LOCR - An Optical Character Recognition Program for Linux

I worked in this project during the Summer of 2000. Then I got busy with other projects and never had a chance to come back to this one. In its current state the recognizing algorithm needs improvement (e.g. to avoid confusing similar looking symbols such as 0 and O, 1 and l, recognizing italics, etc.), but I made it public so that others can take advantage of the ideas in it.

Sources (version 0.1.0)

• locr.c: C source of the program.
• foo.ps: Sample document used for scanning and generating data.
• foo.txt: Plain text version of the sample text (ISO-8859-1 encoding).
• foo.pnm: Scanned version of the sample text.
• foo.dat: Data generated from the sample text.
• gpl.txt: GNU General Public License.

• locr-0.1.0.tgz: The whole thing in a single file.

How to use it

     gcc -O2 locr.c -o locr

• Generate data mode: Used for generating a database of information to be used later in recognizing mode. Example: we have a document "foo.ps" and we want to use it for compiling data. Do the following:
  • Print it.
  • Scan it. Make sure that the output is a P1 or P4 pnm file "foo.pnm". The recommended resolution is 300dpi.

    Alternatively, the two previous steps can be replaced by the following command:

      gs -sDEVICE=pnmraw -r300x300 -sOutputFile=foo.pnm -dNOPAUSE foo.ps quit.ps
    

  • Write a plain text version "foo.txt" of the document.
  • Generate the data file "foo.dat" with the following command:

         ./locr -g -t foo.txt foo.pnm foo.dat
    

  The original document should be very simple (single column, no pictures...), contain a wide range of characters (we want to collect as much information as possible) and be virtually perfect (otherwise the program will make mistakes and the information collected will be useless). Test it by using the program to recognize "foo.pnm" using "foo.dat" as datafile. The output should be identical to "foo.txt", except possibly for the spacing of the characters.

• Recognizing mode: Uses information previously collected in a datafile for recognizing a scanned text. Example, after scanning a document we get a PNM file "document.pnm" and we want to recognize it using "foo.dat" as data file. Do it with the following command (the output goes to "document.out"):

       ./locr -d foo.dat document.pnm document.out
  

Example:

1. Original document (PDF version)
2. Document in PNM format
3. Ouput of locr (ISO-8859-1 encoding)

How it works

1. Load image as an array of 0's and 1's.
2. Remove dust and snow.
3. Make table of blocks. (A block is a set of contiguous pixels. Some characters are made of more than one block, for instance "i" has two blocks: the body and the dot.)
4. Remove atypical blocks (with unrealistic dimensions). In this way pictures are removed.
5. Find columns of text. Make table of columns. Sort blocks by columns.
6. On each column find lines of text. Make table of lines. Sort blocks by lines and by their position on each line.
7. On each line join blocks horizontally overlapped. This transforms the component pieces of each character (such as the body and the dot in an "i") into a single object.
8. Compute attributes of each character (e.g.: number of pieces, number of holes, vertical position on the line, etc.)
9. For each character, compare its attributes to those stored in the data file. Select possible "candidates". Compare the given character to the candidates selected and decide which one is the closest. The final comparison is made by computing the Hamming distance (number of pixels where they differ) between scaled 16x16 versions of the characters. If no candidate is found similar enough to the current character, relax the criteria and try again.
10. In "generate data" mode the previous step is replaced by just dumping the information collected into a file.

Other free OCR projects

• Joerg Schulenburg's GOCR

Emailme to: mlerma at math dot northwestern dot edu