Bull's Head and Mermaid - The Bernstein Project - Österreichische ...

ers other paper features like the paper structure, since this
is present in all pieces of paper, like chain and laid lines. Especially,
the laid line density (the number of laid lines per
centimetre) as well as the chain line distances (the distances
between the chain lines) are important features. Until now
these features were extracted manually by simple measuring
them with a ruler.
Doing this with a computer is also now far from simple.
Also here we have to do with noise. Advanced image processing
techniques are needed to extract the relevant parameters.
The following two illustrations show a process of
chain line detection. Illustration 15 shows how in a number
of various steps (a-d), the X-ray image is processed in such
a way that the resulting image is more suitable for a computer
to look for the chain lines. Then the image d is
scanned line by line. For each line the grayscale values are
represented by numbers and these are translated into a sig-
112
Ill. 14: A page from the printed Piccard collection with one of the isolated watermarks in two forms
nal (e). It is a bit like an electrocardiogram; with the heartbeats
showing up as high point that may be indicate the
position of a chain line. By subjecting all the resulting
‘heartbeat’ signals to a logical algorithm enables the computer
to pinpoint the exact positions of the chain lines (f-h).
Having obtained the line patterns and the corresponding
chain line distances retrieval is now rather simple. The chain
line distances are compared with other series of numbers in
the database. On the basis of some similarity measure the
computer comes up with a number of possibly identical papers
with a decreasing degree of certainty (Ill. 16).
As a matter of fact the best results are obtained when
the information of watermarks as well as of laid line density
and chain line distances are used at the same time.
J.C.A.v.d.L. / H.M.O.