OpenGL mipmapping: level outside the range?

I'm going deeper on OpenGL texture mipmapping.

I noticed in the specification that mipmap levels less than zero and greater than log2(maxSize) + 1 are allowed.

Effectively TexImage2D doesn't specify errors for level parameter. So... Probably开发者_如何学JAVA those mipmaps are not accessed automatically using the standard texture access routines...

How could be effectively used this feature?

For the negative case, the glTexImage2D's man page says:

GL_INVALID_VALUE is generated if level is less than 0.

For the greater than log2(maxsize) case, the specification says what happens to those levels in Raterization/Texturing/Texture Completeness. The short of it is that, yes, they are ignored.