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astMatrixMap - Create a MatrixMap

This function creates a new MatrixMap and optionally initialises its attributes.

A MatrixMap is a form of Mapping which performs a general linear transformation. Each set of input coordinates, regarded as a column-vector, are pre-multiplied by a matrix (whose elements are specified when the MatrixMap is created) to give a new column-vector containing the output coordinates. If appropriate, the inverse transformation may also be performed.

AstMatrixMap $*$astMatrixMap( int nin, int nout, int form, const double matrix[], const char $*$options, ... )
The number of input coordinates, which determines the number of columns in the matrix.
The number of output coordinates, which determines the number of rows in the matrix.
An integer which indicates the form in which the matrix elements will be supplied.

A value of zero indicates that a full "nout" x "nin" matrix of values will be supplied via the "matrix" parameter (below). In this case, the elements should be given in row order (the elements of the first row, followed by the elements of the second row, etc.).

A value of 1 indicates that only the diagonal elements of the matrix will be supplied, and that all others should be zero. In this case, the elements of "matrix" should contain only the diagonal elements, stored consecutively.

A value of 2 indicates that a "unit" matrix is required, whose diagonal elements are set to unity (with all other elements zero). In this case, the "matrix" parameter is ignored and a NULL pointer may be supplied.

The array of matrix elements to be used, stored according to the value of "form".
Pointer to a null-terminated string containing an optional comma-separated list of attribute assignments to be used for initialising the new MatrixMap. The syntax used is identical to that for the astSet function and may include "printf" format specifiers identified by "%" symbols in the normal way.
If the "options" string contains "%" format specifiers, then an optional list of additional arguments may follow it in order to supply values to be substituted for these specifiers. The rules for supplying these are identical to those for the astSet function (and for the C "printf" function).
Returned Value:
A pointer to the new MatrixMap.
  • In general, a MatrixMap's forward transformation will always be available (as indicated by its TranForward attribute), but its inverse transformation (TranInverse attribute) will only be available if the associated matrix is square and non-singular.

  • As an exception to this, the inverse transformation is always available if a unit or diagonal matrix is specified. In this case, if the matrix is not square, one or more of the input coordinate values may not be recoverable from a set of output coordinates. Any coordinates affected in this way will simply be set to the value zero.

  • A null Object pointer (AST__NULL) will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.
Status Handling
The protected interface to this function includes an extra parameter at the end of the parameter list descirbed above. This parameter is a pointer to the integer inherited status variable: "int $*$status".

next up previous 264
Next: astNegate - Negate the area represented by a Region
Up: AST Function Descriptions
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AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
7th December 2012

Copyright (C) 2012 Science \& Technology Facilities Council