+5. run ocropus's binarise over all the pngs
+
+6. foreach binarised scan page
+ 6.1. create a mask from the original (unbinarised) page
+ 6.2. use the mask to trim the binarised page (cutting this off improves unpaper's accuracy)
+ 6.3. run unpaper over the clean binarised page, creating 2 pages (physical page)
+ 6.4. foreach physical page
+ 6.4.1. remask and retrim
+ 6.4.2. attempt to detect if a physical page contains 2 logical pages,
+ 6.4.2.1. if so split with unpaper
+ 6.4.3. do any final processing (resize for bebook)
+7. move all the final pictures into a final picture directory