"Interpolate" seems to have difficulty handling the edge
of the image_map.

Consider the appended scene.  The scene requires an
associated PNG file, called "diagonal.png".  The map I 
used was a 32x32 with a diagonal line.... At risk of
violating list protocol I attach the file, which is 
quite compact.

Note it renders fine with Interpolate set to 0.
However, with 0 or 1, the interpolation wraps around,
generating edge "wraparound" artifacts both in the foreground
"box" and the background plane. 

In the case that the "once" keyword is active, there
should probably be no wrap-around interpolation.

Dan

PII 128MB POV3.1r1 W95

===============  CUT HERE =========================
background { color <0.000,0.000,0.000> } 

#declare Interpolate = 2;
#declare Once = 1;

camera {
  location <0, 0, -5>
  look_at 0
}

light_source {
  <5, 5, -10>
  color rgb 1
}   


plane {
  -z,
  0
  texture {
    pigment {
      image_map {
        png "c:\tmp\diagonal.png"
        interpolate Interpolate 
        #if (Once) once #end
      }
      warp { repeat x flip x }
      warp { repeat y flip y }
    }
  }
}

box {
  <0, 0, -1>,
  <1, 1, -0.99>
  texture {
    pigment {
      image_map {
        png "c:\tmp\diagonal.png"
        interpolate Interpolate
        #if (Once) once #end
      }
    }
  }
}

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Preview of image 'diagonal.png'