GLenum pname,
GLdouble param )
void glTexGenf(
GLenum coord,
GLenum pname,
GLfloat param )
void glTexGeni(
GLenum coord,
GLenum pname,
GLint param )
GLenum pname,
const GLdouble *params )
void glTexGenfv(
GLenum coord,
GLenum pname,
const GLfloat *params )
void glTexGeniv(
GLenum coord,
GLenum pname,
const GLint *params )
If the texture generation function is GL_OBJECT_LINEAR, the function
is used, where $g$ is the value computed for the coordinate named in coord,
$p sub 1$,
$p sub 2$,
$p sub 3$,
and
$p sub 4$ are the four values supplied in params, and
$x sub o$,
$y sub o$,
$z sub o$, and
$w sub o$ are the object coordinates of the vertex.
This function can be used, for example, to texture-map terrain using sea level
as a reference plane
(defined by $p sub 1$, $p sub 2$, $p sub 3$, and $p sub 4$).
The altitude of a terrain vertex is computed by the GL_OBJECT_LINEAR
coordinate generation function as its distance from sea level;
that altitude can then be used to index the texture image to map white snow
onto peaks and green grass onto foothills.
If the texture generation function is GL_EYE_LINEAR, the function
is used, where
and
$x sub e$,
$y sub e$,
$z sub e$, and
$w sub e$ are the eye coordinates of the vertex,
$p sub 1$,
$p sub 2$,
$p sub 3$,
and
$p sub 4$ are the values supplied in params, and
$M$ is the modelview matrix 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 sup prime be the current normal, after transformation to eye coordinates. Let
Finally, let $ m ~=~ 2 sqrt { f sub x sup {~2} ~+~ f sub y sup {~2} ~+~ (f sub z ~+~ 1 ) sup 2}$. Then the values assigned to the $s$ and $t$ texture coordinates are
To enable or disable a texture-coordinate generation function, call 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).
When the GL_ARB_multitexture extension is supported, glTexGen set the texture generation parameters for the currently active texture unit, selected with glActiveTextureARB.
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.