reconstructing shredded documents through feature matching
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Reconstructing shredded documentsthrough feature matching
Authors: Edson Justino, Luiz S. Oliveira, Cinthia FreitasSource: Forensic Science International 160 (2006), pp. 140–147
Date: 2012.12.19Speaker: Meng-Jing Tsai
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Different Kinds of Shredding
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Outline
҉� Introductions҉� Proposed Method҉� Experimental Result҉� Conclusions
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Introductions
• The amount of time necessary to reconstruct a document depends on the size and the number of fragments, and it can be measured in days or even weeks.
• Traditional puzzle solving algorithms usually take into account smooth edges and well defined corners.
• The act of shredding a piece of paper by hand often produces some irregularities in the boundaries.
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Proposed Method
• The block diagram of the proposed methodology
1.
2.
3.
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Proposed Method
• Pre-processing– In order to overcome this kind of problem, we
have tested different algorithms, and the one that brought the best results was the well-known Douglas–Peucker (DP) algorithm.
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Proposed Method• Douglas-Peucker Algorithm
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Proposed Method
• Pre-processing
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Proposed Method
• Feature extraction
Fig. 1 Angle features extracted from the polygon
(180,110)
(180,0)
(10,110)
(10,70) (55,67)
(67,25)
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43.6120°
vuuv
cos
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Proposed Method
• Matching
– Computing the similarity between polygons
– Global search
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Proposed Method
• Computing the similarity between polygons– If the complementarity is verified like in Fig. 2,
then Wangles=1.
Fig. 2 Similarity between angles
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Proposed Method
• Computing the similarity between polygons
Fig. 3 Distance features extracted from the polygon
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Proposed Method
• We consider the relevance of the matching regarding the perimeter of the fragment using the following rules:– If the contour matched represents more than 1/5 of the
perimeter of the fragment, then Wmatching = Wmatching+2.– If the contour matched represents more than 1/10 of
the perimeter of the fragment, then Wmatching = Wmatching+1.
– Otherwise, Wmatching is not increased.
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Proposed Method
• Global search– Let us consider a shredded document D ={F1, F2,..., Fn} composed of n fragments.
– The algorithm compares the fragment F1 with all the other fragments searching for the best matching.
Fig. 4 Best matching (a) fragments i and j and (b) new fragment Fij where three vertices were removed
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Proposed Method
• Steps of the document reconstruction
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Experimental Result
• Examples of a document totally reconstructed: (a) fragments and (b) document reconstructed.
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Experimental Result
• Performance of the proposed methodology in reconstructing documents shredded by hand.
The fragments size range from 1cm × 1cm to 5cm × 5cm.
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Conclusions
• This paper proposed a method for document reconstruction based on feature matching.
• It can be addressed by choosing the most important aspects for the application.
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Thank youfor your listening.
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