vpCurrentMatrix  choose the current transformation matrix
#include <volpack.h>
vpResult
vpCurrentMatrix(vpc, option)

 vpContext *vpc;

 int option;
 vpc
 VolPack context from vpCreateContext.
 option
 A code specifying one of the VolPack transformation matrices (VP_MODEL,
VP_VIEW or VP_PROJECT).
vpCurrentMatrix is used to choose the current transformation matrix. The
option parameter must be one of the following:

 VP_MODEL modeling transformation
VP_VIEW viewing transformation
VP_PROJECT projection transformation
Subsequent calls to the matrix transformation functions (
vpIdentityMatrix,
vpTranslate, vpRotate, vpScale, vpMultMatrix and
vpSetMatrix) are
applied to the new current transformation matrix.
Typically the modeling transformation is used to specify a transformation from
object coordinates to world coordinates. In object coordinates the volume data
is located inside a unit cube centered at the origin (corners at (0.5, 0.5,
0.5) and (0.5, 0.5, 0.5)). The default modeling transformation is the
identity, so the volume is initially located inside the unit cube in world
coordinates. By default, the current matrix is VP_MODEL. The modeling
transformation is also applied to light direction vectors (see vpSetLight(3)).
The viewing transformation is used for transforming world coordinates into eye
coordinates. In eye coordinates the viewer is looking down the Z axis in the
negative direction. The default value of the matrix is also the identity, so
the viewer is looking down the Z axis in world coordinates as well. Typically
this transformation matrix is used to specify the location and direction of
the viewer. It is also possible to combine the modeling and viewing matrices
(as in the OpenGL viewing model), in which case you can store the composite
transformation in either the model or the view matrix set the other matrix to
the identity.
The projection transformation specifies the transformation from eye coordinates
to clip coordinates. In the clip coordinate system the entire viewing frustum
lies inside a cube defined by the planes X=1, X=1, Y=1, Y=1, Z=1 and Z=1.
This transformation also specifies the type of projection used to produce a 2D
image. The 2D image is computed by projecting along the Z axis in the clip
coordinate system and then scaling in the X and Y dimensions to the final
image size. The routines
vpWindow and
vpWindowPHIGS are useful
for computing projection transformations.
The current transformation matrix code can be retrieved with the following state
variable codes (see vpGeti(3)): VP_CURRENT_MATRIX.
The normal return value is VP_OK. The following error return value is possible:
 VPERROR_BAD_OPTION
 The option argument is invalid.
VolPack(3), vpCreateContext(3), vpGetMatrix(3)