Name
gl.TexGen -- control the generation of texture coordinates
Synopsis
gl.TexGen(coord, pname, param)
Function
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.

Inputs
coord
specifies a texture coordinate; must be one of #GL_S, #GL_T, #GL_R, or #GL_Q
pname
specifies the symbolic name of the texture-coordinate generation function or function parameters (see above)
param
single value or table containing parameters for pname (see above)
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 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().

Associated gets
gl.GetTexGen()

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


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