next up previous
Next: PermMap - Coordinate permutation Mapping
Up: AST Class Descriptions
Previous: Object - Base class for all AST Objects

   
PcdMap - Apply 2-dimensional pincushion/barrel distortion

Description:
A PcdMap is a non-linear Mapping which transforms 2-dimensional positions to correct for the radial distortion introduced by some cameras and telescopes. This can take the form either of pincushion or barrel distortion, and is characterized by a single distortion coefficient.

A PcdMap is specified by giving this distortion coefficient and the coordinates of the centre of the radial distortion. The forward transformation of a PcdMap applies the distortion:

RD = R * ( 1 + C * R * R )

where R is the undistorted radial distance from the distortion centre (specified by attribute PcdCen), RD is the radial distance from the same centre in the presence of distortion, and C is the distortion coefficient (given by attribute Disco).

The inverse transformation of a PcdMap removes the distortion produced by the forward transformation. The expression used to derive R from RD is an approximate inverse of the expression above.

Constructor Function:
AST_PCDMAP

Inheritance
The PcdMap class inherits from the Mapping class.

Attributes
In addition to those attributes common to all Mappings, every PcdMap also has the following attributes:

  • Disco: PcdMap pincushion/barrel distortion coefficient
  • PcdCen(axis): Centre coordinates of pincushion/barrel distortion

Functions
The PcdMap class does not define any new routines beyond those which are applicable to all Mappings.



next up previous
Next: PermMap - Coordinate permutation Mapping
Up: AST Class Descriptions
Previous: Object - Base class for all AST Objects

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
30th April 2003
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2003 Central Laboratory of the Research Councils