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

Calculating the Mipmap Level

The computation of which mipmap level of a texture to use for a particular pixel depends on the scale factor between the texture image and the size of the polygon to be textured (in pixels). Let’s call this scale factor ρ, and also define a second value, λ, where λ = log₂ ρ + lod_bias. (Since texture images can be multidimensional, it is important to clarify that ρ is the maximum scale factor of all dimensions.)

lod_bias is the level-of-detail bias for the sampler, a constant value set by calling glSamplerParameteri() with the pname parameter set to GL_TEXTURE_LOD_BIAS and is used to adjust λ. By default, lod_bias = 0.0, which has no effect. It’s best to start with this default value and adjust in small amounts, if needed. ...