glTexGen

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 GL_S, GL_T, GL_R, or GL_Q.
pname	Specifies the symbolic name of the texture-coordinate generation function. Must be GL_TEXTURE_GEN_MODE.
param	Specifies a single-valued texture generation parameter, one of GL_OBJECT_LINEAR, GL_EYE_LINEAR, or GL_SPHERE_MAP.

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 GL_S, GL_T, GL_R, or GL_Q.
pname	Specifies the symbolic name of the texture-coordinate generation function or function parameters. Must be GL_TEXTURE_GEN_MODE, GL_OBJECT_PLANE, or GL_EYE_PLANE.
params	Specifies a pointer to an array of texture generation parameters. If pname is GL_TEXTURE_GEN_MODE, the array must contain a single symbolic constant, one of GL_OBJECT_LINEAR, GL_EYE_LINEAR, or GL_SPHERE_MAP. 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 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 = p₁x_w + p₂ y_o + p₃z_o + p₄w_o

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 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 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'x_e + p₂' y_e + p₃'z_e + p₄'w_e

is used, where

(p₁' p₂' p₃' and p₄') = (p₁ p₂ p₃ and p₄)M^-1

and x_e, y_e, z_e, and w_e 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 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' 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 - 2n'n'^Tu

Finally, let m = 2 * SQRT(f_x² + f_y² + (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)

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