GLTEXGEN(3gl)GLTEXGEN(3gl)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 genera‐
tion function. Must be GL_TEXTURE_GEN_MODE.
param Specifies a single-valued texture generation parameter, one of
GL_OBJECT_LINEAR, GL_EYE_LINEAR, GL_SPHERE_MAP, GL_REFLEC‐
TION_MAP or GL_NORMAL_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 parame‐
ters. If pname is GL_TEXTURE_GEN_MODE, then the array must con‐
tain a single symbolic constant, one of GL_OBJECT_LINEAR,
GL_EYE_LINEAR, GL_SPHERE_MAP, GL_REFLECTION_MAP, or GL_NOR‐
MAL_MAP. Otherwise, params holds the coefficients for the tex‐
ture-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_TEX‐
TURE_GEN_MODE, GL_OBJECT_PLANE, or GL_EYE_PLANE. If pname is GL_TEX‐
TURE_GEN_MODE, then params chooses a mode, one of GL_OBJECT_LINEAR,
GL_EYE_LINEAR, GL_SPHERE_MAP, GL_REFLECTION_MAP, or GL_NORMAL_MAP. If
pname is either GL_OBJECT_PLANE or GL_EYE_PLANE, params contains coef‐
ficients for the corresponding texture generation function.
If the texture generation function is GL_OBJECT_LINEAR, the function
g=p1xo+p2yo+p3zo+p4wo
is used, where g is the value computed for the coordinate named in
coord, p1, p2, p3, and p4 are the four values supplied in params, and
xo, yo, zo, and wo 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 p1, p2, p3, and p4). The alti‐
tude 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=p1′xe+p2′ye+p3′ze+p4′we
is used, where
(p1′p2′p3′p4′)=(p1p2p3p4)M−1
and xe, ye, ze, and we are the eye coordinates of the vertex, p1, p2,
p3, and p4 are the values supplied in params, and M is the modelview
matrix when glTexGen is invoked. If M is poorly conditioned or singu‐
lar, texture coordinates generated by the resulting function may be
inaccurate or undefined.
Note that the values in params define a reference plane in eye coordi‐
nates. 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=(fxfyfz)T
be the reflection vector such that
f=u−2n′n′Tu
Finally, let m=2√fx+fy+(fz+1)2 . Then the values assigned to the s and
t texture coordinates are
s=__+_
t=__+_
If pname is GL_REFLECTION_MAP, a reflection vector r is computed as
described for the GL_SPHERE_MAP mode. Then the value assigned to
s coordinate is s = rx; the value assigned to a t coordinate is
t = ry; and the value assigned to the r coordinate is r = rz.
If pname is GL_NORMAL_MAP, a normal vector nf is computed. Then the
value assigned to s coordinate is s = nfx; the value assigned to t
coordinate is t = nfy; and the value assigned to the r coordinate is r
= nfz. (the values nfx,nfy,and nfz are the components of nf.)
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_TEX‐
TURE_GEN_Q) as the argument. When enabled, the specified texture coor‐
dinate 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. Ini‐
tially, 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.
GL_INVALID_ENUM is generated when TexGen is called with a coord of GL_Q
when pname indicates GL_REFLECTION_MAP, or GL_NORMAL_MAP.
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, glCopyTexSubIm‐
age2D, glTexEnv, glTexImage1D, glTexImage2D, glTexParameter, glTex‐
SubImage1D, glTexSubImage2D
15 Mar 97 GLTEXGEN(3gl)