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

tel-00912566, version 1 - 2 Dec 2013
Figure 2.12: Left image shows segmented lines from a document in [7] and Right
image displays projection profiles of vertical strips.
segmentation contests.
Hough-based methods can also be considered as projection based methods.
They are based on Hough transform but instead of projecting pixel values into
x or y axis, they project intensities onto parameter’s space. In general, the
purpose of Hough transform is to find imperfect instances of objects within a
certain class of shapes by a voting procedure. The classical Hough transform is
concerned with the identification of lines in an image, and text lines in the case
of document image. However, it is not limited to lines, and any kind of shape
can be found inside an image. In document image analysis, Hough transform
is used in a variety of situations. Some methods use it to detect text lines or
the skew angle of the text lines, or to drive different characteristics such as the
direction of a connected handwritten word, or even to detect table lines. Here
we note two methods that use Hough transform specifically for detecting text
lines.
In [58], G. Louloudis et al. have proposed a Hough transform based handwritten
segmentation method. Their earliest results have been published in
[56, 57]. The method is adapted to deal with challenges of handwritten documents,
including arbitrary slanted and skewed text lines, accent marks above
or below the text lines and touching lines. After dividing the document image
into equally sized blocks, the aim of Hough transform becomes to find dominant
direction of connected components in each block. The algorithm also keeps
track of the dominant direction of all components inside the document. To form
text lines, for each connected component a decision is held based on rules that
compare the direction of each component in a block with its adjacent blocks.
An additional constraint is applied upon which, a text line is valid only if the
corresponding skew angle of the line deviates from the dominant direction by
30