Ubuntu Manpage: STUMTA - Given the coordinates of a point on the streamline in data, user, and NDC space,

NAME

       STUMTA - Given the coordinates of a point on the streamline in data, user, and NDC space,
       and the interpolated, normalized components of the vector at the point relative to data
       coordinate space, the user-modifiable routine STUMTA finds the directional angle of the
       streamline relative to NDC space at the point.

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.

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