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Manual Reference Pages  - GLTEXGEN (3G)

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glTexGend, glTexGenf, glTexGeni, glTexGendv, glTexGenfv, glTexGeniv - control the generation of texture coordinates

CONTENTS

C Specification
Parameters
C Specification
Parameters
Description
Errors
Associated Gets
See Also

void glTexGend( GLenum coord,

              GLenum pname,
              GLdouble param )

void glTexGenf( GLenum coord,

              GLenum pname,
              GLfloat param )

void glTexGeni( GLenum coord,

              GLenum pname,
              GLint param )

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.

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 )

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, then 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.

 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 = p₁ x_o + p₂ y_o + p₃ z_o + p₄ w_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

( p sub 1 sup prime ~~p sub 2 sup prime~~p sub 3 sup prime~~ (p₁^' p₂^' p₃^' p₄^' ) = (p₁ p₂ p₃ 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 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 - 2n^' n^{' T}u

Finally, let m = 2 sqrt 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

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).

When the  GL_ARB_multitexture extension is supported,  glTexGen set the texture generation parameters for the currently active texture unit, selected with  glActiveTextureARB.

 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.

 GL_INVALID_OPERATION is generated if  glTexGen is executed between the execution of  glBegin and the corresponding execution of  glEnd.

 glGetTexGen
 glIsEnabled with argument  GL_TEXTURE_GEN_S
 glIsEnabled with argument  GL_TEXTURE_GEN_T
 glIsEnabled with argument  GL_TEXTURE_GEN_R
 glIsEnabled with argument  GL_TEXTURE_GEN_Q

 glActiveTextureARB(3G),  glCopyPixels(3G),  glCopyTexImage2D(3G),  glCopyTexSubImage1D(3G),  glCopyTexSubImage2D(3G),  glCopyTexSubImage3D(3G),  glTexEnv(3G),  glTexImage1D(3G),  glTexImage2D(3G),  glTexImage3D(3G),  glTexParameter(3G),  glTexSubImage1D(3G),  glTexSubImage2D(3G),  glTexSubImage3D(3G)

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