SYNOPSIS

CALL STUMTA (XDA,YDA,XUS,YUS,XND,YND,DU,DV,TA,IST)

DESCRIPTION

XDA

(REAL, input): The X coordinate of a point on the streamline in the data coordinate system.

YDA

(REAL, input); The Y coordinate of a point on the streamline in the data coordinate system.

XUS

(REAL, input): The X coordinate of the point in the user coordinate system.

YUS

(REAL, input): The Y coordinate of the point in the user coordinate system.

XND

(REAL, input): The X coordinate of the point in NDC space.

YND

(REAL, input): The Y coordinate of the point in NDC space.

DU

(REAL, input): The interpolated value of the normalized component of the vector at the point, with direction parallel to the X axis of the data coordinate system.

DV

(REAL, input): The interpolated value of the normalized component of the vector at the point, with direction parallel to the Y axis of the data coordinate system.

TA

(REAL, output): The directional angle of the streamline at the point relative to NDC space.

IST

(REAL, output); Status of the mapping operation: 0 indicates success, negative values indicate that the mapping failed; positive values are reserved and should not be used by the implementor of a mapping routine.

USAGE

STUMTA is a user-modifiable routine provided to support custom mappings of the data coordinate space. The user does not invoke it directly. Instead, whenever the parameter MAP specifies a mapping not handled by Streamlines internally (i.e., when MAP is set to a value other than 0, 1, or 2), Streamlines calls STUMTA once for each incremental step in the creation of a streamline. The default version of STUMTA simply returns the angle implied by the incremental vector components passed to it: that is, it returns ATAN2(DV,DU). In order to implement a custom mapping, you must pick a unique mapping code (a positive integer greater than 2), and then modify each of the three routines, STUMXY, STUIXY, and STUMTA to recognize and respond consistently to the chosen code. In the standard distribution of NCAR Graphics, these three routines reside in a single file, 'stumxy.f'. STUMXY maps a point from data to user coordinate space and STUIXY inversely maps a point from user to data coordinate space. STUMTA, which is likely to be the most difficult to implement, finds the tangent angle of the streamline at a point in NDC space.

STUMTA has access to a common block called STMAP that contains a number of variables used to record the current transformation state. In order to accommodate a variety of mapping implementations, STMAP provides more information than normally required. Consider the values stored in STMAP as strictly read-only. One essential member of this common block is IMAP, which contains the value currently assigned to the MAP parameter.

When implementing a non-linear mapping, an iterative differential technique will most likely be required. Look at the routine, STMPTA, in 'stmpxy.f', which handles the pre-defined mappings, for examples of the method. Both the default transformation (MAP set to 0), in order to account for possible log scaling of the user coordinate axes, and also the Ezmap projection (MAP set to 1) use such a technique. Basically the idea is that the vector components must be proportionally reduced in size enough that an effectively "instantaneous" angle can be calculated, although they must not become so small that the calculation is adversely affected by the floating point precision available for the machine. Additionally, checks must be put in place to prevent the increment from stepping off the edge of the coordinate system space. The pre-defined mappings step in the opposite direction to find the angle whenever an increment in the original direction would fall off the edge.

ACCESS

To use STUMTA, load the NCAR Graphics libraries ncarg, ncarg_gks, and ncarg_c, preferably in that order.

COPYRIGHT

Copyright (C) 1987-2009
University Corporation for Atmospheric Research
The use of this Software is governed by a License Agreement.