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   Computes the pixel map conversion factor.
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   Form:
   f = ComputePixelMapScale( pixelWidth, fOV )
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   ------
   Inputs
   ------
   pixelWidth    (1,1)   Width in pixels
   fOV           (1,1)   Field of view in same direction (rad)

   -------
   Outputs
   -------
   f             (1,1)   Scale

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