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.
.P
If the texture generation function is GL_OBJECT_LINEAR ,
the function
.ce
g = p sub 1 x sub o + p sub 2 y sub o + p sub 3 z sub o + p sub 4 w sub o
is used, where g is the value computed for the coordinate named in coord,
p sub 1,
p sub 2,
p sub 3,
and
p sub 4 are the four values supplied in params, and
x sub o,
y sub o,
z sub o, and
w sub 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 sub 1, p sub 2, p sub 3, and p sub 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
.ce
g = {p sub 1} sup prime ~x sub e + {p sub 2} sup prime ~y sub e + {p sub 3} sup prime ~z sub e + {p sub 4} sup prime ~w sub e
is used, where
.ce
$( {p sub 1} sup prime
~~{p sub 2} sup prime~~{p sub 3} sup prime~~
{{p sub 4}sup prime}) = ( p sub 1~~ p sub 2~~ p sub 3~~ p sub 4 ) ~M sup -1$
and
x sub e,
y sub e,
z sub e, and
w sub e are the eye coordinates of the vertex,
p sub 1,
p sub 2,
p sub 3,
and
p sub 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
.ce
f ~=~ ( f sub x~~f sub y~~f sub z ) sup T
be the reflection vector such that
.ce
f ~=~ u ~-~ 2 n sup prime n sup prime sup T u
Finally, let m ~=~ 2 sqrt { f sub x sup {~2} + f sub y sup {~2} + (f sub z + 1 ) sup 2}.
Then the values assigned to the s and t texture coordinates are
.ce 1
s ~=~ f sub x over m ~+~ 1 over 2
.sp
.ce 1
t ~=~ f sub 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_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).
GL.glTexEnv()