Function glTexGendv

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 = 𝐩₁ × 𝐱ₒ + 𝐩₂ × 𝐲ₒ + 𝐩₃ × 𝐳ₒ + 𝐩₄ × 𝐰ₒ

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 𝐱ₒ, 𝐲ₒ, 𝐳ₒ, and 𝐰ₒ 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′′ × 𝐱ₑ + 𝐩₂′′ × 𝐲ₑ + 𝐩₃′′ × 𝐳ₑ + 𝐩₄′′ × 𝐰ₑ

is used, where

(𝐩₁′′𝐩₂′′𝐩₃′′𝐩₄′′) = (𝐩₁𝐩₂𝐩₃𝐩₄)ᴹ^⁻¹

and 𝐱ₑ, 𝐲ₑ, 𝐳ₑ, and 𝐰ₑ 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 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 = (𝐟ₓ𝐟ᵧ𝐟𝑧)ᵀ be the reflection vector such that

𝐟 = 𝐮 - 2𝐧′′𝐧′′ᵀ𝐮

Finally, let 𝐦 = 2√(𝐟ₓ² + 𝐟ᵧ² + (𝐟𝑧 + 1)²). Then the values assigned to the 𝐬 and 𝐭 texture coordinates are

𝐬 = 𝐟ₓ / 𝐦 + 1 / 2

𝐭 = 𝐟ᵧ / 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.

Summary

control the generation of texture coordinates

See

glTexGen