An evaluation survey of binarization algorithms on historical documents.
ABSTRACT Document binarization is an active research area for many years. There are many difficulties associated with satisfactory binarization of document images and especially in cases of degraded historical documents. In this paper, we try to answer the question ldquohow well an existing binarization algorithm can binarize a degraded document image?rdquo We propose a new technique for the validation of document binarization algorithms. Our method is simple in its implementation and can be performed on any binarization algorithm since it doesnpsilat require anything more than the binarization stage. Then we apply the proposed technique to 30 existing binarization algorithms. Experimental results and conclusions are presented.
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ABSTRACT: This article presents a new technique for the binarisation of historical document images characterized by deteriorations and damages making their automatic processing difficult at several levels. The proposed method is based on hybrid thresholding in order to combines the advantages of global and local thresholding methods, and on the mixture of several binarization techniques. Two stages have been included. In the first stage, global thresholding is applied on the entire image and two different thresholds are determined from which the most of pixels are classified into foreground or background. In the second stage, the remaining pixels are assigned to foreground or background classes based on local analysis. In this stage, several local thresholding methods are combined and the final binary value of each remaining pixel is chosen as the most probable one. The proposed technique has been tested on a large collection of standard and synthetic documents and compared with well known methods using standard measures and showed powerful.Advances in Multimedia 01/2014;
An Evaluation Survey of Binarization Algorithms on Historical Documents
Pavlos Stathis1, Ergina Kavallieratou2 and Nikos Papamarkos1
1Image Processing and Multimedia Laboratory
Department of Electrical & Computer Engineering
Democritus University of Thrace, 67100 Xanthi, Greece
2Dept. of Information and Communication Systems Engineering
University of the Aegean, 83200 Karlovassi, Samos, Greece
Document binarization is an active research area
for many years. There are many difficulties associated
with satisfactory binarization of document images and
especially in cases of degraded historical documents.
In this paper, we try to answer the question “how well
an existing binarization algorithm can binarize a
degraded document image?” We propose a new
technique for the validation of document binarization
algorithms. Our method is simple in its implementation
and can be performed on any binarization algorithm
since it doesn’t require anything more than the
binarization stage. Then we apply the proposed
technique to 30 existing binarization algorithms.
Experimental results and conclusions are presented.
The document binarization is a preprocessing task,
very useful to document processing systems. It
automatically converts the document images in a bi-
level form in such way that the foreground information
is represented by black pixels and the background by
This simple procedure has been proved to be a very
difficult task, especially in the case of historical
documents where very specialized problems arise such
as variation in contrast and illumination, smearing and
smudging of text, seeping of ink to the other side of
the page, and general degradation of the paper and ink
due to aging.
Several algorithms have been proposed for the
document binarization task [9-38]. However, the
selection of the most appropriate one is not a simple
procedure. The evaluation and comparison of these
algorithms is proved to be another difficult task since
there is no objective way to compare the results.
Leedham et al.  compared five binarization
algorithms by using the precision and recall analysis of
the resultant words in the foreground. He et al. 
compared six algorithms by evaluating their effect on
end-to-end word recognition performance utilizing a
commercial OCR engine. Sezgin and Sankur 
described 40 thresholding algorithms and categorized
them according to the used information content. They
measured and ranked their performance comparatively
in two different contexts of images.
All the mentioned works presented some very
interesting conclusions. However, the problem is that
almost in every case, they try to use results from
ensuing tasks in document processing hierarchy, in
order to estimate the performance of the binarization
algorithm. In case of historical documents where their
quality obstructs the recognition, and sometimes the
word segmentation as well, this method of evaluation
can be proved problematic. On the other hand, we need
a different, more direct evaluation technique dealing
only with the binarization stage.
The ideal way of evaluation should be able to
decide, for each pixel, if it has finally succeeded the
right color (black or white) after the binarization. This
task is implemented in this paper in an automatic way.
A wide range of binarization algorithms, from the
oldest  till the newest ones , are examined and
their performance is measured using a collection of
artificial historical documents. More information about
the tested algorithms can be found in the respective
references [9-38]. The proposed procedure is described
in detail in the section 2, while the experimental results
and the conclusions are given in sections 3 and 4,
978-1-4244-2175-6/08/$25.00 ©2008 IEEE
2. The proposed technique
Our experiments were performed on artificial
historical documents that imitate the common
problems of historical documents. The artificial
historical documents were constructed by using
techniques of image mosaicing and combining old
blank document pages with noise-free pdf documents.
This way, during the evaluation, it is possible to
objectively decide for every single pixel if its value is
correct comparing its color with the corresponding
pixel in the original pdf document.
Two sets of images were combined by using image
mosaicing techniques. The first set consists of ten
document images in pdf format, including tables,
graphics, columns, and many of the typical elements
that can be found in a document. The second set
consists of fifteen old blank images, taken from a
digitized document archive of the 18th century. These
documents include most kinds of problems that can be
met in old documents: presence of stains, background
of big variations and uneven illumination, etc.
The two sets were combined by applying image
mosaicing superimposing techniques for blending .
Two different sets of 150 document images each were
prepared. In more detail, we used as target images the
pdf documents and we resized all the noise images to
A4 size. Then, we used two different techniques for
the blending: the maximum intensity and the image
In the first case, the maximum intensity technique
(max_int), the new image was constructing by
selecting for each pixel in the new image the darkest
corresponding pixel of the two images. This means
that in case of foreground pixels, the pdf document
image will have an advantage over the noise image. On
the other hand, in the background pixels, the noise
image will have an advantage since it is almost always
darker than the document background that is
absolutely white. This technique has a good optical
result but it is not very natural. The foreground would
be always the darkest, since it is not affected at all
from the noise. This setting permits us to check how
much of the background can be detracted by a
However, in order to have a more natural result, we
also used the image averaging technique (ave-int),
where each pixel in the new image is the average of
the two corresponding ones in the original images. In
this case, the result presents a lighter background than
that of the maximum intensity technique but the
foreground is also affected by the image noise level.
3. Experimental results
In the evaluation, we used statistical measures of
image quality description . More specifically, we
used the square error (MSE), the signal to noise ratio
(SNR) and the peak signal to noise ratio (PSNR).
However, as already mentioned, our intention was to
be able to check for every pixel if it is right or wrong
and we also used this measure for the evaluation. Thus,
we call pixel error the total amount of pixels of the
output image that has been assigned to incorrect color:
that is, black if white in original document and
viceversa. Thus, the pixel error rate (PERR) will be:
Let x(i,j) represent the value of the i-th row and j-th
column pixel in the original document and let y(i,j)
represent the value of the corresponding pixel in the
output image y:MxN. Since we deal with black and
white images, both values will be either 0 (black) or
255 (white). The local error is e(i,j)=x(i,j)-y(i,j) and
the total square error rate:
Notice that if a pixel has been assigned to the right
color, the value of e(i,j)2 will be 0, while in case the
pixel is assigned to the wrong color it will be 2552.
Thus, taking into account the PERR definition, it will
SNR  is defined as the ratio of average signal
power to average noise power and for an MxN image it
) , (
The peak measure, PSNR, depends on the word-
length of the image pixels, and is defined as the ratio
of peak signal power to average noise power. For 8-bit
images, as in our case, it is:
Table 1. The resulted measures for max-int and ave-int techniques
ALGORITHMS MAX-INT TECHNIQUE
MSE SNR PSNR
SNR PERR MSE PSNR PERR
We applied all the methods to both sets described in
section 2. The pixels that changed value (white-to-
black or vice versa) were counted by comparing the
output image with the original pdf document image. It
should be mentioned that the majority of the pixel
errors are white-to-black conversions with a max of
0.02‰ black-to-white conversion in both techniques.
In table 1 you can see all the above mentioned
measures for max-int and ave-int techniques and we
can remark the following:
1) The majority of the algorithms (21 out of 30)
have better performance on the test of ave-int
technique. This way we can distinguish the methods in
those that perform considerably better when there is
clear outstanding of the foreground (e.g Lloyd) and
others in the opposite case (e.g Otsu).
2) Although there is a slight better performance
of the local binarization methods in comparison to the
global binarization methods, there are global methods
with a very good performance and local methods close
to the worst method.
A technique was proposed for the evaluation of
binarization algorithms. This method is appropriate for
document images that are difficult to be evaluated
using techniques based on segmentation or recognition
of the text. In order to survey the algorithm
performance we used 30 binarization algorithms. We
performed experiments on document archives made by
using two different techniques of image mosaicing and
combining old blank document pages with noise-free
pdf documents. This way, after the application of the
binarization algorithms to the synthetic images, it is
easy to evaluate the results by comparing the resulted
image with the original document.
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