DragonFly On-Line Manual Pages
GLTEXGEN() GLTEXGEN()
NAME
glTexGend, glTexGenf, glTexGeni, glTexGendv, glTexGenfv, glTexGeniv -
control the generation of texture coordinates
C SPECIFICATION
void glTexGend( GLenum coord,
GLenum pname,
GLdouble param )
void glTexGenf( GLenum coord,
GLenum pname,
GLfloat param )
void glTexGeni( GLenum coord,
GLenum pname,
GLint param )
PARAMETERS
coord Specifies a texture coordinate. Must be one of 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.
C SPECIFICATION
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 )
PARAMETERS
coord Specifies a texture coordinate. Must be one of 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.
DESCRIPTION
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; it must be one of the 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_1 x_o + p_2 y_o + p_3 z_o + p_4 w_o
is used, where g is the value computed for the coordinate named in
coord, p_1, p_2, p_3, and p_4 are the four values supplied in params,
and x_o, y_o, z_o, and w_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_1, p_2, p_3, and p_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
g = (p_1)^' x_e + (p_2)^' y_e + (p_3)^' z_e + (p_4)^' w_e
is used, where
((p_1)^' (p_2)^' (p_3)^' ((p_4)^')) = (p_1 p_2 p_3 p_4) M^(-1)
and x_e, y_e, z_e, and w_e are the eye coordinates of the vertex, p_1,
p_2, p_3, and p_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
f = (f_x f_y f_z)^T be the reflection vector such that
f = u - 2 n^' n^('^T) u
Finally, let m = 2 <sqrt>(f_x^(2) + f_y^(2) + (f_z + 1)^2). Then the
values assigned to the s and t texture coordinates are
s = (f_x)/m + 1/2
t = (f_y)/m + 1/2
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).
ERRORS
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.
ASSOCIATED GETS
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
SEE ALSO
glCopyPixels, glCopyTexImage2D, glCopyTexSubImage1D,
glCopyTexSubImage2D, glTexEnv, glTexImage1D, glTexImage2D,
glTexParameter, glTexSubImage1D, glTexSubImage2D
GLTEXGEN()