title: what is the fwidth glsl function actually used for?
body: <p>Every time I drunk browse so I see an unanswered <code>fwidth</code> question.
  And it makes me wonder what it actually was designed to do.</p>
  <p>Reading the docs it is:
  <code>abs(dFdx(p)) + abs(dFdy(p))</code></p>
  <p>So it is not classic mip selection which is <code>max(dx,dy)</code>.
  Is it for alternative mip selection? But I fail to find a case where <code>abs(dx) + abs(dy)</code> 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.<br />
  But what?
  I am sure somebody here knows and once you see it it's obvious.
  So: What algorithm uses <code>abs(dx) + abs(dy)</code>?</p>