gl.TexGen(coord, pname, param)
gl.TexGen()
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
param
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
, param
contains coefficients for the corresponding texture generation function.
If the texture generation function is #GL_OBJECT_LINEAR
, the function
g = p1x0 + p2y0 + p3z0 + p4w0 |
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 param
,
and x0, y0, z0, and w0 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 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 = p1'x0 + p2'y0 + p3'z0 + p4'w0 |
is used, where
(p1' p2' p3' p4') = ( p1 p2 p3 p4) M^-1 |
and x0, y0, z0, and w0 are the eye coordinates of the vertex, p1, p2, p3, and p4 are the values supplied in param
, and M is the
modelview matrix when gl.TexGen()
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 param
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' be the current normal,
after transformation to eye coordinates. Let f = (fx fy fz)^T be the reflection vector such that
f = u - 2n' n'^Tu |
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 gl.Enable() or gl.Disable()
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).
Please consult an OpenGL reference manual for more information.
#GL_S
, #GL_T
, #GL_R
, or #GL_Q
pname
(see above)#GL_INVALID_ENUM
is generated when coord
or pname
is not an accepted defined value, or when pname
is #GL_TEXTURE_GEN_MODE
and param
is not an accepted defined value.
#GL_INVALID_ENUM
is generated when pname
is #GL_TEXTURE_GEN_MODE
, param
is #GL_SPHERE_MAP
, and coord
is either #GL_R
or #GL_Q
.
#GL_INVALID_OPERATION
is generated if gl.TexGen()
is executed between the execution of gl.Begin() and the corresponding execution of gl.End().
gl.IsEnabled() with argument #GL_TEXTURE_GEN_S
gl.IsEnabled() with argument #GL_TEXTURE_GEN_T
gl.IsEnabled() with argument #GL_TEXTURE_GEN_R
gl.IsEnabled() with argument #GL_TEXTURE_GEN_Q