man TDINIT (3): Initialization routine for TDPACK, called to define the position of

SYNOPSIS

CALL TDINIT (UMID, VMID, WMID, UORI, VORI, WORI, UTHI, VTHI, WTHI, OTEP)

C-BINDING SYNOPSIS

#include <ncarg/ncargC.h>

void c_tdinit(float umid, float vmid, float wmid, float uori, float vori, float wori, float uthi, float vthi, float wthi, float otep)

DESCRIPTION

This initialization routine is called to define the position of the eye, the position of the point looked at, which way is up, and whether or not a stereo view is to be done. TDINIT precomputes some quantities in TDPACK common blocks that will subsequently be used in projecting points from 3-space (U, V, and W coordinates) to 2-space (X and Y coordinates).

By default (that is to say, if the internal parameter 'SET' has its default value), TDINIT also calls the SPPS routine SET to define the mapping from the "user" coordinate system (the X/Y system) to the "fractional" coordinate system (in GKS terms, NDC space). This is done in such a way as to show in the projection plane a field of view of 'FOV' degrees. The viewport to be used in the fractional coordinate system is that defined by the values of the internal parameters 'VPL', 'VPR', 'VPB', and 'VPT'. In some situations, it may be desirable, after calling TDINIT, to call the SPPS routine GETSET to retrieve the arguments with which TDINIT called SET and then recall SET with a different set of arguments.

Normally, the creation of an image with TDPACK starts with a call to TDINIT (perhaps preceded by calls to TDSETI, TDSETR, and/or TDSTRS to reset internal parameters of the package) and continues with calls to draw objects. (Of course, if all of the arguments in a call to TDINIT have the same values as in the last call and the SET call that was done as a result is still in effect, then it's not necessary to repeat the call to TDINIT.)

For stereo views, one calls TDINIT with a negative OTEP, executes the object-drawing calls, calls TDINIT again with a positive OTEP, and then repeats all of the object-drawing calls. The exact way in which stereo views are drawn is also affected (slightly) by the value of the internal parameter 'STE'.

The arguments of TDINIT are as follows:

UMID, VMID, and WMID

(input expressions of type REAL) - the coordinates of a point, E, at the eye position (if a single view is being drawn), or of a point midway between the two eyes (if a stereo view is being drawn).

UORI, VORI, and WORI

(input expressions of type REAL) - the coordinates of a point, O, that the eye is looking at. That point defines the origin of the XY projection plane. The line of sight is the line from E to O. The projection plane passes through O and is perpendicular to the line of sight.

UTHI, VTHI, and WTHI

(input expressions of type REAL) - the coordinates of a third point, T, needed to completely specify the orientation of the X and Y axes in the projection plane. The Y axis of the projection plane is its intersection with the plane passing though the points E, O, and T. The X axis of the projection plane passes through the point O and is perpendicular to the Y axis.

OTEP

(an input expression of type REAL) - set non-zero if and only if a stereo view is to be drawn. Make the value negative to draw a left-eye view, positive to draw a right-eye view. The magnitude of OTEP is the distance from either eye to the point midway between the eyes; if R represents the approximate distance from the eye to the objects being drawn, then OTEP may be set using a statement like

  OTEP = (+ or -) R *
         TAN(.017453292519943*ANGD/2.)

where the constant (.01745...) is just pi/180 and ANGD is the desired difference in the angle between the two views, in degrees; use a value of about 1 or 2 degrees for ANGD.

Note that, if the origin is approximately in the middle of the objects being viewed, then it's probably appropriate to use a value of R computed as follows:

  R = SQRT((UMID-UORI)**2
           (VMID-VORI)**2+
           (WMID-WORI)**2)

C-BINDING DESCRIPTION

The C-binding argument descriptions are the same as the FORTRAN argument descriptions.

ACCESS

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

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