I am submitting another bug report related to the so called Stacking distortion in gscan2pdf - ID: 3382109. This bug, whose effect can be described as a shift along the horizontal direction of large/small rectangular blocks of the image itself, has been elusive but regularly in the use of gscan2pdf as a front-end for digitizing documents. I've succeded in reproducing it deterministically, and I've added the log file of the session, a copy of the standard error of the session, and a precise description of the steps I've done to unveil it. In order to describe those steps, let's precise the structure of my system: I'm running a Ubuntu 12.10 fully updated on Vaio VGN-FE48M, and I've installed gscan2pdf 1.1.0 (the latest stable?), and the file I am working on is a 1857 paper of Faà di Bruno, taken from a copy of the Quarterly Journal of Pure and Applied Mathematics made freely available by Google books:
1) I downloaded a PDF copy of the whole Journal from the link http://books.google.com/books?id=7BELAAAAYAAJ&pg=PA359
2) I lauched gscan2pdf from the command line: gscan2pdf --log=log 2> std_err_log
3) I opened the downloaded PDF file and exctrated the two pages I'm interested to, i.e. pages 425 and 426.
4) I removed Google's watermarks
5) I renumbered the pages. I think this step is not significant, since during the recorded test I have to do it twice since I chosen the a wrong initial value (i.e 2 instead of 1) for the page number.
6) I cleaned the two pages with unpaper using the standard (i.e. factory default) options. This step, the following one and their relative order of execution may be important in the fixing of the bug, see below.
7) I then cleaned the same two pages with gimp (version 2.8.2), in order to remove the upper part of the text (the concluding remarks of the preceeding paper) on the first page and to remove the lower black streams on the second page plus some big black points unpaper cannot remove on both pages.
8) I saved the file in PDF format, and the saved file shows stacking distortion.
I repeated the test three times. The first one succeded in reproducing the bug while the second one did not, but in that test I did step 7 before step 8: I also noticed that the files in gimp where in PNG instead of PNM format. For the third, last test, to which the logs and the result file included here are related, I powerd off the computer, restarted it and then executed exactly the sequence described above. Well, I hope all this will help fixig the bug: if you need me to experiment more, please let me know. Thank you for your attention.
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