> Hi,
>
>     I've been trying to add a checkered plane to an image. The plane itself can
> be generated with a POV-Ray file comprising:
>
> camera {
>     perspective
>     location<15, -12, 72>
>     look_at<-1,3,35>
>     sky<1,-1,0>
>     }
> light_source {
>     <50, -20, 35>, color rgb<1.000,1.000,1.000>
>     parallel
>     point_at<0,0,20>
>     }
> plane {<1,-1,-0.1>, -30
>     pigment { checker rgb<0.8, 0.8, 1.0>, rgb<1.0, 1.0, 1.0>  scale 20}
>     }
>
> With POV-Ray 3.6.2 under Windows XP (Mac Parallels VM), I see overlapping lines
> of checkers that look like they're going to different perspective points (sorry
> I can't attach a file here, please let me know an e-mail address and I'll be
> happy to send along a PNG of what I see). I was expecting a simple checkered
> plane rather than two overlapping ones. What am I doing wrong here?
>
> Thanks for any help!
>
>
>
>

This is the reason for what you see: plane {<1,-1,-0.1>

The checker pattern is composed of alterning unit cube areas or two 
pigments or patterns, alligned to the base axis.
AND
Your plane is tilted relative to the checker pattern.

It would be beter to set your plane as follow:

plane{-1*y, -30
  pigment { checker rgb<0.8, 0.8, 1.0>, rgb<1.0, 1.0, 1.0> scale 20}
  rotate<-45,0,1>
}

Adjust the rotation as needed.

You could also rotate the checker pattern so that it's orientation 
matches that of your plane.

Another possibility, is to shift the pattern and scale it by some large 
value in ine direction:
pigment { checker rgb<0.8, 0.8, 1.0>, rgb<1.0, 1.0, 1.0>translate -0.5*y 
scale<20, 99999, 20>}

And, no, the use of negative values have nothing to do with your result.



Alain

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