needleRotation + " °N"
color: platformStyle.colorNormalLight
font.pixelSize: 35
anchors.top: parent.top
anchors.topMargin: 60
anchors.right: parent.right
anchors.rightMargin: 30
}

WP7 Project (MainPage.xaml)

To build a similar interface like Qt, let’s first create a circle using Ellipse Class which defines the outline of the compass.

<Ellipse StrokeThickness="2" x:Name="circle" Width="400" Height="400" 
Margin="0,90,0,0" Fill="Black">
<Ellipse.Stroke>
<SolidColorBrush Color="{StaticResource PhoneAccentColor}" />
</Ellipse.Stroke>
</Ellipse>

To create the compass needle we used Line Class.

<Line x:Name="line" X1="240" Y1="350" X2="240" Y2="270"  Stroke="{StaticResource PhoneForegroundBrush}" StrokeThickness="4"></Line>

Now we add the reference Microsoft.Devices.Sensors to initialize and detect the compass presence in the device.

Compass compass; 
if (Compass.IsSupported)
{
compass = new Compass();
compass.TimeBetweenUpdates = TimeSpan. FromSeconds(1);
compass.CurrentValueChanged += new EventHandler<SensorReadingEventArgs<CompassReading>>(compass_CurrentValueChanged);
compass.Start();
}
else
{
MessageBox.Show("Device doesn't suport compass");
}

TimeBetweenUpdates property will update the compass data in every second. CurrentValueChanged event handler will give the compass value each time there is any change in the compass needle position. And then we call the Start() method to start the compass. The change in compass value is then passed to the compass_CurrentValueChanged() method. Which then pass to a separate thread using Dispatcher.BeginInvoke() method.

void compass_CurrentValueChanged(object sender, SensorReadingEventArgs<CompassReading> e)
{
Dispatcher.BeginInvoke(() => UpdateLine(e.SensorReading));
}
private void UpdateLine(CompassReading e)
{
degreeText.Text = e.MagneticHeading.ToString("0");
line.X2 = line.X1 - (200 * Math.Sin(MathHelper.ToRadians((float)e.MagneticHeading)));
line.Y2 = line.Y1 - (200 * Math.Cos(MathHelper.ToRadians((float)e.MagneticHeading)));
}

The MagneticHeading gives the magnetic north pole value of the compass and is displayed in a TextBlock. We also add Microsoft.Xna.Framework reference into our project to find out the needle direction using MathHelper class.

Source Code

This page was last modified on 26 June 2013, at 09:56.
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