GLUNURBSPROPERTY() MachTen Programmer’s Manual GLUNURBSPROPERTY()

NAME
gluNurbsProperty - set a NURBS property

C SPECIFICATION
void gluNurbsProperty( GLUnurbs* nurb,
GLenum property,
GLfloat value )

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PARAMETERS
nurb Specifies the NURBS object (created with
gluNewNurbsRenderer).

property Specifies the property to be set. Valid values
are GLU_SAMPLING_TOLERANCE, GLU_DISPLAY_MODE,
GLU_CULLING, GLU_AUTO_LOAD_MATRIX,
GLU_PARAMETRIC_TOLERANCE, GLU_SAMPLING_METHOD,
GLU_U_STEP, GLU_V_STEP, or GLU_NURBS_MODE_EXT.

value Specifies the value of the indicated property.
It may be a numeric value, or one of
GLU_OUTLINE_POLYGON, GLU_FILL,
GLU_OUTLINE_PATCH, GL_TRUE, GL_FALSE,
GLU_PATH_LENGTH, GLU_PARAMETRIC_ERROR,
GLU_DOMAIN_DISTANCE, GLU_NURBS_RENDERER_EXT, or
GLU_NURBS_TESSELLATOR_EXT.

DESCRIPTION
gluNurbsProperty is used to control properties stored in a
NURBS object. These properties affect the way that a NURBS
curve is rendered. The accepted values for property are as
follows:

GLU_NURBS_MODE_EXT
value should be set to be either
GLU_NURBS_RENDERER_EXT or
GLU_NURBS_TESSELLATOR_EXT. When set to
GLU_NURBS_RENDERER_EXT, NURBS objects are
tessellated into openGL primitives and sent
to the pipeline for rendering. When set to
GLU_NURBS_TESSELLATOR_EXT, NURBS objects
are tessellated into openGL primitives but
the vertices, normals, colors, and/or tex-
tures are retrieved back through a callback
interface (see gluNurbsCallback). This
allows the user to cache the tessellated
results for further processing.

GLU_SAMPLING_METHOD
Specifies how a NURBS surface should be
tessellated. value may be one of
GLU_PATH_LENGTH, GLU_PARAMETRIC_ERROR,
GLU_DOMAIN_DISTANCE,
GLU_OBJECT_PATH_LENGTH_EXT, or
GLU_OBJECT_PARAMETRIC_ERROR_EXT. When set
to GLU_PATH_LENGTH, the surface is rendered
so that the maximum length, in pixels, of
the edges of the tessellation polygons is
no greater than what is specified by
GLU_SAMPLING_TOLERANCE.

GLU_PARAMETRIC_ERROR specifies that the
surface is rendered in such a way that the
value specified by GLU_PARAMETRIC_TOLERANCE
describes the maximum distance, in pixels,
between the tessellation polygons and the
surfaces they approximate.

GLU_DOMAIN_DISTANCE allows users to spec-
ify, in parametric coordinates, how many
sample points per unit length are taken in
u, v direction.

GLU_OBJECT_PATH_LENGTH_EXT is similar to
GLU_PATH_LENGTH except that it is view
independent, that is, the surface is ren-
dered so that the maximum length, in object
space, of edges of the tessellation poly-
gons is no greater than what is specified
by GLU_SAMPLING_TOLERANCE.

GLU_OBJECT_PARAMETRIC_ERROR_EXT is similar
to GLU_PARAMETRIC_ERROR except that it is
view independent, that is, the surface is
rendered in such a way that the value spec-
ified by GLU_PARAMETRIC_TOLERANCE describes
the maximum distance, in object space,
between the tessellation polygons and the
surfaces they approximate.

The initial value of GLU_SAMPLING_METHOD is
GLU_PATH_LENGTH.

GLU_SAMPLING_TOLERANCE
Specifies the maximum length, in pixels or
in object space length unit, to use when
the sampling method is set to
GLU_PATH_LENGTH or
GLU_OBJECT_PATH_LENGTH_EXT. The NURBS code
is conservative when rendering a curve or
surface, so the actual length can be some-
what shorter. The initial value is 50.0
pixels.

GLU_PARAMETRIC_TOLERANCE
Specifies the maximum distance, in pixels
or in object space length unit, to use when
the sampling method is GLU_PARAMETRIC_ERROR
or GLU_OBJECT_PARAMETRIC_ERROR_EXT. The
initial value is 0.5.

GLU_U_STEP Specifies the number of sample points per
unit length taken along the u axis in para-
metric coordinates. It is needed when
GLU_SAMPLING_METHOD is set to
GLU_DOMAIN_DISTANCE. The initial value is
100.

GLU_V_STEP Specifies the number of sample points per
unit length taken along the v axis in para-
metric coordinate. It is needed when
GLU_SAMPLING_METHOD is set to
GLU_DOMAIN_DISTANCE. The initial value is
100.

GLU_DISPLAY_MODE
value can be set to GLU_OUTLINE_POLYGON,
GLU_FILL, or GLU_OUTLINE_PATCH. When
GLU_NURBS_MODE_EXT is set to be
GLU_NURBS_RENDERER_EXT, value defines how a
NURBS surface should be rendered. When
value is set to GLU_FILL, the surface is
rendered as a set of polygons. When value
is set to GLU_OUTLINE_POLYGON, the NURBS
library draws only the outlines of the
polygons created by tessellation. When
value is set to GLU_OUTLINE_PATCH just the
outlines of patches and trim curves defined
by the user are drawn.

When GLU_NURBS_MODE_EXT is set to be
GLU_NURBS_TESSELLATOR_EXT, value defines
how a NURBS surface should be tessellated.
When GLU_DISPLAY_MODE is set to GLU_FILL or
GLU_OUTLINE_POLY, the NURBS surface is tes-
sellated into openGL triangle primitives
which can be retrieved back through call-
back functions. If GLU_DISPLAY_MODE is set
to GLU_OUTLINE_PATCH, only the outlines of
the patches and trim curves are generated
as a sequence of line strips which can be
retrieved back through callback functions.

The initial value is GLU_FILL.

GLU_CULLING value is a boolean value that, when set to
GL_TRUE, indicates that a NURBS curve
should be discarded prior to tessellation
if its control points lie outside the cur-
rent viewport. The initial value is
GL_FALSE.

GLU_AUTO_LOAD_MATRIX
value is a boolean value. When set to
GL_TRUE, the NURBS code downloads the pro-
jection matrix, the modelview matrix, and
the viewport from the GL server to compute
sampling and culling matrices for each
NURBS curve that is rendered. Sampling and
culling matrices are required to determine
the tessellation of a NURBS surface into
line segments or polygons and to cull a
NURBS surface if it lies outside the view-
port.

If this mode is set to GL_FALSE, then the
program needs to provide a projection
matrix, a modelview matrix, and a viewport
for the NURBS renderer to use to construct
sampling and culling matrices. This can be
done with the gluLoadSamplingMatrices func-
tion. This mode is initially set to
GL_TRUE. Changing it from GL_TRUE to
GL_FALSE does not affect the sampling and
culling matrices until gluLoadSamplingMa-
trices is called.

NOTES
If GLU_AUTO_LOAD_MATRIX is true, sampling and culling may
be executed incorrectly if NURBS routines are compiled
into a display list.

A property of GLU_PARAMETRIC_TOLERANCE,
GLU_SAMPLING_METHOD, GLU_U_STEP, or GLU_V_STEP, or a value
of GLU_PATH_LENGTH, GLU_PARAMETRIC_ERROR,
GLU_DOMAIN_DISTANCE are only available if the GLU version
is 1.1 or greater. They are not valid parameters in GLU
1.0.

gluGetString can be used to determine the GLU version.

SEE ALSO
gluGetNurbsProperty, gluLoadSamplingMatrices, gluNewNurb-
sRenderer, gluGetString, gluNurbsCallback

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