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_C_ Original Code. The Original Code is: OpenGL Sample Implementation,
_C_ Version 1.2.1, released January 26, 2000, developed by Silicon Graphics,
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_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(Light,set light source parameters)
_names(Light,[fi])
.EQ
delim $$
.EN
.SH PARAMETERS
_phead(_param1)
Specifies a light.
The number of lights depends on the implementation,
but at least eight lights are supported.
They are identified by symbolic names of the form _const(LIGHT)$i$
where 0 \(<= $ i $ < _const(MAX_LIGHTS).
_phead(_param2)
Specifies a single-valued light source parameter for _param1.
_const(SPOT_EXPONENT),
_const(SPOT_CUTOFF),
_const(CONSTANT_ATTENUATION),
_const(LINEAR_ATTENUATION), and
_const(QUADRATIC_ATTENUATION) are accepted.
_phead(_param3)
Specifies the value that parameter _param2 of light source _param1
will be set to.
_names(Light,[fi]v)
.SH PARAMETERS
_phead(_param1)
Specifies a light.
The number of lights depends on the implementation, but
at least eight lights are supported.
They are identified by symbolic names of the form _const(LIGHT)$i$
where 0 \(<= $ i $ < _const(MAX_LIGHTS).
_phead(_param2)
Specifies a light source parameter for _param1.
_const(AMBIENT),
_const(DIFFUSE),
_const(SPECULAR),
_const(POSITION),
_const(SPOT_CUTOFF),
_const(SPOT_DIRECTION),
_const(SPOT_EXPONENT),
_const(CONSTANT_ATTENUATION),
_const(LINEAR_ATTENUATION), and
_const(QUADRATIC_ATTENUATION) are accepted.
_phead(_param3)
Specifies a pointer to the value or values that parameter _param2
of light source _param1 will be set to.
.SH DESCRIPTION
_cmnd sets the values of individual light source parameters.
_param1 names the light and is a symbolic name of the form _const(LIGHT)$i$,
where 0 \(<= i < _const(MAX_LIGHTS).
_param2 specifies one of ten light source parameters,
again by symbolic name.
_param3 is either a single value or a pointer to an array that contains
the new values.
.P
To enable and disable lighting calculation, call _cmnd(Enable)
and _cmnd(Disable) with argument _const(LIGHTING). Lighting is
initially disabled.
When it is enabled,
light sources that are enabled contribute to the lighting calculation.
Light source $i$ is enabled and disabled using _cmnd(Enable) and
_cmnd(Disable) with argument _const(LIGHT)$i$. 
.P
The ten light parameters are as follows:
.TP 20
_const(AMBIENT)
_param3 contains four integer or floating-point values that specify
the ambient RGBA intensity of the light.
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 light intensity is (0, 0, 0, 1).
.TP
_const(DIFFUSE)
_param3 contains four integer or floating-point values that specify
the diffuse RGBA intensity of the light.
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 value
for _const(LIGHT0) is (1, 1, 1, 1); for other lights, the
initial value is (0, 0, 0, 0). 
.TP
_const(SPECULAR)
_param3 contains four integer or floating-point values that specify
the specular RGBA intensity of the light.
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 value
for _const(LIGHT0) is (1, 1, 1, 1); for other lights, the
initial value is (0, 0, 0, 0). 
.TP
_const(POSITION)
_param3 contains four integer or floating-point values that specify
the position of the light in homogeneous object coordinates.
Both integer and floating-point values are mapped directly.
Neither integer nor floating-point values are clamped.
.IP
The position is transformed by the modelview matrix when
_cmnd is called (just as if it were a point),
and it is stored in eye coordinates.
If the $w$ component of the position is 0,
the light is treated as a directional source.
Diffuse and specular lighting calculations take the light's direction,
but not its actual position,
into account,
and attenuation is disabled.
Otherwise,
diffuse and specular lighting calculations are based on the actual location
of the light in eye coordinates,
and attenuation is enabled.
The initial position is (0, 0, 1, 0);
thus, the initial light source is directional,
parallel to, and in the direction of the $-z$ axis.
.TP
_const(SPOT_DIRECTION)
_param3 contains three integer or floating-point values that specify
the direction of the light in homogeneous object coordinates.
Both integer and floating-point values are mapped directly.
Neither integer nor floating-point values are clamped.
.IP
The spot direction is transformed by the inverse of the modelview matrix when
_cmnd is called (just as if it were a normal),
and it is stored in eye coordinates.
It is significant only when _const(SPOT_CUTOFF) is not 180,
which it is initially.
The initial direction is (0, 0, \-1).
.TP
_const(SPOT_EXPONENT)
_param3 is a single integer or floating-point value that specifies
the intensity distribution of the light.
Integer and floating-point values are mapped directly.
Only values in the range [0,128] are accepted.
.IP
Effective light intensity is attenuated by the cosine of the angle between
the direction of the light and the direction from the light to the vertex
being lighted,
raised to the power of the spot exponent.
Thus, higher spot exponents result in a more focused light source,
regardless of the spot cutoff angle (see _const(SPOT_CUTOFF), next paragraph).
The initial spot exponent is 0,
resulting in uniform light distribution.
.TP
_const(SPOT_CUTOFF)
_param3 is a single integer or floating-point value that specifies
the maximum spread angle of a light source.
Integer and floating-point values are mapped directly.
Only values in the range [0,90] and the special value 180
are accepted.
If the angle between the direction of the light and the direction from the
light to the vertex being lighted is greater than the spot cutoff angle,
the light is completely masked.
.BP
Otherwise, its intensity is controlled by the spot exponent and the
attenuation factors.
The initial spot cutoff is 180,
resulting in uniform light distribution.
.TP
_const(CONSTANT_ATTENUATION)
.TP
_const(LINEAR_ATTENUATION )
.TP
_const(QUADRATIC_ATTENUATION)
_param3 is a single integer or floating-point value that specifies
one of the three light attenuation factors.
Integer and floating-point values are mapped directly.
Only nonnegative values are accepted.
If the light is positional,
rather than directional,
its intensity is attenuated by the reciprocal of the sum of the constant
factor, the linear factor times the distance between the light
and the vertex being lighted,
and the quadratic factor times the square of the same distance.
The initial attenuation factors are (1, 0, 0),
resulting in no attenuation.
.SH NOTES
It is always the case that _const(LIGHT)$i$ = _const(LIGHT0) + $i$.
.SH ERRORS
_const(INVALID_ENUM) is generated if either _param1 or _param2
is not an accepted value.
.P
_const(INVALID_VALUE) is generated if a spot exponent value is specified
outside the range [0,128],
or if spot cutoff is specified outside the range [0,90] (except for the
special value 180),
or if a negative attenuation factor is specified.
.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(GetLight)
.br
_cmnd(IsEnabled) with argument _const(LIGHTING)
.SH SEE ALSO
_cmnd(ColorMaterial),
_cmnd(LightModel),
_cmnd(Material)
