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.
SEE ALSO¶
Online: stgetc, stgeti, stgetr, stinit, stream, streamlines, streamlines_params,
strset, stsetc, stseti, stsetr, stuixy, stumsl, stumxy, ncarg_cbind.
Hardcopy: NCAR Graphics Fundamentals, UNIX Version
COPYRIGHT¶
Copyright (C) 1987-2009
University Corporation for Atmospheric Research
The use of this Software is governed by a License Agreement.