Bull's Head and Mermaid - The Bernstein Project - Österreichische ...
Bull's Head and Mermaid - The Bernstein Project - Österreichische ...
Bull's Head and Mermaid - The Bernstein Project - Österreichische ...
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Ill. 12: Watermark detection in a very noisy image<br />
tion. All these elements need first to be identified in order to<br />
properly isolate the watermark. <strong>The</strong> main problem for the<br />
computer is to decide what belongs to the watermark <strong>and</strong><br />
what not.<br />
In our method first groups of connected black pixels are<br />
detected, the so-called Blobs (binary large objects). E.g. a<br />
window in the castle of Ill. 13 is a blob, the group of laid<br />
lines is a blob et cetera. <strong>The</strong>reafter all these blobs are studied<br />
by the computer by looking to their size, their mutual<br />
distances et cetera. Specific properties like many lines with<br />
a certain length are for the computer an indication that it<br />
probably has to do with a blob with laid lines. This blob is<br />
then removed. In the same way blobs with text can be detected<br />
<strong>and</strong> removed et cetera. <strong>The</strong> final result will be the<br />
isolated digital watermark without all its additional information.<br />
In experiments the performance was 84%. Of course,<br />
sometimes it is going wrong; e.g. a chain line is detected as<br />
a part of the watermark.<br />
With respect to the printed collections we have similar<br />
problems. <strong>The</strong> printed Piccard (see Chapter V) consists of<br />
45,000 watermarks, represented in 17 volumes. As a matter<br />
of fact these watermarks should also be in the digital<br />
database. All the pages are now scanned, but since there<br />
are a number of watermarks on each page, further digital<br />
processing of the individual is impossible without a method<br />
that first isolates the individual watermarks. Of course this<br />
can be done manually (e.g. by using Photoshop), but for<br />
45,000 watermarks this is a time consuming job. Also here<br />
the computer can assist. First the number of watermarks is<br />
determined by locating the numbers of the watermarks in a<br />
way as done for Piccard-Online <strong>and</strong> then counting them.<br />
And since these numbers are always below the watermark<br />
the positions of the watermark can be determined <strong>and</strong> finally<br />
the watermark which then can be isolated. In about<br />
88% of the cases this goes correct. In the other cases the<br />
user has to correct the result manually. But still this method<br />
saves much time (Ill. 14).<br />
Matching <strong>and</strong> retrieval of watermarks<br />
Once the digital watermarks have been extracted they can<br />
be stored in a database <strong>and</strong> used for retrieval purposes. Assume<br />
you have an artwork with a watermark <strong>and</strong> want to<br />
Ill. 13: Watermark with additional information, from the Piccard-<br />
Online database<br />
know whether there is an identical watermark in Piccard.<br />
For the expert it is a time consuming task, even if he should<br />
only compare it with watermarks of the same category.<br />
<strong>The</strong>ir amount can be large <strong>and</strong> the differences small. For<br />
the human eye it is rather difficult to decide whether two<br />
watermarks are identical. A very simple method is the following.<br />
<strong>The</strong> computer puts a grid on the watermark, consisting<br />
of blocks of a certain size. Within each blocks the<br />
number of pixels belonging to the watermark are counted.<br />
By doing this for all blocks a series of numbers is obtained,<br />
a so-called vector. And this is again done for all watermarks.<br />
By comparing the series of numbers – which goes<br />
very fast – identical or nearly similar watermarks can easily<br />
be detected. From experiments with the Piccard database it<br />
works out for 91% of the cases.<br />
Chain lines <strong>and</strong> laid lines<br />
Although according to the experts watermarks are the most<br />
important paper feature in order to discover identical pieces<br />
of paper, they are not always present in a piece of paper. If<br />
a sheet of paper with one watermark is divided in smaller<br />
pieces, e.g. in the case of making prints of an etching plate,<br />
in general just one piece of paper contains a watermark,<br />
the other ones do not. For that very reason one also consid-<br />
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