Segmentation of heterogeneous document images : an ... - Tel

them.
In such circumstances, it would be logical to take advantage of methods that
work on a block or a region of document instead of the connected components.
The idea is that regions of text have a particular texture that can be revealed
with methods such as wavelet analysis, co-occurrence matrices, Radon transform,
etc.
Shape of the regions can be rectangular or free form. They can be strips of
overlapping blocks or coming from a multi-resolution structure like quad-tree or
image pyramids. To name a few methods, Journet [44] uses a method based on
orientation of auto-correlation and frequency features to segment regions of text
in historical document images. In [46], Kim et al propose an approach based on
support vector machine (SVM) and CAMSHIFT algorithm.
tel-00912566, version 1 - 2 Dec 2013
In CC-based methods, the geometry of the components is clearly defined.
However, no matter how well the methods work, there is always an issue of
confidence at the boundaries of regions. This issue becomes larger as the gap
between text and non-textual components gets smaller. Using large block implies
high confidence on the computed features, but poor confidence on regions’
boundaries. Using small blocks implies just the opposite. One solution is to use
overlapping blocks, but then the problem becomes to assign a label to pixels
when two or more overlapping blocks have a disagreement about the label of the
pixels. Confidence based methods for combination of evidence (e.g. Dempster-
Shafer ) also may fail. Consider the case where two blocks have overlapping
pixels. One block is located on the mostly text area of the document, and the
other block is located on the mostly graphical one. The pixels in between get
evidences from two sources that have a high degree of conflict. The Dempster-
Shafer theory is criticized for being incompetence in the mentioned situation
[104].
The other problem with region-based methods is their ineffectiveness to detect
frames and lines that belong to a table. Thus, the result for parts of a table
that contain text, comes back as text for every pixel of that region, whereas it
includes separating rule lines.
2.1.3 Conclusion
Whenever it is feasible to compute the connected components of a document,
it is preferable to assign the label to the connected component itself and avoid
using a region-based method. However, to overcome some of the drawbacks
of the component based methods, a hybrid approach should be adapted that
also takes advantage of texture features around the component. Our method is
based on this strategy and is described in chapter 3.
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