Example 5. Transforming a cursor position.

The following shows TRN_TR2R being used to transform 2-dimensional coordinate data obtained by reading a cursor. It is assumed that the identifier ID is associated with a compiled { $2 \rightarrow 2$} mapping which has been established to convert cursor coordinates into ``user'' coordinates:

*  Declare variables.
      INTEGER STATUS, N, ID
      REAL XCUR( 1 ), YCUR( 1 ), XUSER( 1 ), YUSER( 1 )

*  Set up cursor choice device.
      CALL SGS_SELCH( 0 )
      CALL SGS_DEFCH( '.' )

*  Loop to read cursor positions.
      DO WHILE ( STATUS .EQ. SAI__OK )
         CALL SGS_REQCU( XCUR( 1 ), YCUR( 1 ), N )      [1]
         IF( N .NE. 0 ) GO TO 1

*  Transform the cursor coordinates.
         CALL TRN_TR2R( .FALSE., 1, XCUR, YCUR, ID,     [2]
     :                  XUSER, YUSER, STATUS )

*  Display the position in user coordinates.
         CALL MSG_SETR( 'X', XUSER( 1 ) )
         CALL MSG_SETR( 'Y', YUSER( 1 ) )
         CALL MSG_OUT( ' ', 'X = ^X ; Y = ^Y', STATUS ) [3]
      ENDDO
    1 CONTINUE

Programming notes:

1. The SGS cursor position is read repeatedly until the user terminates interaction (by pressing the keyboard `.' key in this case).

2. TRN_TR2R is used to transform each cursor position in turn into ``user'' coordinates. Only a single data point is transformed here, but in a non-interactive application it would be more efficient to transform a whole set of data points in a single call to this routine.

3. The transformed positions are displayed in ``user'' coordinates.