Limit characters tesseract is looking for

Question

Is it possible to limit the set of characters that tesseract is looking for (e.g. search only for letters a-z)? That would improve my results greatly.

Answer 1

Create a config file (e.g "letters") in tessdata/configs directory - usually /usr/share/tesseract/tessdata/configs
or
/usr/share/tesseract-ocr/tessdata/configs

And add this line to the config file:

tessedit_char_whitelist abcdefghijklmnopqrstuvwxyz  

...or maybe [a-z] works.. dunno :-)
Then call tesseract similar to this:

tesseract input.tif output nobatch letters  

That will limit tesseract to recognize only the wanted characters

Answer 2

To use whitelist in a config file or using the -c tessedit_char_whitelist=... command-line switch, in the newest 4.0 version you will have to set OCR Engine mode to the "Original Tesseract only". This is because the new "Neural nets LSTM" mode doesn't respect the whitelist setting. Example of proper command-line for 4.0 version:

tesseract input_file output_file --oem 0 -c tessedit_char_whitelist=abc123

UPDATE: In newer versions (4.0) there's corrupted eng.traineddata file installed by default by Windows and some Linux installers. Temporary solution is to replace tessdata\eng.traineddata file with one from older version. This file should be about 30MB. Otherwise you'll get Error: "Tesseract couldn't load any languages!" or similar.

Answer 3

In addition to the config file, is the -c flag:

tesseract stdin stdout -c tessedit_char_whitelist=abcdefghijklmnopqrstuvwxyz -psm 6

Answer 4

Just adding this for anyone using tesseract on Android. In your readOCR function where you set the language etc. add the following line;

tesseract.setVariable("tessedit_char_whitelist","ABCDEFGHIJKLMNOPQRSTUVWXYZ");

you can also do blackList for characters to exclude.

Answer 5

In Tesseract version 4.00, this can't be done. You only can fine-tune your model or use regex to remove extra characters from the prediction.