Gl.TexGeni (gb.opengl)

Control the generation of texture coordinates.

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.

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

Gl.TexGen 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

is used, where $\mathit{g}$ is the value computed for the coordinate named in coord, ${\mathit{p}}_1$, ${\mathit{p}}_2$, ${\mathit{p}}_3$, and ${\mathit{p}}_4$ are the four values supplied in params, and ${\mathit{x}}_{\mathit{o}}$, ${\mathit{y}}_{\mathit{o}}$, ${\mathit{z}}_{\mathit{o}}$, and ${\mathit{w}}_{\mathit{o}}$ 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 ${\mathit{p}}_1$, ${\mathit{p}}_2$, ${\mathit{p}}_3$, and ${\mathit{p}}_4$). 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

is used, where

and ${\mathit{x}}_{\mathit{e}}$, ${\mathit{y}}_{\mathit{e}}$, ${\mathit{z}}_{\mathit{e}}$, and ${\mathit{w}}_{\mathit{e}}$ are the eye coordinates of the vertex, ${\mathit{p}}_1$, ${\mathit{p}}_2$, ${\mathit{p}}_3$, and ${\mathit{p}}_4$ are the values supplied in params, and $\mathit{M}$ is the modelview matrix when Gl.TexGen is invoked. If $\mathit{M}$ 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, $\mathit{s}$ and $\mathit{t}$ 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

be the reflection vector such that

Finally, let . Then the values assigned to the $\mathit{s}$ and $\mathit{t}$ texture coordinates are

To enable or disable a texture-coordinate generation function, call Gl.Enable or Gl.Disable 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 $\mathit{s}$ plane equations are (1, 0, 0, 0), both $\mathit{t}$ plane equations are (0, 1, 0, 0), and all $\mathit{r}$ and $\mathit{q}$ plane equations are (0, 0, 0, 0).

When the ARB_multitexture extension is supported, Gl.TexGen sets the texture generation parameters for the currently active texture unit, selected with Gl.ActiveTexture.

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 Gl.TexGen is executed between the execution of Gl.Begin and the corresponding execution of Gl.End.

Associated Gets

Gl.GetTexGen

Gl.IsEnabled with argument Gl.TEXTURE_GEN_S

Gl.IsEnabled with argument Gl.TEXTURE_GEN_T

Gl.IsEnabled with argument Gl.TEXTURE_GEN_R

Gl.IsEnabled with argument Gl.TEXTURE_GEN_Q

See also

See original documentation on OpenGL website