Multiple PS Eye Cameras Calibration

Calibration is a process of computing accurate camera positions and orientations from a video of user waving a small glowing object called marker (for color/color+depth cameras). This step is essential and required for multi-camera system setup.

Important! Once you calibrated the camera system, you should not move your cameras for subsequent video shoots. If you move at least one camera, you need to perform calibration again.

Tip: We recommend to run calibration twice - before and after capture session. If any camera was moved during capture session, calibration made after the session can give you correct camera positions.

Importance of high frame rate

You should record calibration video at the same resolution as your action video and at the same (or higher) frame rate.

Calibration at a different resolution may lead to reduced accuracy because cameras usually have different minor distortions at different resolutions (caused by internal scaling algorithm).

Calibration at low frame rate may lead to reduced accuracy because of increased synchronization errors.

Glowing Marker

Mini Maglite flashlight is recommended for calibration. This is a very common flashlight in US and many other countries. Removing flashlight reflector converts it into an ideal glowing marker easily detectable by motion capture software.

If you cannot get a Mini Maglite, you can use some other similar flashlight.

Alternatively, you can use Sony Move motion controller with white light turned on.

Step 1: Running iPi Recorder in Calibration Mode

Run iPi Recorder and choose one of the darkening modes in "darkening for calibration" list (for Sony PS Eye cameras)

or set Exposure to reasonably small value (for DirectShow-compatible web cameras)

This is important because it helps to reduce motion blur during calibration.

Video will look dim in calibration mode.

Step 2: Record Calibration Video

Start video recording.

Move the marker slowly through your entire capture volume (front-top-right-bottom-left-back-top-right-bottom-left). Start from top and move the marker in a descending spiral motion.

Tip: The exact trajectory of the marker is not so important, just try to cover the whole capture volume, or at least its perimeter.

Tip: For successful calibration, several hundred frames are sufficient. Much more frames (like several thousands) will not produce more accurate results, but instead will increase the processing time and possible number of marker detection errors. So generally it is not recommended to record calibration videos of more than 1 minute length.

Tip: You should make the marker visible to as much cameras as possible at all times. Hold the marker in the straight arm away from your body. In a circle configuration, when approaching the boundary of the capture area, position the marker inside the area, and your body outside.

Put the marker to the ground at each corner and at the center of capture volume. At least 4-5 ground points are needed for the correct detection of the groundplane.

Step 3: Stop Recording and Check Recorded Video

• There is no significant motion blur (image of marker looks like a round spot rather than an ellipse or a luminescent line)
• Most of the time marker is visible in at least 3 cameras and not completely obscured by human body

Step 4: Take Note of Height of Your First Camera Over the Ground.

Take note of height of your first camera over the ground. You will need this parameter later. If you cannot measure this height accurately, then at least make a rough estimation.

Step 5: Process Calibration Video in iPi Mocap Studio

Strictly speaking, you can postpone processing calibration video until after you finished recording other videos (e.g. your action videos). However, it is a good idea to process your calibration video as soon as it was recorded because it helps you ensure that you have good calibration. (Later on, incorrectly recorded calibration video may affect your ability to process action videos. )

To process calibration video please do the following:

• Load your calibration video to iPi Mocap Studio
• Important. Adjust the Region of Interest to cover the part of video that contains the glowing marker.
• Set the Diagonal Field of View (FOV) for your cameras on the “Scene” tab. If you use Sony PlayStation Eye or Logitech QuickCam 9000 cameras, leave the FOV value at the default 75 degrees.
• Go to “Calibration” tab. Check “Auto detect initial camera positions” checkbox. Click “Calibrate” button and wait while the system finishes calibration.
• Calibration algorithm may occasionally fail to find correct camera positions. If this happens, you should manually adjust initial camera positions to roughly match your configuration. The main thing is the correct order of the cameras around the capture area, and their approximate view directions.
  • Reset camera positions with one of the standard half-circle or full circle configurations that best suits your configuration.
  • For each camera that requires adjusting, switch to this camera using the toolbar button, and correct its position using controls on the “Scene” tab.
  • Uncheck “Auto detect initial camera positions” checkbox.
  • Click “Calibrate” button. This will rerun calibration process, without recomputing marker positions.

Step 6: Check Calibration Quality

Resulting scene should look like this:

Make sure you have Good or Perfect calibration result.

Important! Failed calibration is not recommended to use, as you will not be able to get accurate tracking results. However, sometimes Failed status can be misdetected. If detected marker positions are close to marker image on video in all frames for all sensors, you can use this calibration for tracking (5).

Step 7: Check Ground Plane

• Ground plane should be detected automatically. Ground points are marked by yellow color.
• Make sure that ground points are actually near the ground plane.
• If ground plane is detected incorrectly, select ground points manually.
  • For each point, click on it in 3D view and press Mark as ground button.
  • You can cancel marking point as ground point by pressing Unmark ground button.

Important! If you do not mark ground points then the ground plane is incorrect and there is no sense in using Foot tracking option and camera heights values.

Step 8: Set Scene Scale Using Сamera Height as Reference

Now cameras in your scene are properly oriented relative to other cameras and relative to ground plane. But you still need to find one more parameter: scene scale.

Use camera #1 height over ground to set correct scene scale.

• Save results to calibration project file or using Save scene... button on Scene tab (6).