riktpunkt

Riktpunkt is the Swedish term for the principal point of a camera or optical system. It is the point where the optical axis intersects the image plane and thus represents the projection of the camera center onto the sensor. In an ideal pinhole model the riktpunkt coincides with the image center, but in real systems it can be offset due to manufacturing tolerances, sensor placement, and lens mounting.

In camera calibration and computer vision, the riktpunkt is one of the intrinsic parameters that describe how 3D points are mapped to the image. It is typically represented by the coordinates (cx, cy) in the camera’s intrinsic matrix, alongside the focal lengths (fx, fy) and sometimes a skew term. The intrinsic matrix is often written as [ [fx, s, cx], [0, fy, cy], [0, 0, 1] ], where cx and cy denote the riktpunkt.

Determining the riktpunkt occurs during calibration by analyzing images of known patterns (for example, checkerboards) to minimize reprojection error. The exact location can be influenced by sensor alignment, lens mounting, and post-processing steps, and may shift slightly when distortion corrections are applied.

In practice, each camera in a system has its own riktpunkt. In stereo setups, the relative positions of the riktpunkter affect epipolar geometry and 3D reconstruction accuracy. The term is commonly used in Swedish-language opto-geometric literature; in English, it is called the principal point. See also principal point, focal length.