glTexGen(3G)glTexGen(3G)NAME
glTexGen, glTexGend, glTexGenf, glTexGeni, glTexGendv, glTexGenfv,
glTexGeniv - control the generation of texture coordinates
SYNOPSIS
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
Specifies a texture coordinate. Must be one of GL_S, GL_T, GL_R, or
GL_Q. Specifies the symbolic name of the texture-coordinate generation
function. Must be GL_TEXTURE_GEN_MODE. Specifies a single-valued tex‐
ture generation parameter, one of GL_OBJECT_LINEAR, GL_EYE_LINEAR, or
GL_SPHERE_MAP.
SYNOPSIS
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
Specifies a texture coordinate. Must be one of GL_S, GL_T, GL_R, or
GL_Q. 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. 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.
DESCRIPTIONglTexGen() 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, or GL_SPHERE_MAP. If pname is either GL_OBJECT_PLANE or
GL_EYE_PLANE, params contains coefficients for the corresponding tex‐
ture 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 ter‐
rain 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 ]prime x[e] + p[2 ]prime y[e] + p[3 ]prime z[e] + p[4 ]prime w[e]
g = p[1]prime x[e] + p[2]prime y[e] + p[3]prime z[e] + p[4] prime w[e]
is used, where
( p[1]prime p[2 ]prime P[3] prime p[4 ]prime) = ( 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 condi‐
tioned 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 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 = ( f[x]f[y]f[z] )^T
be the reflection vector such that f = u - 2 n prime n prime^T u
Finally, let m = 2 sqrt f[x^2] + f[y^2]+ (f[z] + 1 )^2. Then the val‐
ues assigned to the s and t texture coordinates are s = f[x] over m + 1
over 2
t = f[y] over m + 1 over 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_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).
When the GL_ARB_multitexture extension is supported, glTexGen() set the
texture generation parameters for the currently active texture unit,
selected with glActiveTextureARB().
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 GETSglGetTexGen()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 ALSOglActiveTextureARB(3), glCopyPixels(3), glCopyTexImage2D(3), glCopyTex‐
SubImage1D(3), glCopyTexSubImage2D(3), glCopyTexSubImage3D(3), glTex‐
Env(3), glTexImage1D(3), glTexImage2D(3), glTexImage3D(3), , glTexPa‐
rameter(3), glTexSubImage1D(3), glTexSubImage2D(3), glTexSubImage3D(3)glTexGen(3G)