(b) 
Without seep thru:
The gradient approximates the euclidean distance along the sheet's
surface, the fold is irrelevant.
(a) (b)
With seep thru:
The gradient approximates the euclidean distance on the folded sheet
((b) shows the gradient values after unfolding the sheet)
(a) 
(b) 
NOTE: the image on the right reminds me of butterfly wings. this might
be how the perfectly symmetric wing images are produced - the winges are
folded together when the pattern forms and it seeps through both wings.
when the wings are opened (unfolded) then you get perfect mirror images...