Consider, as a practical example of CmpMaps, two images of the sky. Suppose that for each image we have a Mapping which converts from pixel coordinates to a standard celestial coordinate system, say FK5 (J2000.0). If we wish to inter-compare these images, we can do so by using this celestial coordinate system to align them. That is, we first convert from pixel coordinates in the first image into FK5 coordinates and we then convert from FK5 coordinates into pixel coordinates in the second image.
If MAPA and MAPB are pointers to our two original Mappings, we could form a CmpMap which transforms directly between the pixel coordinates of the first and second images by combining these Mappings, as follows:
INTEGER ALIGNMAP, MAPA, MAPB ... CALL AST_INVERT( MAPB, STATUS ) ALIGNMAP = AST_CMPMAP( MAPA, MAPB, .TRUE., ' ', STATUS ) CALL AST_INVERT( MAPB, STATUS )
Here, we have used AST_INVERT () to invert MAPB before inserting it into the CmpMap because, as supplied, it converted in the wrong direction. Afterwards, we invert it again to return it to its original state. The CmpMap, however, will ignore this subsequent change ().
The forward transformation of the resulting CmpMap will now transform
from pixel coordinates in the first image to pixel coordinates in the
second image, while its inverse transformation will convert in the
AST A Library for Handling World Coordinate Systems in Astronomy