The line
#local fchecker = function { pigment { checker } }
probably should have been
#local fchecker = function { pigment { checker 0 1} };
The default colors for a checker pigment are green
and blue. Since you later evaluate the gray values,
which should be nearly equal, I think somewhere
the pov engine gets confused.

After making this change it rendered fine for me
at 1000x1000 no AA and default radiosity (23 sec)

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