Let us first consider how to create a ZoomMap. This is done very simply as follows:
#include "ast.h" AstZoomMap *zoommap; ... zoommap = astZoomMap( 2, 5.0, "" )
The first step is to include the header file ``ast.h'' which declares the interface to the AST library. We then declare a pointer of type AstZoomMap* to receive the result and invoke the function astZoomMap to create the ZoomMap. The pattern is the same for all other classes of AST Object--you simply prefix ``ast'' to the class name to obtain the function that creates the Object and prefix ``Ast'' to obtain the type of the returned pointer.
These functions are called constructor functions, or simply
constructors (you can find an individual description of all AST
functions in ) and the arguments
passed to the constructor are used to initialise the new Object. In
this case, we specify 2 as the number of coordinates (i.e. we
are going to work in a 2-dimensional
space) and 5.0 as the Zoom factor to be applied. Note that this is a C
double value. We will return to the final argument, an empty string,
shortly (
).
The value returned by the constructor is termed an Object pointer or, in this case, a ZoomMap pointer and is used to refer to the Object. You perform all subsequent operations on the Object by passing this pointer to other AST functions.
AST A Library for Handling World Coordinate Systems in Astronomy