How to Calibrate Camera Optics

Introduction

Calibration of camera optics is required to find out the values of the following intrinsic parameters:

• Focal length
• Shift of camera principal point from the center of image
• Radial distortion parameters (2 — 4 coefficients)
• Pixel aspect ratio (optional)
• Pixel shear (optional)

This procedure is strongly mandatory for modern wide-angle cameras including action cameras. The thing is that these cameras has significant distortion.

Note: As to web-cameras usable with iPi Motion Capture (Sony PS3 Eye and Logitech C922), they have optics with low distortion and principal point shift is not significant as a rule. For this reason camera optics calibration is not required for them. But if you want, you can calibrate them and apply results of this calibration to recorded video using Build Combined Video tool of iPi Recorder application.

Recording of Calibration Video

• Mark camera by some label to distinguish it from others.
• Set view-angle and resolution you're going to use during motion capture session(s).
• 

  Tip: 720p (1280 x 720) resolution is recommended for full body motion capture needs in most cases (except very large capture area).
• 

  Important! If you use GoPro action camera, please don't use SuperView mode because in this mode calibration will fail (thanks to special "dynamic stretches" from 4:3 to 16:9 format made by camera software).
• If your camera has zoom and/or auto-focus features then these features must be turned off before calibration because this procedure is valid only for fixed optics.
• Run iPi Recorder and click Calibrate Camera Optics button in toolbar:

• To calibrate camera optics special calibration pattern is used. You have to record this pattern shown on computer screen from different view points.
• Click Show Pattern button in toolbar of PATTERN tab of Camera Optics Calibration tool:

• Start video recording and shoot this pattern from different view angles. For each view:
  • Ensure the pattern fills most of the frame. It's OK if the whole screen doesn't feet into the frame.
  • Hold the camera still for a second.
• We recommend to use the following five view angles:

View Angle	How it Looks from Side	How it Looks from Top	How it Looks in Camera
1. Front
2. From the left
3. From the right
4. From the top
5. From the bottom

You can download and examine the following sample calibration video recorded by GoPro HERO4 Silver action camera (720p, Wide view): cam1-720p-wide.MP4 (13 MB)