Segmentation

[~] Skew angle estimation

source: project OCR by group algfk-21 at Chalmers

A document obtained by a digitizer may exhibit various kinds of geometrical distorsions. For instance if the document is not properly aligned on the scanner the document could become skewed (rotated). This problem can however be corrected by estimating the skew angle.

One way of doing this is by hypothesis-testing --
- projec the image along a number of axes
- compute the orientation-dependante histograms
- look for the direction that maximizes an alignement criterion A(a)
- Finally, the estimated skew angle is given as the angle that maximizes A(a)





[~] Character extraction

source: project OCR by group algfk-21 at Chalmers


X-Y-Tree Decomposition / Iterative Projection Profile Cuttings method:

basic idea is that the document contains vertical and/or horizontal structure. Often documents contain blocks of text that contains rows of characters. The basic operation of this algorithm is a projection of the document image on a horizontal or vertical axis.

-1 Compute the horizontal projection of the entire page
-2 Analyse the projection profile to extract the lines
-3 For each line, compute the vertical projection profile
-4 Analyse the projection profile obtained in step 3 to extract the characters

In some cases two projections is not enough, as for more complex documents

This segmentation algorithm is simple and have problems ---
with characters that is disjoint, these characters can get segmented into two different characters
On the other hand if two characters are connected they will be segmented into one block