glTexGen(3G) OpenGL Reference glTexGen(3G)
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 the following:
GLS, GLT, GLR, or GLQ.
pname Specifies the symbolic name of the texture-coordinate generation
function. Must be GLTEXTUREGENMODE.
param Specifies a single-valued texture generation parameter, one of
GLOBJECTLINEAR, GLEYELINEAR, or GLSPHEREMAP.
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 the following:
GLS, GLT, GLR, or GLQ.
pname
Specifies the symbolic name of the texture-coordinate generation
function or function parameters. Must be GLTEXTUREGENMODE,
GLOBJECTPLANE, or GLEYEPLANE.
params
Specifies a pointer to an array of texture generation parameters.
If pname is GLTEXTUREGENMODE, then the array must contain a
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glTexGen(3G) OpenGL Reference glTexGen(3G)
single symbolic constant, one of GLOBJECTLINEAR, GLEYELINEAR, or
GLSPHEREMAP. 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, and it must be one of these symbols: GLS, GLT,
GLR, or GLQ. pname must be one of three symbolic constants:
GLTEXTUREGENMODE, GLOBJECTPLANE, or GLEYEPLANE. If pname is
GLTEXTUREGENMODE, then params chooses a mode, one of GLOBJECTLINEAR,
GLEYELINEAR, or GLSPHEREMAP. If pname is either GLOBJECTPLANE or
GLEYEPLANE, params contains coefficients for the corresponding texture
generation function.
If the texture generation function is GLOBJECTLINEAR, 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 ,
1 2 3 4 o o
z , and w are the object coordinates of the vertex. This function can
o o
be used to texture-map terrain using sea level as a reference plane
(defined by p , p , p , and p ). The altitude of a terrain vertex is
1 2 3 4
computed by the GLOBJECTLINEAR 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 GLEYELINEAR, 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 GLSPHEREMAP and coord is either GLS or GLT, 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
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glTexGen(3G) OpenGL Reference glTexGen(3G)
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
\| x y z
and 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
(GLTEXTUREGENS, GLTEXTUREGENT, GLTEXTUREGENR, or
GLTEXTUREGENQ) 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 GLEYELINEAR 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
GLINVALIDENUM is generated when coord or pname is not an accepted
defined value, or when pname is GLTEXTUREGENMODE and params is not an
accepted defined value.
GLINVALIDENUM is generated when pname is GLTEXTUREGENMODE, params is
GLSPHEREMAP, and coord is either GLR or GLQ.
GLINVALIDOPERATION is generated if glTexGen is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGetTexGen
glIsEnabled with argument GLTEXTUREGENS
glIsEnabled with argument GLTEXTUREGENT
glIsEnabled with argument GLTEXTUREGENR
glIsEnabled with argument GLTEXTUREGENQ
SEE ALSO
glTexEnv, glTexImage1D, glTexImage2D, glTexParameter
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