Fraunhofer IPK

WHEN the shredding machines failed, and the mob was at the gates, the spooks at East Germany's State Security Service, better known as the Stasi, tried turning their files into mush by dunking them in water. But the number of bathtubs in their headquarters in Normannenstrasse was as unequal to the task as the machines had been. In the end, they resorted to tearing each page up by hand. The fact that many of the resulting shreds are only a few millimetres across is testament to just how much the soon-to-be-ex-members of the intelligence service did not want their work to fall into the public domain.

If Bertram Nickolay of the Fraunhofer Institute for Production Systems and Design Technology, in Berlin, has his way, however, the public domain is where they soon will be. Using the institute's expertise in pattern-matching technology, he and his group are about to embark on one of the biggest jigsaw puzzles of all time—or, rather, 45m of them. For that is the number of pages which the 600m fragments of paper, stored in more than 16,000 bags, that were recovered from Normannenstrasse are thought to represent.

At first glance, the task of joining that many shreds looks impossible, even when each shred has been scanned so that the matching can be done by machine. The secret, as with any computing task, is to break the problem into manageable chunks.

The first stage of this breakdown was done, unwittingly, by the Stasi themselves. Since they were in a hurry, the shredders tended to stuff all the bits of a given document into a single bag. That simplifies the problem from nearly impossible to merely unwieldy.

The next stage is to group the shreds from a single bag according to various criteria. These include the colour and texture of the paper a shred is made from, whether that paper is lined or not, the colour of the ink used, whether that ink represents a picture, a piece of typewritten text or a piece of handwriting, and—if it is handwriting—what style.

Only when a group of related shreds has been found using these criteria does the actual puzzling begin. That is done the way human puzzlers do it, by paying attention to shapes and sizes of the pieces, and the contours of their edges. If two shreds can be connected, they are regarded as one larger shred, and are thrown back into the heap of images to be analysed and compared with the others. Thus, as with a real jigsaw, areas get progressively filled in until the whole picture is complete.

Like a human, the program that does the puzzling is capable of learning. It spawns slightly altered versions of itself that compete for computer time on the basis of their success at finding matches. The most successful are then mutated again, in a process similar to biological evolution. This is necessary because unlike a real jigsaw—or a machine-shredded document—in which the pieces fit perfectly, the shreds of a torn document are slightly distorted and frayed at the edges. Deciding what matches what therefore requires judgment, which is notoriously difficult to program in advance. Indeed, if the program really cannot make up its mind whether two shreds match or not, it refers the matter to a human operator.

Dr Nickolay aims to process about 400 bags over the course of the next two years, as a final test of the technology. If that works, it will just be a question of adding more scanners and computers to expose the truth about East Germany's dark past.