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

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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