Limit characters tesseract is looking for

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.

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. I don't know. Then call tesseract similar to this:

tesseract input.tif output nobatch letters  

That will limit tesseract to recognize only the wanted characters.

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

tesseract stdin stdout -c tessedit_char_whitelist=abcdefghijklmnopqrstuvwxyz -psm 6

update

confirmed working on versions:

• 4.1.1

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.

Update from tesseract 4.1.1

• However, in tesseract 4.1.1 the above bug is fixed, that is, in tesseract 4.1.1 the following works like a charm

  tesseract my_image.jpg stdout -l mylang configfile myconfig

Where "myconfig" is a plaintext file located in TESSDATA/configs

load_system_dawg false
load_freq_dawg false
tessedit_char_whitelist ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789