L(x, y) = (0.5 + 0.5*fade*sin(2.0*Pi*a*exp(5*a) ))
where a = x / double(nx);
and fade = (1.0 - y / double(ny))^3;
where x and y are coordinates
in the system where the lower-left corner of the image is (0,0) and the
upper right is (nx,ny).
And an image of a black and white infinite axis-aligned checkerboard
on the xy (z=0) plane, with the eye at
(0,0,1), looking in direction (1, 1, 0) and a vertical field-of-view
of 90 degrees.
One way to define checker color is by looking at the sign of the value sin(x/pi)*sin(y/pi).