.\" Copyright (c) 1994 The Board of Trustees of The Leland Stanford
.\" Junior University.  All rights reserved.
.\" 
.\" Permission to use, copy, modify and distribute this software and its
.\" documentation for any purpose is hereby granted without fee, provided
.\" that the above copyright notice and this permission notice appear in
.\" all copies of this software and that you do not sell the software.
.\" Commercial licensing is available by contacting the author.
.\" 
.\" THE SOFTWARE IS PROVIDED "AS IS" AND WITHOUT WARRANTY OF ANY KIND,
.\" EXPRESS, IMPLIED OR OTHERWISE, INCLUDING WITHOUT LIMITATION, ANY
.\" WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
.\" 
.\" Author:
.\"    Phil Lacroute
.\"    Computer Systems Laboratory
.\"    Electrical Engineering Dept.
.\"    Stanford University
.\" 
.\" $Date: 1994/12/31 19:49:53 $
.\" $Revision: 1.1 $
.\"
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.TH vpShadeTable 3 "" VolPack
.SH NAME
vpShadeTable \- compute the contents of the shading lookup table
.SH SYNOPSIS
#include <volpack.h>
.sp
.FS vpResult
\fBvpShadeTable(\fIvpc\fB)\fR
.FA
vpContext *\fIvpc;\fR
.FE
.SH ARGUMENTS
.IP \fIvpc\fR
VolPack context from \fBvpCreateContext.\fR
.SH DESCRIPTION
\fBvpShadeTable\fR computes the contents of the shading lookup table
previously specified with \fBvpLookupShader\fR.  For each entry in the
table, the Phong shading equation is evaluated using the current
lighting properties (as specified with \fBvpSetLight\fR), material
properties (as specified with \fBvpSetMaterial\fR), and viewing
parameters (as specified with the view transformation commands).  See
vpSetMaterial(3) for a description of the shading calculation.
.PP
This function should be called before rendering a volume if any of the
lighting or material properties have changed or if the viewpoint has
changed since the last call to \fBvpShadeTable\fR.  It should not be
called if shading is performed using a callback function or if the
shading lookup tables are initialized by a user-defined routine.
.PP
There is one state variable that affects the operation of
\fBvpShadeTable\fR.  If VP_CLAMP_SHADE_TABLE is enabled (see
vpEnable(3)), then values are clamped to the range 0.0-255.0 before
they are stored in the table.  If this option is not enabled then no
clamping is performed.  Clamping is enabled by default, but can be
turned off to achieve special effects.  For example, if all of the
voxels in a data set have very low opacity then the rendered image may
be very dark.  To brighten it, multiply all of the material parameter
coefficients by a constant, possibly making the values exceed 1.0, and
turn off clamping.
.SH ERRORS
The normal return value is VP_OK.  The following error return values
are possible:
.IP VPERROR_SINGULAR
One of the current view transformation matrices is singular.
.IP VPERROR_BAD_SHADER
The shading lookup tables are sized incorrectly, or one of the voxel
fields used to index the tables is invalid, or the number of material
types is invalid.
.IP VPERROR_BAD_SHADOW
The angle between the shadow light vector and the viewing direction is
too large (must be less than 45 degrees).  Either change the light
direction or disable shadows (see vpSetShadowLookupShader).
.SH SEE ALSO
VolPack(3), vpCreateContext(3), vpCurrentMatrix(3),
vpSetLookupShader(3), vpSetLight(3), vpSetMaterial(3)