1991-2006 Silicon Graphics, Inc.
glTexGen 3G
glTexGen control the generation of texture coordinates

## C Specification

void `glTexGeni` GLenum coord GLenum pname GLint param void `glTexGenf` GLenum coord GLenum pname GLfloat param void `glTexGend` GLenum coord GLenum pname GLdouble 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`, `GL_SPHERE_MAP`, `GL_NORMAL_MAP`, or `GL_REFLECTION_MAP`.

## C Specification

void `glTexGeniv` GLenum coord GLenum pname const GLint * params void `glTexGenfv` GLenum coord GLenum pname const GLfloat * params void `glTexGendv` GLenum coord GLenum pname const GLdouble * 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`, then the array must contain a single symbolic constant, one of `GL_OBJECT_LINEAR`, `GL_EYE_LINEAR`, `GL_SPHERE_MAP`, `GL_NORMAL_MAP`, or `GL_REFLECTION_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`, `GL_SPHERE_MAP`, `GL_NORMAL_MAP`, or `GL_REFLECTION_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 the texture generation function 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

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 `ARB_multitexture` extension is supported, `glTexGen` sets the texture generation parameters for the currently active texture unit, selected with glActiveTexture.

## 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`.

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

## Associated Gets

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`

## See Also

glActiveTexture, glCopyPixels, glCopyTexImage2D, glCopyTexSubImage1D, glCopyTexSubImage2D, glCopyTexSubImage3D, glTexEnv, glTexImage1D, glTexImage2D, glTexImage3D, glTexParameter, glTexSubImage1D, glTexSubImage2D, glTexSubImage3D

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.