Projective AR calibration

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Motivation

There are different alternatives for visualizing Augmented Reality (AR) contents. Next to screens or head mounted displays an interesting alternative is given by projector technology which allows to overlay additional information onto real world objects by projecting them on available surfaces. An example of a projective AR scenario can be found at the very bottom of this article. The image shows a real car and a projective augmentation of virtual interior parts of the side door giving the illusion of an X-ray view.

In order to set up a projective AR scenario, the projector needs to be calibrated to identify

• The intrinsic parameters of the projector: e.g. focal length, principal point

• The extrinsic parameters of the projector: the offset between the projector's internal coordinate system and an external real world coordinate system (e.g. the camera coordinate system of the camera attached to the projector)

The following steps describe how to calibrate a projector using the metaio projector calibration software. The outcome is a calibration file including both the intrinsic and the extrinsic parameters of the projector calibration. This result file can then be used to set up a projective AR scenario in the Unifeye application.

Preparing the Setup

1. Connect the Projector and mount the camera rigidly on it.

2. Adjust the focus in a way that fits your scenario. Make sure not to change it after calibration.

3. Prepare projection

on Windows:

• Get the resolution of your projector. Enable the projector as an extended monitor and set its resolution so that it matches the projectors resolution.
• Open the installation directory of your Unifeye. The folder “…/ProjectorCalibration/projectorImages” contains several images with different resolutions. Choose the one that corresponds to the resolution of your projector. If none of the provided images has exactly the same resolution as your projector, you will need to rescale it such that it fits.
• For the calibration you will need to project the image in fullscreen mode and corresponding to your projectors resolution. For this reason, define the chosen image as Windows Desktop Background and set the value for ‘picture position’ to ‘centered’.

Recording image material for the calibration

1. Print a metaio marker ID1 and attach it to a planar (bright/white) surface. Please note: It is advisable to take the images used for calibration in the area where the later tracking should take place.

2. Ensure that the camera parameters (e.g. shutter and illumination) are set to values that allow all/most markers to be visible in good quality in the camera image.

3. Record approximate 10 images as some of them may be of bad quality and can’t be used for calibration later on. Vary the projectors position to obtain images shot from various perspectives.

4. The image projected by your projector should contain an empty quarter in the lower right. This is the area where the real marker should be. Make sure the real marker can be fully recognized. That means it has to be equally lighted and should not overlap with the projected markers. (see figure below)

Running the Projector-Camera Calibration Tool

1. Start the Projector-Camera Calibration Tool (Start -> Program files -> metaio -> Unifeye -> Calibration -> Projector Calibration)

2. Enter the required parameters:

	The path for your camera calibration file (xml/bin/ior)
	The path for the image you projected
	The number of images you want to use for calibration
	The physical size of your real marker
	The path for the resulting .xml file

3. Subsequently, add your images. You can select single images or multi-select a bunch of images and add them at once.

4. When the determined number of images to use was reached, the tool will compute the calibration, save the resulting .xml file to the specified direction and give you feedback about the reprojection error.

Please note:

• Compared to the Sextant Camera Calibration Tool the resulting reprojection error will be very high. The reason is the non-existing support of the camera lens undistortion of the rendering systems.
• The principal point in y-direction often is negative. This is okay, as the projector’s rays are tilted upwards.

Starting a Projective AR Application

1. Adjust the resolution of your main monitor: It has to be set to the same resolution as the projector which is your second monitor.

2. Start Unifeye (Start -> Program files -> metaio -> Unifeye -> Unifeye) and proceed as usual:

• Load the camera
• Load camera settings and camera calibration file
• Load the tracking data for your real marker

3. Load the models to be displayed.

• Load the test-model: projective_ar_testmodel.zip

OR

• Load your own models. Please note: The models should not be positioned on the COS-origin of your marker, since displaying it with the projector will negatively affect the marker tracking of the camera.

4. Open the "OST Configuration" tool in the Configuration panel of the Unifeye GUI and load the projector-camera calibration file. Afterwards click the "Activate OST" button. (see Figure below)

5. Move the whole Unifeye window on the second monitor (projector) and activate the fullscreen mode.

6. Check your Result: The models you defined should be placed at the specified position relative to the real marker. They should maintain their position even if you move or rotate the projector. If you used the test-model you can easily verify the projector-camera calibration result: The projected marker has to be displayed 20 cm left of the real one. (see figures below)

Projective AR Applications displaying the test-model (left) and customized models (right) relative to a real marker.

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-- SupportMetaio - 2011-01-21