GLLIGHT() MachTen Programmer’s Manual GLLIGHT()

NAME
glLightf, glLighti, glLightfv, glLightiv - set light
source parameters

C SPECIFICATION
void glLightf( GLenum light,
GLenum pname,
GLfloat param )
void glLighti( GLenum light,
GLenum pname,
GLint param )

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PARAMETERS
light 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 GL_LIGHT$i$ where 0 <= $ i $ <
GL_MAX_LIGHTS.

pname Specifies a single-valued light source parameter
for light. GL_SPOT_EXPONENT, GL_SPOT_CUTOFF,
GL_CONSTANT_ATTENUATION, GL_LINEAR_ATTENUATION,
and GL_QUADRATIC_ATTENUATION are accepted.

param Specifies the value that parameter pname of light
source light will be set to.

C SPECIFICATION
void glLightfv( GLenum light,
GLenum pname,
const GLfloat *params )
void glLightiv( GLenum light,
GLenum pname,
const GLint *params )

PARAMETERS
light 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 GL_LIGHT$i$ where 0 <= $ i $ <
GL_MAX_LIGHTS.

pname Specifies a light source parameter for light.
GL_AMBIENT, GL_DIFFUSE, GL_SPECULAR, GL_POSITION,
GL_SPOT_CUTOFF, GL_SPOT_DIRECTION,
GL_SPOT_EXPONENT, GL_CONSTANT_ATTENUATION,
GL_LINEAR_ATTENUATION, and GL_QUADRATIC_ATTENUATION
are accepted.

params Specifies a pointer to the value or values that
parameter pname of light source light will be set
to.

DESCRIPTION
glLight sets the values of individual light source parame-
ters. light names the light and is a symbolic name of the
form GL_LIGHT$i$, where 0 <= i < GL_MAX_LIGHTS. pname
specifies one of ten light source parameters, again by
symbolic name. params is either a single value or a
pointer to an array that contains the new values.

To enable and disable lighting calculation, call glEnable
and glDisable with argument GL_LIGHTING. Lighting is ini-
tially disabled. When it is enabled, light sources that
are enabled contribute to the lighting calculation. Light
source $i$ is enabled and disabled using glEnable and glD-
isable with argument GL_LIGHT$i$.

The ten light parameters are as follows:

GL_AMBIENT params contains four integer or float-
ing-point values that specify the
ambient RGBA intensity of the light.
Integer values are mapped linearly
such that the most positive repre-
sentable 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).

GL_DIFFUSE params contains four integer or float-
ing-point values that specify the dif-
fuse RGBA intensity of the light.
Integer values are mapped linearly
such that the most positive repre-
sentable 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 GL_LIGHT0 is (1,
1, 1, 1); for other lights, the ini-
tial value is (0, 0, 0, 0).

GL_SPECULAR params contains four integer or float-
ing-point values that specify the
specular RGBA intensity of the light.
Integer values are mapped linearly
such that the most positive repre-
sentable 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 GL_LIGHT0 is (1,
1, 1, 1); for other lights, the ini-
tial value is (0, 0, 0, 0).

GL_POSITION params contains four integer or float-
ing-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 float-
ing-point values are clamped.

The position is transformed by the
modelview matrix when glLight 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 direc-
tional source. Diffuse and specular
lighting calculations take the light’s
direction, but not its actual posi-
tion, 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 attenua-
tion 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.

GL_SPOT_DIRECTION params 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 float-
ing-point values are clamped.

The spot direction is transformed by
the inverse of the modelview matrix
when glLight is called (just as if it
were a normal), and it is stored in
eye coordinates. It is significant
only when GL_SPOT_CUTOFF is not 180,
which it is initially. The initial
direction is (0, 0, -1).

GL_SPOT_EXPONENT params is a single integer or float-
ing-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.

Effective light intensity is attenu-
ated 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
GL_SPOT_CUTOFF, next paragraph). The
initial spot exponent is 0, resulting
in uniform light distribution.

GL_SPOT_CUTOFF params is a single integer or float-
ing-point value that specifies the
maximum spread angle of a light
source. Integer and floating-point
values are mapped directly. Only val-
ues in the range [0,90] and the spe-
cial 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. Otherwise, its
intensity is controlled by the spot
exponent and the attenuation factors.
The initial spot cutoff is 180,
resulting in uniform light distribu-
tion.

GL_CONSTANT_ATTENUATION

GL_LINEAR_ATTENUATION

GL_QUADRATIC_ATTENUATION
params is a single integer or float-
ing-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 atten-
uation factors are (1, 0, 0), result-
ing in no attenuation.

NOTES
It is always the case that GL_LIGHT$i$ = GL_LIGHT0 + $i$.

ERRORS
GL_INVALID_ENUM is generated if either light or pname is
not an accepted value.

GL_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 speci-
fied.

GL_INVALID_OPERATION is generated if glLight is executed
between the execution of glBegin and the corresponding
execution of glEnd.

ASSOCIATED GETS
glGetLight
glIsEnabled with argument GL_LIGHTING

SEE ALSO
glColorMaterial, glLightModel, glMaterial

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