NAME

       vpCurrentMatrix - choose the current transformation matrix

SYNOPSIS

       #include <volpack.h>

       vpResult
       vpCurrentMatrix(vpc, option)
           vpContext *vpc;
           int option;

ARGUMENTS

       vpc    VolPack context from vpCreateContext.

       option A  code specifying one of the VolPack transformation matrices (VP_MODEL, VP_VIEW or
              VP_PROJECT).

DESCRIPTION

       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.

STATE VARIABLES

       The current transformation matrix code can be retrieved with the following state  variable
       codes (see vpGeti(3)): VP_CURRENT_MATRIX.

ERRORS

       The normal return value is VP_OK.  The following error return value is possible:

       VPERROR_BAD_OPTION
              The option argument is invalid.

SEE ALSO

       VolPack(3), vpCreateContext(3), vpGetMatrix(3)