1STUMTA(3NCARG) NCAR GRAPHICS STUMTA(3NCARG)
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6 STUMTA - Given the coordinates of a point on the streamline in data,
7 user, and NDC space, and the interpolated, normalized components of the
8 vector at the point relative to data coordinate space, the user-
9 modifiable routine STUMTA finds the directional angle of the streamline
10 relative to NDC space at the point.
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13 CALL STUMTA (XDA,YDA,XUS,YUS,XND,YND,DU,DV,TA,IST)
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16 XDA (REAL, input): The X coordinate of a point on the
17 streamline in the data coordinate system.
18 YDA (REAL, input); The Y coordinate of a point on the
20 streamline in the data coordinate system.
21 XUS (REAL, input): The X coordinate of the point in the user
23 coordinate system.
24 YUS (REAL, input): The Y coordinate of the point in the user
26 coordinate system.
27 XND (REAL, input): The X coordinate of the point in NDC space.
29 YND (REAL, input): The Y coordinate of the point in NDC space.
31 DU (REAL, input): The interpolated value of the normalized
33 component of the vector at the point, with direction
34 parallel to the X axis of the data coordinate system.
35 DV (REAL, input): The interpolated value of the normalized
37 component of the vector at the point, with direction
38 parallel to the Y axis of the data coordinate system.
39 TA (REAL, output): The directional angle of the streamline at
41 the point relative to NDC space.
42 IST (REAL, output); Status of the mapping operation: 0
44 indicates success, negative values indicate that the
45 mapping failed; positive values are reserved and should not
46 be used by the implementor of a mapping routine.
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49 STUMTA is a user-modifiable routine provided to support custom mappings
50 of the data coordinate space. The user does not invoke it directly.
51 Instead, whenever the parameter MAP specifies a mapping not handled by
52 Streamlines internally (i.e., when MAP is set to a value other than 0,
53 1, or 2), Streamlines calls STUMTA once for each incremental step in
54 the creation of a streamline. The default version of STUMTA simply
55 returns the angle implied by the incremental vector components passed
56 to it: that is, it returns ATAN2(DV,DU). In order to implement a custom
57 mapping, you must pick a unique mapping code (a positive integer
58 greater than 2), and then modify each of the three routines, STUMXY,
59 STUIXY, and STUMTA to recognize and respond consistently to the chosen
60 code. In the standard distribution of NCAR Graphics, these three
61 routines reside in a single file, ´stumxy.f´. STUMXY maps a point from
62 data to user coordinate space and STUIXY inversely maps a point from
63 user to data coordinate space. STUMTA, which is likely to be the most
64 difficult to implement, finds the tangent angle of the streamline at a
65 point in NDC space.
66 STUMTA has access to a common block called STMAP that contains a number
68 of variables used to record the current transformation state. In order
69 to accommodate a variety of mapping implementations, STMAP provides
70 more information than normally required. Consider the values stored in
71 STMAP as strictly read-only. One essential member of this common block
72 is IMAP, which contains the value currently assigned to the MAP
73 parameter.
74 When implementing a non-linear mapping, an iterative differential
76 technique will most likely be required. Look at the routine, STMPTA, in
77 ´stmpxy.f´, which handles the pre-defined mappings, for examples of the
78 method. Both the default transformation (MAP set to 0), in order to
79 account for possible log scaling of the user coordinate axes, and also
80 the Ezmap projection (MAP set to 1) use such a technique. Basically
81 the idea is that the vector components must be proportionally reduced
82 in size enough that an effectively "instantaneous" angle can be
83 calculated, although they must not become so small that the calculation
84 is adversely affected by the floating point precision available for the
85 machine. Additionally, checks must be put in place to prevent the
86 increment from stepping off the edge of the coordinate system space.
87 The pre-defined mappings step in the opposite direction to find the
88 angle whenever an increment in the original direction would fall off
89 the edge.
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92 To use STUMTA, load the NCAR Graphics libraries ncarg, ncarg_gks, and
93 ncarg_c, preferably in that order.
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96 Online: stgetc, stgeti, stgetr, stinit, stream, streamlines,
97 streamlines_params, strset, stsetc, stseti, stsetr, stuixy, stumsl,
98 stumxy, ncarg_cbind.
99 Hardcopy: NCAR Graphics Fundamentals, UNIX Version
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103 Copyright (C) 1987-2009
104 University Corporation for Atmospheric Research
105 The use of this Software is governed by a License Agreement.
106UNIX April 1993 STUMTA(3NCARG)