jp.keijiro.apriltag - AprilTag package for Unity

jp.keijiro.apriltag is a Unity package that provides an implementation of the AprilTag tracker.

AprilTag is a marker based tracking system developed by the APRIL Robotics Laboratory at the University of Michigan. Please see the AprilTag web page for further details.

System requirements

• Unity 2020.2

At the moment, this package supports the following systems:

• Windows (x86-64), macOS (x86-64), Linux (x86-64), iOS (arm64), Android (arm64)

How to try the sample project

Clone this repository and play the WebCamTest.unity scene on Unity Editor.

The current version of the TagDetector component only supports the tagStandard41h12 tag set. You can download those tag images from the apriltag-imgs repository. Print some of them using a printer. You also can use a smartphone screen to display the tags.

The WebCamTest component uses the Field of View value of the main camera to estimate tag positions. You can try the sample without adjusting it, but it may give incorrect depth information. To get accurate tag positions, you should match the FoV value with the actual camera FoV.

For example, I'm using Zoom Q2n-4K video camera for testing, which gives about 78 degrees horizontal FoV at the mid-angle mode. So I changed the FOV Axis to "Horizontal" and the Field of View value to 78.

How to install the package

This package uses the scoped registry feature to import dependent packages. Please add the following sections to the package manifest file (Packages/manifest.json).

To the scopedRegistries section:

{
  "name": "Unity NuGet",
  "url": "https://unitynuget-registry.azurewebsites.net",
  "scopes": [ "org.nuget" ]
},
{
  "name": "Keijiro",
  "url": "https://registry.npmjs.com",
  "scopes": [ "jp.keijiro" ]
}

To the dependencies section:

"jp.keijiro.apriltag": "1.0.1"

After changes, the manifest file should look like below:

{
  "scopedRegistries": [
    {
      "name": "Unity NuGet",
      "url": "https://unitynuget-registry.azurewebsites.net",
      "scopes": [ "org.nuget" ]
    },
    {
      "name": "Keijiro",
      "url": "https://registry.npmjs.com",
      "scopes": [ "jp.keijiro" ]
    }
  ],
  "dependencies": {
    "jp.keijiro.apriltag": "1.0.1",
...

How to detect tags

At first, create the AprilTag.TagDetector object specifying the input image dimensions. You can run the detector in a lower resolution by specifying a decimation factor. It may improve the speed at the cost of accuracy and detection rate.

detector = new AprilTag.TagDetector(imageWidth, imageHeight, decimation);

Call the ProcessImage method every frame to detect tags from an input image. You can use a Color32 array to give an image. At the same time, you have to specify the camera FoV (horizontal) in degrees and the tag size in meters.

webcamTexture.GetPixels32(buffer);
detector.ProcessImage(buffer, fov, tagSize);

You can retrieve the detected tags from the DetectedTags property.

foreach (var tag in detector.DetectedTags)
    Debug.Log($"{tag.ID} {tag.Position} {tag.Rotation}");

Dispose the detector object when you no longer need it.

detector.Dispose();

For details, please check the WebCamTest.cs example, which shows how to detect tags from images with WebCamTexture.

Related repositories

• The original AprilTag project repository: https://github.com/AprilRobotics/apriltag
• Pre-generated tag images: https://github.com/AprilRobotics/apriltag-imgs
• A fork of the AprilTag repository used to build Unity plugin binaries. https://github.com/keijiro/apriltag