GLTEXGEN(3G) 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 GLS, GLT,
GLR, or GLQ.
pname Specifies the symbolic name of the texture-coordinate genera-
tion 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 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 parame-
ters. If pname is GLTEXTUREGENMODE, then the array must con-
tain a 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; it must be one of the 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 = 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 GLOBJECTLINEAR 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 GLEYELINEAR, the function
g = p1'xe ] p2'ye ] p3'ze ] p4'we
is used, where
(p1' p2' p3' p4') = (p1 p2 p3 p4) 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 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 n sup prime be the current normal, after transformation to eye
coordinates. Let
f = (fx fy fz)^T
be the reflection vector such that
f = u - 2 n' n'^T u
Finally, let m = 2 sqrt( fx^2 ] fy^2 ] (fz ] 1)^2). Then the values
assigned to the s and t texture coordinates are
s = (fx/m) ] (1/2)
t = (fy/m) ] (1/2)
To enable or disable a texture-coordinate generation function, call
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 coordi-
nates. 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).
When the GLARBmultitexture extension is supported, glTexGen set the
texture generation parameters for the currently active texture unit,
selected with glActiveTextureARB.
ERORS
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.
ASOCIATED GETS
glGetTexGen
glIsEnabled with argument GLTEXTUREGENS
glIsEnabled with argument GLTEXTUREGENT
glIsEnabled with argument GLTEXTUREGENR
glIsEnabled with argument GLTEXTUREGENQ
SEE ALSO
glActiveTextureARB, glCopyPixels, glCopyTexImage2D,
glCopyTexSubImage1D, glCopyTexSubImage2D, glCopyTexSubImage3D,
glTexEnv, glTexImage1D, glTexImage2D, glTexImage3D, glTexParameter,
glTexSubImage1D, glTexSubImage2D, glTexSubImage3D
GLTEXGEN(3G)
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