Using QR Codes

Intro

QR codes are a super fast and easy way to to locate an object, provide information from the environment, or localize two devices to the same coordinate space! HoloLens 2 and WMR headsets have a really convenient way to grab and use this data. They can use the tracking cameras of the device, at the driver level to provide QR codes from the environment, pretty much for free!

The only caveat is that tracking cameras are lower resolution, so they need big QR codes, or to be very close to the codes. They also only update around 2 times a second. But if that suits your needs? Then you’re in luck!

Pre-Requisites

QR code support is not built directly in to StereoKit, but it is quite trivial to implement! For this, we use the Microsoft MixedReality QR code library through its NuGet package. This will require a UWP StereoKit project, and the Webcam capability in the project’s .appxmanifest file.

So! That’s the pre-reqs for this guide!

Then in your code, you’ll be able to add this using statement and get access to the QRCodeWatcher, the main interface to the QR code functionality.

using Microsoft.MixedReality.QR;

Code

For code, we’ll start with our own representation of what a QR code means. Nothing fancy, we just want to show the orientation and contents of each code! So, pose, size, and data as text.

We’ll also include a function to convert the WMR QR code into our own. The only fancy stuff happening here is grabbing the Pose! The SpatialGraphNodeId contains a pose, but it’s in UWPs coordinate space. Pose.FromSpatialNode is a bridge function that will convert from UWP’s coordinates into our own.

struct QRData
{ 
    public Pose   pose;
    public float  size;
    public string text;
    public static QRData FromCode(QRCode qr)
    {
        QRData result = new QRData();
        result.pose = Pose.FromSpatialNode(qr.SpatialGraphNodeId);
        result.size = qr.PhysicalSideLength;
        result.text = qr.Data == null ? "" : qr.Data;
        return result;
    }
}

Ok, cool! Now here’s the data we’ll be tracking for this demo, the QRCodeWatcher is the object that’ll provide us QR data, watcherStart will let us filter out QR codes from other sesions, and poses is our list of unique QR codes that we can iterate through and draw.

QRCodeWatcher watcher;
DateTime      watcherStart;
Dictionary<Guid, QRData> poses = new Dictionary<Guid, QRData>();

Initialization is just a matter of asking for permission, and then hooking up to the QRCodeWatcher’s events. QRCodeWatcher.RequestAccessAsync is an async call, so you could re-arrange this code to be non-blocking!

You’ll also notice there’s some code here for filtering out QR codes. The default behavior for the QR code library is to provide all QR codes that it knows about, and that includes ones that were found before the session began. We don’t need that, so we’re ignoring those.

public void Initialize()
{
    // Ask for permission to use the QR code tracking system
    var status = QRCodeWatcher.RequestAccessAsync().Result;
    if (status != QRCodeWatcherAccessStatus.Allowed)
        return;

    // Set up the watcher, and listen for QR code events.
    watcherStart = DateTime.Now;
    watcher      = new QRCodeWatcher();
    watcher.Added   += (o, qr) => {
        // QRCodeWatcher will provide QR codes from before session start,
        // so we often want to filter those out.
        if (qr.Code.LastDetectedTime > watcherStart) 
            poses.Add(qr.Code.Id, QRData.FromCode(qr.Code)); 
    };
    watcher.Updated += (o, qr) => poses[qr.Code.Id] = QRData.FromCode(qr.Code);
    watcher.Removed += (o, qr) => poses.Remove(qr.Code.Id);
    watcher.Start();
}

// For shutdown, we just need to stop the watcher
public void Shutdown() => watcher?.Stop();

Now all we need to do is show the QR codes! In this case, we’re just displaying an axis widget, and the contents of the QR code as text.

With the text, all we’re doing is squeezing the text into the bounds of the QR code, and shifting it to be a little forward, in front of the code!

public void Update()
{
    foreach(QRData d in poses.Values)
    { 
        Lines.AddAxis(d.pose, d.size);
        Text .Add(
            d.text, 
            Matrix.TRS(
                d.pose.position + d.pose.Forward*d.size*0.1f, 
                Quat.FromAngles(0, 0, 180)*d.pose.orientation),
            Vec2.One * d.size,
            TextFit.Squeeze,
            TextAlign.XLeft | TextAlign.YTop);
    }
}

And that’s all there is to it! You can find all this code in context here on Github.