Game Development Reference
In-Depth Information
Figure 6.4 Augmented reality using ARToolkit. (Illustration: Yvan Poupyrev, with permission)
camera's own parameters are also called its intrinsic parameters. Generally, they include
the distance between the optical centre and the image plane (focal length), and the
projection of the optical centre in the image plane. Parameters of distortion created
by the lens can also be added. All these parameters are obtained by a procedure called
calibration. Several methods are available, but the principle of all these methods is the
acquisition of images of an object whose geometrical structure is perfectly known. As
for the relative position of the two cameras with respect to each other, we talk about
the extrinsic parameters. For each pair of cameras, these parameters are rotation (R)
and translation (T).
The techniques used to calculate these parameters are similar to those used for
intrinsic parameters. To calculate the position of point M, it is still necessary to match
points m and m'. These correspondences can be found, more or less robustly, using
various techniques. For example, let's consider the epipolar geometry constraint. There
is a wealth of literature on the subject of calibration and 3D reconstruction (Faugeras,
1993; Horaud & Monga, 1995).
The problem can be seen in various cameras. The systems available in the market
often use more than 2 cameras thereby making it possible to increase the volume of
work and also to level the major problem of optical systems - the non-visibility of
the target. To sum up, we can say that all you need is two cameras which can see the
target.
6.4.3 Classification of trackers
The trackers are generally grouped on one of the following two principles: outside-in or
inside-out. In case of outside-in, the sensor is fixed and the markers are embedded in the
individual. The difficulty of this principle is that the reference points move relatively
less than the rotation of the target, which results in less accurate measurements in
rotation. In case of inside-out, the markers are fixed and the sensor is embedded. The