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/.