When images or sequences are captured with consumer level cameras at short focal lengths, distortion is introduced into the images by the system of lenses in the camera. Due to this distortion, images of straight-lines appear to be slightly curved. This distortion must be removed to ensure accuracy in later calibration and reconstruction phases. Typically, consumer grade cameras experience “barrel distortion” at short focal lengths, and small amounts of “pin-cushion distortion” at long focal lengths

The amount of geometric distortion in each image/video sequence is estimated by having the user identify lines in the image that are supposed to be straight. The user may draw multiple lines in different images, or in different frames of a video sequence. After at least one line has been placed, ICARUS can calculate the distortion parameters required to straighten them. If the user has marked a number of lines in different frames of a video sequence, distortion parameters may be calculated independently for each frame. The parameters for in-between frames are then interpolated from these results. Alternatively, if the video sequence has been captured at approximately a constant focal length, a single set of “average” parameters may be estimated for the entire sequence.

There are several different types of distortion parameter that may be calculated (see the reference literature for further details):

1.The centre of lens distortion,
2.Low order radial distortion,
3.Higher order radial distortion,
4.Low order tangential distortion,
5.Higher order tangential distortion.

The only parameters which normal users of the system need worry about are the first three. Generally, the centre of the lens distortion can be kept at the centre of the image. Tangential distortion may sometimes be necessary for non-standard lenses.

As well an removing lens distortion from a sequence (as described below), the distortion module may be used to reapply the lens distortion. This can be achieved by loading in a previous distortion file and changing the “Action” option in movie or image export dialog. Distortion files can be loaded and saved using the options in the “Project” menu.


HOW TO USE THE DISTORTION MODULE

Now we know the basics and it´s time to use our knowledge in the ICARUS Distortion Module.
Open the software, and make a new project. Load your image, and than draw some lines on your image. You need to set 3 points in one line (minimum for the calculation).