- define a one-dimensional evaluator
C SPECIFICATION
PARAMETERS
_param1 Specifies the kind of values that are generated
by the evaluator. Symbolic constants
GL_MAP1_VERTEX_3, GL_MAP1_VERTEX_4,
GL_MAP1_INDEX, GL_MAP1_COLOR_4, GL_MAP1_NORMAL,
GL_MAP1_TEXTURE_COORD_1, GL_MAP1_TEXTURE_COORD_2,
GL_MAP1_TEXTURE_COORD_3, and
GL_MAP1_TEXTURE_COORD_4 are accepted.
_param2, _param3
Specify a linear mapping of u, as presented to
glEvalCoord1, to <I>u, the variable that is evalu-
ated by the equations specified by this command.
_param4 Specifies the number of floats or doubles between
the beginning of one control point and the begin-
ning of the next one in the data structure refer-
enced in _param6. This allows control points to
be embedded in arbitrary data structures. The
only constraint is that the values for a particu-
lar control point must occupy contiguous memory
locations.
_param5 Specifies the number of control points. Must be
positive.
_param6 Specifies a pointer to the array of control
points.
DESCRIPTION
Evaluators provide a way to use polynomial or rational
polynomial mapping to produce vertices, normals, texture
coordinates, and colors. The values produced by an evalu-
ator are sent to further stages of GL processing just as
if they had been presented using glVertex, glNormal,
glTexCoord, and glColor commands, except that the gener-
ated values do not update the current normal, texture
coordinates, or color.
All polynomial or rational polynomial splines of any
degree (up to the maximum degree supported by the GL
implementation) can be described using evaluators. These
include almost all splines used in computer graphics: B-
splines, Bezier curves, Hermite splines, and so on.
Evaluators define curves based on Bernstein polynomials.
Define p(<I>u) as
Bn<I>i(<I>u)=( )<I>ui(1-<I>u)n-i
i
Recall that
n
00==1 and ( )==1
0
glMap1 is used to define the basis and to specify what
kind of values are produced. Once defined, a map can be
enabled and disabled by calling glEnable and glDisable
with the map name, one of the nine predefined values for
_param1 described below. glEvalCoord1 evaluates the one-
dimensional maps that are enabled. When
glEvalCoord1 presents a value u, the Bernstein functions
are evaluated using <I>u, where
<I>u=_e_q_n_p_u<I>a_<I>r_<I>a_e<I>m_q_n_p_a<I>e_r<I>q_a<I>n_m<I>p_<I>a_r_a_m__
_param1 is a symbolic constant that indicates what kind of
control points are provided in _param6, and what output is
generated when the map is evaluated. It can assume one of
GL_MAP1_INDEX Each control point is a single
floating-point value representing
a color index. Internal glIndex
commands are generated when the
map is evaluated but the current
index is not updated with the
value of these glIndex commands.
GL_MAP1_COLOR_4 Each control point is four float-
ing-point values representing
red, green, blue, and alpha.
Internal glColor4 commands are
generated when the map is evalu-
ated but the current color is not
updated with the value of these
glColor4 commands.
GL_MAP1_NORMAL Each control point is three
floating-point values represent-
ing the x, y, and z components of
a normal vector. Internal
glNormal commands are generated
when the map is evaluated but the
current normal is not updated
with the value of these glNormal
commands.
GL_MAP1_TEXTURE_COORD_1 Each control point is a single
floating-point value representing
the s texture coordinate. Inter-
nal
glTexCoord1 commands are gener-
ated when the map is evaluated
but the current texture coordi-
nates are not updated with the
value of these glTexCoord com-
mands.
GL_MAP1_TEXTURE_COORD_2 Each control point is two float-
ing-point values representing the
s and t texture coordinates.
Internal
glTexCoord2 commands are gener-
ated when the map is evaluated
but the current texture coordi-
nates are not updated with the
value of these glTexCoord com-
mands.
GL_MAP1_TEXTURE_COORD_3 Each control point is three
floating-point values represent-
ing the s, t, and r texture coor-
nates are not updated with the
value of these glTexCoord com-
mands.
_param4, _param5, and _param6 define the array addressing
for accessing the control points. _param6 is the location
of the first control point, which occupies one, two,
three, or four contiguous memory locations, depending on
which map is being defined. _param5 is the number of con-
trol points in the array. _param4 specifies how many
float or double locations to advance the internal memory
pointer to reach the next control point.
NOTES
As is the case with all GL commands that accept pointers
to data, it is as if the contents of _param6 were copied
by glMap1 before glMap1 returns. Changes to the contents
of _param6 have no effect after glMap1 is called.
ERRORS
GL_INVALID_ENUM is generated if _param1 is not an accepted
value.
GL_INVALID_VALUE is generated if _param2 is equal to
_param3.
GL_INVALID_VALUE is generated if _param4 is less than the
number of values in a control point.
GL_INVALID_VALUE is generated if _param5 is less than 1 or
greater than the return value of GL_MAX_EVAL_ORDER.
GL_INVALID_OPERATION is generated if glMap1 is executed
between the execution of glBegin and the corresponding
execution of glEnd.
When the GL_ARB_multitexture extension is supported,
GL_INVALID_OPERATION is generated if glMap1 is called and
the value of GL_ACTIVE_TEXTURE_ARB is not GL_TEXTURE0_ARB.
ASSOCIATED GETS
glGetMap
glGet with argument GL_MAX_EVAL_ORDER
glIsEnabled with argument GL_MAP1_VERTEX_3
glIsEnabled with argument GL_MAP1_VERTEX_4
glIsEnabled with argument GL_MAP1_INDEX
glIsEnabled with argument GL_MAP1_COLOR_4
glIsEnabled with argument GL_MAP1_NORMAL
glIsEnabled with argument GL_MAP1_TEXTURE_COORD_1
glIsEnabled with argument GL_MAP1_TEXTURE_COORD_2
glIsEnabled with argument GL_MAP1_TEXTURE_COORD_3
glIsEnabled with argument GL_MAP1_TEXTURE_COORD_4