Parameters

Parameters

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 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.

If the texture generation function is GL_OBJECT_LINEAR, the function

g = p 1 x o + p 2 y o + p 3 z o + p 4 w o

is used, where g is the value computed for the coordinate named in coord, p 1 , p 2 , p 3 , and p 4 are the four values supplied in params, and x o , y o , z o , and w 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 1 , p 2 , p 3 , and p 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

g = p 1 ' x e + p 2 ' y e + p 3 ' z e + p 4 ' w e

is used, where

( p 1 ' p 2 ' p 3 ' p 4 ' ) = ( p 1 p 2 p 3 p 4 ) M -1

and x e , y e , z e , and w e are the eye coordinates of the vertex, p 1 , p 2 , p 3 , and p 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

f = ( f x f y f z ) T be the reflection vector such that

f = u - 2 n ' n ' T u

Finally, let m = 2 f x 2 + f y 2 + ( f z + 1 ) 2 . Then the values assigned to the s and t texture coordinates are

s = f x m + 1 2 t = f y m + 1 2

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 params is not an accepted defined value.

GL_INVALID_ENUM is generated when pname is GL_TEXTURE_GEN_MODE, params is GL_SPHERE_MAP, and coord is either GL_R or GL_Q.

Associated Gets

glGetTexGen

See Also