OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Explicit Gradient Specification

It is also possible to override the level-of-detail calculation for mipmapping at an earlier part of the process rather than explicitly giving the level-of-detail parameter directly. When the gradient texture functions are used, the partial derivative of the texture coordinates is given as a parameter. Some key prototypes are listed below. (A full list is in Appendix C, “Built-in GLSL Variables and Functions”.)

gvec4 textureGrad(gsampler1D tex, float P, float dPdx, float dPdy);gvec4 textureGrad(gsampler2D tex, vec2 P, vec2 dPdx, vec2 dPdy);gvec4 textureGrad(gsampler3D tex, vec3 P, vec3 dPdx, vec3 dPdy);gvec4 textureGrad(gsamplerCube tex, vec3 P, vec3 dPdx,                         vec3 dPdy);gvec4 textureGrad