ARUWPMarker Options in HoloLensARToolKit v0.2

This post is part of documentation of HoloLensARToolKit, version v0.2. The ARUWPMarker options documentation for v0.1 is here.

ARUWPMarker

ARUWPMarker.cs is one of the main scripts used in HoloLensARToolKit. In this post, the options of this script are listed and discussed, along with common usecases.

In Unity project using HoloLensARToolKit package, each actual marker must have one corresponding ARUWPMarker script.

ARUWPMarker is very similar to ARMarker in ARToolKit. However, because currently HoloLensARToolKit does not support NFT (Natural Feature Marker) of ARToolKit, that part of attributes are not included in ARUWPMarker.

When ARUWPMarker script is attached to some Unity GameObject, its inspector window looks like this:

Type

This field configures the type of the current marker. Available options are: single, single_barcode, single_buffer, and multi.

• single: single pattern marker, e.g. Hiro marker and Kanji marker. If single is selected, then the field File Name will appear. Root path of file name is Assets/StreamingAssets/.
• single_barcode: single barcode marker (as known as single matrix marker). If it is selected, then the field Barcode ID will appear.
• single_buffer: single pattern marker, but directly specified by a byte buffer. It should be set at runtime using function ARUWPMarker.SetSingleBufferBuffer().
• multi: multi marker consists of multiple barcode marker. If it is selected, then the field File Name will appear to ask for the configuration file. Root path of the configuration file is Assets/StreamingAssets/ as well.

Size in mm

You need to specify the size of the marker (outer border), when single or single_barcode marker is used.

Visualization Target

Marker pose provided by the algorithm contains position and orientation. This information should be applied to some object in the scene so that augmented reality experience is created. This field controls the target GameObject that receives the pose of marker. The common practice is to create an GameObject acting like a sub-scene, and actual 3D model to be overlaid can be attached to this sub-scene. In this case, the root of the sub-scene should be the Visualization Target.

Advanced Options

When this checkbox is checked, more options will appear in the inspector that enables you to configure the behavior of the marker with more freedom. The full list of options are shown below:

Filtered

If Filtered is checked, filtering options of marker tracking can be configured, including: Sample Rate and Cutoff Frequency. Please refer to ARToolKit documentation for details about filtering.

Continuous Pose Estimation

Please refer to ARToolKit documentation for details about Continuous Pose Estimation.

Confidence Cutoff

Please refer to ARToolKit documentation for details about Confidence Cutoff.

Anchored to World

HoloLens is constantly localizing itself in the room.

The default augmented reality experience of HoloLens is creating an AR scenario within the room, and let the user explore it. Therefore, HoloLensARToolKit package takes advantage of its localization algorithm, to make the virtual object appear in the world coordinate system. The benefit of doing it is that, when the marker is occluded, it stays at the same position in the room. It is desired when the line-of-sight of the HoloLens camera is blocked, but the tracked object is not moving in the environment.

If this field is not checked, then all the pose update is achieved in the camera coordinate system. The virtual object stays at the same pose with respect to the camera, when the tracking is lost.

Apply Rotation

Whether rotation of tracked marker should be applied to the Visualization Target.

Apply Translation

Whether translation (position) of tracked marker should be applied to the Visualization Target.

Apply Undistortion Function

The object to overlay and the marker seens by humans eye sometimes misalign, because the tracking system and the display system are never calibrated with respect to each other. However, if you are able to calculate the misalignment between them, it is possible to manually cancel that misalignment by putting an additional transformation on top of the tracking result. This field is the switch to enable or disable such undistortion. In addition to that, please use ARUWPMarker.magicMatrix1 to set the undistortion matrix of translation, and use ARUWPMarker.magicMatrix2 to optimize the rotation.

For more details about this technique, please refer to our arxiv paper.

Examples

Below are basic usage of three typical markers used in HoloLensARToolKit:

Hiro Marker

Matrix Marker (Number 0)

Cube Marker

Finally

You can access more articles describing the implementation details of HoloLensARToolKit in my blog, simply clicking on the tag: hololens-artoolkit.

Thanks for reading!

Coorindate Systems in HoloLensARToolKit v0.2

This post is part of documentation of HoloLensARToolKit, version v0.2. The documentation for the coordinate system in HoloLensARToolKit v0.1 is here.

Coordinate System of Unity3D

Unity3D uses left-hand coordinate systems to define transformations.

The above is a screenshot of a Unity transformation. The x, y, z axis directions are visualized with different colors, and are following left-land rules. In addition, rotation along axis is applied clockwise, unlike counter-clockwise in right-hand coordinate systems. If you are accustomed to right-hand coordinate systems as I do, please pay more attention when working with raw 4x4 matrices.

Coordinate System of ARToolKit

ARToolKit uses right-hand coordinate systems. Since OpenGL is right-handed as well, it is very comfortable to visualize the tracking result of ARToolKit by OpenGL.

In the official Unity package of ARToolKit, arunity, there exists a utility function

public static Matrix4x4 LHMatrixFromRHMatrix(Matrix4x4 rhm)

that converts the tracking result of ARToolKit to Unity environment.

Coordinate System of HoloLensARToolKit

It took me a while to figure out the complicated conversions happened inside ARToolKit, inside Unity, and in-between. Actually in HoloLensARToolKit v0.1, the coordinate system is not very clear nor consistent between different kinds of markers. In the current version v0.2, the design of coordinate system is improved, and a special Unity scene is designed to help visualize the coordinate system associated with the marker.

In this project, the tracking result is converted into left-handed coordinate system, with y-axis flipped. This is performed in the function ARUWPUtils.ConvertARUWPFloatArrayToMatrix4x4(). When the tracking result is applied to a Visualization Target of the current marker, there is no need to worry about any conversion.

The following examples show the coordinate system associated with each kind of marker.

Single Pattern Marker

Hiro marker and Kanji marker are examples of single pattern marker. They are defined using binary files.

The coordinate system associated with a single pattern marker looks like this:

Single Matrix Marker

Single matrix marker is pre-defined in ARToolKit. User is only required to provide the ID and type of matrix code associated when using them in applications. A full list of matrix markers are available in ARToolKit repository. The coordinate system associated with a matrix marker looks like this:

Multi Matrix Marker

Multi matrix marker is defined by a configuration file, that contains

• total number of matrix markers
• the ID of each matrix marker
• the transformation from the multi-matrix marker to individual matrix marker.

Multi-matrix marker is useful for robust tracking, thus is much more useful for accuracy-critical augmented reality applications. Iterative Closest Point (ICP) algorithm is the underlying method for the fusion of tracking information.

For example, for a multi-barcode-4x3 configuration, the coordinate system of the multi-marker looks like this:

Marker Configuration File

In HoloLensARToolKit v0.1, users need to adjust the multi-marker configuration file provided by ARToolKit, like this, to make it work with HoloLensARToolKit. Starting from v0.2, this step is not needed. The multi marker configuration file of ARToolKit and HoloLensARToolKit is EXACTLY SAME, which means, the description of multi marker is actually using right-hand coordinate system.

Finally

You can access more articles describing the implementation details of HoloLensARToolKit in my blog, simply clicking on the tag: hololens-artoolkit.

Thanks for reading!