Normals appear to have an inherent directional bias independent of 
pattern.  To eliminate as a variable any bias in a built-in pattern, I 
used an explicit normal function.  The two images should be mirror 
images of each other, but they are not.  The bias shows in all versions 
from 3.5 to 3.8.

----------[BEGIN CODE]----------
// +KFF2
#version max (3.5, min (3.8, version));

global_settings { assumed_gamma 1 }

light_source { <(frame_number = 2? 1: -1), 1, -1.4> * 1000, rgb 1 }

camera
{ location -6 * z
   right x
   angle 30
}

box
{ 0, 3
   pigment { rgb <1, 0.5, 0.5> }
   normal
   { function
     { max
       ( select (x - 1, 1, select (x - 2, 0, 1)),
         select (y - 1, 1, select (y - 2, 0, 1))
       )
     }
     accuracy 0.1
   }
   translate <-1.5, -1.5, 0>
}
-----------[END CODE]-----------

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