http://bugs.freedesktop.org/show_bug.cgi?id=21694


Brian Paul <[email protected]> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX




--- Comment #3 from Brian Paul <[email protected]>  2009-05-11 
15:41:12 PST ---
It's not an LOD clamping bug.  It's an issue with LOD computation with cube
maps.

The original texcoord (S,T,R,Q) is examined to determine which of the cube
faces is to be sampled from.  The determination is made by finding which of S,
T, R have the greatest magnitude.  After that, a new texture coordinate
(S',T',0,1) is computed which is basically a 2D coordinate within the chosen
cube face.

Note that the partial derivatives of the original texcoord (S,T,R,Q) cannot be
used to chose the mipmap LOD since it's a direction, not a face coordinate.  So
the only way to correctly determine LOD within a cube face is to examine the
new texcoords (S',T',0,1) for some number of adjacent pixels (typically a
2x2-pixel quad).

We can't easily do that in Mesa's software rasterizer since each texture sample
is considered in isolation.  But with gallium's software renderer we always
work on a 2x2 "quad" of fragments so we can do the proper LOD computation
within each cube face (at least for samples all taken from the same cube face).

So, I'm afraid there's no simple fix for this in swrast.  I'll have to mark
this as 'wontfix'


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