glTexGend, glTexGenf, glTexGeni, glTexGendv, glTexGenfv, glTexGeniv - control the generation of texture coordinates
void glTexGend( GLenum coord, GLenum pname, GLdouble param ) void glTexGenf( GLenum coord, GLenum pname, GLfloat param ) void glTexGeni( GLenum coord, GLenum pname, GLint param )
coord Specifies a texture coordinate. Must be one of the following: GL_S, GL_T, GL_R, or GL_Q. pname Specifies the symbolic name of the texture-coordinate generation function. Must be GL_TEXTURE_GEN_MODE. param Specifies a single-valued texture generation parameter, one of GL_OBJECT_LINEAR, GL_EYE_LINEAR, or GL_SPHERE_MAP.
void glTexGendv( GLenum coord, GLenum pname, const GLdouble *params ) void glTexGenfv( GLenum coord, GLenum pname, const GLfloat *params ) void glTexGeniv( GLenum coord, GLenum pname, const GLint *params )
coord
Specifies a texture coordinate. Must be one of the following: GL_S,
GL_T, GL_R, or GL_Q.
pname
Specifies the symbolic name of the texture-coordinate generation
function or function parameters. Must be GL_TEXTURE_GEN_MODE,
GL_OBJECT_PLANE, or GL_EYE_PLANE.
params
Specifies a pointer to an array of texture generation parameters. If
pname is GL_TEXTURE_GEN_MODE, then the array must contain a single
symbolic constant, one of GL_OBJECT_LINEAR, GL_EYE_LINEAR, or
GL_SPHERE_MAP. Otherwise, params holds the coefficients for the
texture-coordinate generation function specified by pname.
glTexGen selects a texture-coordinate generation function or supplies
coefficients for one of the functions. coord names one of the (s,t,r,q)
texture coordinates, and it must be one of these symbols: GL_S, GL_T, GL_R,
or GL_Q. pname must be one of three symbolic constants:
GL_TEXTURE_GEN_MODE, GL_OBJECT_PLANE, or GL_EYE_PLANE. If pname is
GL_TEXTURE_GEN_MODE, then params chooses a mode, one of GL_OBJECT_LINEAR,
GL_EYE_LINEAR, or GL_SPHERE_MAP. If pname is either GL_OBJECT_PLANE or
GL_EYE_PLANE, params contains coefficients for the corresponding texture
generation function.
If the texture generation function is GL_OBJECT_LINEAR, the function
g=p x +p y +p z +p w
1 o 2 o 3 o 4 o
is used, where g is the value computed for the coordinate named in coord,
p , p , p , and p are the four values supplied in params, and x , y , z ,
1 2 3 4 o o o
and w are the object coordinates of the vertex. This function can be used
o
to texture-map terrain using sea level as a reference plane (defined by p ,
1
p , p , and p ). The altitude of a terrain vertex is computed by the
2 3 4
GL_OBJECT_LINEAR coordinate generation function as its distance from sea
level; that altitude is used to index the texture image to map white snow
onto peaks and green grass onto foothills, for example.
If the texture generation function is GL_EYE_LINEAR, the function
g=p'x' + p'y + p'z + p'w
1 e 2 e 3 e 4 e
is used, where
-1
(p' p' p' p') = (p p p p )·M
1 2 3 4 1 2 3 4
and x , y , z , and w are the eye coordinates of the vertex, p , p , p ,
e e e e 1 2 3
and p are the values supplied in params, and M is the modelview matrix
4
when glTexGen is invoked. If M is poorly conditioned or singular, texture
coordinates generated by the resulting function may be inaccurate or
undefined.
Note that the values in params define a reference plane in eye coordinates.
The modelview matrix that is applied to them may not be the same one in
effect when the polygon vertices are transformed. This function establishes
a field of texture coordinates that can produce dynamic contour lines on
moving objects.
If pname is GL_SPHERE_MAP and coord is either GL_S or GL_T, s and t
texture coordinates are generated as follows. Let u be the unit vector
pointing from the origin to the polygon vertex (in eye coordinates). Let n'
be the current normal, after transformation to eye coordinates. Let
T
f = (f f f ) be the reflection vector such that
x y z
T
f = u - 2n'n' u
_____________
| 2 2 2
Finally, let m = 2\|f +f +(f +1) . Then the values assigned to the s and
x y z
t texture coordinates are
f
x 1
s = --- + -
m 2
f
y 1
t = --- + -
m 2
A texture-coordinate generation function is enabled or disabled using
glEnable or glDisable with one of the symbolic texture-coordinate names
(GL_TEXTURE_GEN_S, GL_TEXTURE_GEN_T, GL_TEXTURE_GEN_R, or GL_TEXTURE_GEN_Q)
as the argument. When enabled, the specified texture coordinate is computed
according to the generating function associated with that coordinate. When
disabled, subsequent vertices take the specified texture coordinate from
the current set of texture coordinates. Initially, all texture generation
functions are set to GL_EYE_LINEAR and are disabled. Both s plane
equations are (1,0,0,0), both t plane equations are (0,1,0,0), and all r
and q plane equations are (0,0,0,0).
GL_INVALID_ENUM is generated when coord or pname is not an accepted defined value, or when pname is GL_TEXTURE_GEN_MODE and params is not an accepted defined value. GL_INVALID_ENUM is generated when pname is GL_TEXTURE_GEN_MODE, params is GL_SPHERE_MAP, and coord is either GL_R or GL_Q. GL_INVALID_OPERATION is generated if glTexGen is executed between the execution of glBegin and the corresponding execution of glEnd.
glGetTexGen glIsEnabled with argument GL_TEXTURE_GEN_S glIsEnabled with argument GL_TEXTURE_GEN_T glIsEnabled with argument GL_TEXTURE_GEN_R glIsEnabled with argument GL_TEXTURE_GEN_Q
glTexEnv, glTexImage1D, glTexImage2D, glTexParameter
Introduction | Alphabetic | Specification
Last Edited: Tue, May 23, 1995