_C_ License Applicability. Except to the extent portions of this file are
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_C_ Software License B, Version 1.1 (the "License"), the contents of this
_C_ file are subject only to the provisions of the License. You may not use
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_C_ Note that, as provided in the License, the Software is distributed on an
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_C_ 
_C_ Original Code. The Original Code is: OpenGL Sample Implementation,
_C_ Version 1.2.1, released January 26, 2000, developed by Silicon Graphics,
_C_ Inc. The Original Code is Copyright (c) 1991-2000 Silicon Graphics, Inc.
_C_ Copyright in any portions created by third parties is as indicated
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_C_ Additional Notice Provisions: The application programming interfaces
_C_ established by SGI in conjunction with the Original Code are The
_C_ OpenGL(R) Graphics System: A Specification (Version 1.2.1), released
_C_ April 1, 1999; The OpenGL(R) Graphics System Utility Library (Version
_C_ 1.3), released November 4, 1998; and OpenGL(R) Graphics with the X
_C_ Window System(R) (Version 1.3), released October 19, 1998. This software
_C_ was created using the OpenGL(R) version 1.2.1 Sample Implementation
_C_ published by SGI, but has not been independently verified as being
_C_ compliant with the OpenGL(R) version 1.2.1 Specification.
_C_
_C_ $Date$ $Revision$
_C_ The first character in this file must be an '_'!
_C_ Anything on a line after _C_ is ignored
_define(_filters,tbl|eqn)_C_
_C_      eqn is automatically replaced with neqn for nroff
_header(LightModel,set the lighting model parameters)
_names(LightModel,[fi])
.EQ
delim $$
.EN
.SH PARAMETERS
_phead(_param1)
Specifies a single-valued lighting model parameter.
_const(LIGHT_MODEL_LOCAL_VIEWER),
_const(LIGHT_MODEL_COLOR_CONTROL), and
_const(LIGHT_MODEL_TWO_SIDE) are accepted.
_phead(_param2)
Specifies the value that _param2 will be set to.
_names(LightModel,[fi]v)
.SH PARAMETERS
_phead(_param1)
Specifies a lighting model parameter.
_const(LIGHT_MODEL_AMBIENT),
_const(LIGHT_MODEL_COLOR_CONTROL),
_const(LIGHT_MODEL_LOCAL_VIEWER), and
_const(LIGHT_MODEL_TWO_SIDE) are accepted.
_phead(_param2)
Specifies a pointer to the value or values that _param2 will be set to.
.SH DESCRIPTION
_cmnd sets the lighting model parameter.
_param1 names a parameter and _param2 gives the new value.
There are three lighting model parameters:
.TP 10
_const(LIGHT_MODEL_AMBIENT)
_param2 contains four integer or floating-point values that specify
the ambient RGBA intensity of the entire scene.
Integer values are mapped linearly such that the most positive representable
value maps to 1.0,
and the most negative representable value maps to \-1.0.
Floating-point values are mapped directly.
Neither integer nor floating-point values are clamped.
The initial ambient scene intensity is (0.2, 0.2, 0.2, 1.0).
.TP
_const(LIGHT_MODEL_COLOR_CONTROL)
_param2 must be either _const(SEPARATE_SPECULAR_COLOR) or
_const(SINGLE_COLOR).
_const(SINGLE_COLOR) specifies that a single color is generated from the
lighting computation for a vertex.  _const(SEPARATE_SPECULAR_COLOR)
specifies that the specular color computation of lighting be stored
separately from the remainder of the lighting computation.  The specular
color is summed into the generated fragment's color after the application
of texture mapping (if enabled).  The initial value is _const(SINGLE_COLOR).
.TP
_const(LIGHT_MODEL_LOCAL_VIEWER)
_param2 is a single integer or floating-point value that specifies
how specular reflection angles are computed.
If _param2 is 0 (or 0.0), specular reflection angles take the
view direction to be parallel to and in the direction of the -\f2z\fP axis,
regardless of the location of the vertex in eye coordinates.
Otherwise, specular reflections are computed from the origin
of the eye coordinate system.
The initial value is 0.
.TP
_const(LIGHT_MODEL_TWO_SIDE)
_param2 is a single integer or floating-point value that specifies
whether one- or two-sided lighting calculations are done for polygons.
It has no effect on the lighting calculations for points,
lines,
or bitmaps.
If _param2 is 0 (or 0.0), one-sided lighting is specified,
and only the \f2front\fP material parameters are used in the
lighting equation.
Otherwise, two-sided lighting is specified.
In this case, vertices of back-facing polygons are lighted using the
\f2back\fP material parameters,
and have their normals reversed before the lighting equation is evaluated.
Vertices of front-facing polygons are always lighted using the
\f2front\fP material parameters,
with no change to their normals. The initial value is 0.
.P
In RGBA mode, the lighted color of a vertex is the sum of
the material emission intensity,
the product of the material ambient reflectance and the lighting model full-scene 
ambient intensity,
and the contribution of each enabled light source.
Each light source contributes the sum of three terms:
ambient, diffuse, and specular.
The ambient light source contribution is the product of the material ambient
reflectance and the light's ambient intensity.
The diffuse light source contribution is the product of the material diffuse
reflectance,
the light's diffuse intensity,
and the dot product of the vertex's normal with the normalized vector from
the vertex to the light source.
The specular light source contribution is the product of the material specular
reflectance,
the light's specular intensity,
and the dot product of the normalized vertex-to-eye and vertex-to-light
vectors,
raised to the power of the shininess of the material.
All three light source contributions are attenuated equally based on
the distance from the vertex to the light source and on light source
direction, spread exponent, and spread cutoff angle.
All dot products are replaced with 0 if they evaluate to a negative value.
.P
The alpha component of the resulting lighted color is set to the alpha value
of the material diffuse reflectance.
.P
In color index mode,
the value of the lighted index of a vertex ranges from the ambient
to the specular values passed to _cmnd(Material) using _const(COLOR_INDEXES).
Diffuse and specular coefficients,
computed with a (.30, .59, .11) weighting of the lights' colors,
the shininess of the material,
and the same reflection and attenuation equations as in the RGBA case,
determine how much above ambient the resulting index is.
.SH NOTES
_const(LIGHT_MODEL_COLOR_CONTROL) is available only if the GL version is
1.2 or greater.
.SH ERRORS
_const(INVALID_ENUM) is generated if _param1 is not an accepted value.
.P
_const(INVALID_ENUM) is generated if _param1 is
_const(LIGHT_MODEL_COLOR_CONTROL) and _param2 is not one of
_const(SINGLE_COLOR) or _const(SEPARATE_SPECULAR_COLOR).
.P
_const(INVALID_OPERATION) is generated if _cmnd is executed between
the execution of _cmnd(Begin) and the corresponding execution of _cmnd(End).
.SH ASSOCIATED GETS
_cmnd(Get) with argument _const(LIGHT_MODEL_AMBIENT)
.br
_cmnd(Get) with argument _const(LIGHT_MODEL_COLOR_CONTROL)
.br
_cmnd(Get) with argument _const(LIGHT_MODEL_LOCAL_VIEWER)
.br
_cmnd(Get) with argument _const(LIGHT_MODEL_TWO_SIDE)
.br
_cmnd(IsEnabled) with argument _const(LIGHTING)
.SH SEE ALSO
_cmnd(Light),
_cmnd(Material)
