Every time I drunk browse so I see an unanswered fwidth question.
And it makes me wonder what it actually was designed to do.
Reading the docs it is:
abs(dFdx(p)) + abs(dFdy(p))
So it is not classic mip selection which is max(dx,dy).
Is it for alternative mip selection? But I fail to find a case where abs(dx) + abs(dy) would be better.
There must be some siggraph paper or common algorithm I am completely missing that uses that function. And it must be really popular because it made it into GLSL.
The only thing I can think of is some 2d post filter I am missing.
But what?
I am sure somebody here knows and once you see it it's obvious.
So: What algorithm uses abs(dx) + abs(dy)?