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API example for user patterns

User patterns can be useful when recognizing ID type of fields which have non-dictionary words but follow specific patterns of alphabets and digits e.g. \A\A\d\d\d\d\A or \A\A\d\d\d\A

This documentation provides a simple example on how to use the tesseract-ocr API (4.x) in C++ for applying user patterns for improving recognition. It is expected that tesseract-ocr is correctly installed including all dependencies. It is expected the user is familiar with C++, compiling and linking program on their platform.

This is based on an example provided in tesseract-ocr forum and updated for the recent implementation of the feature for tesseract 4.x.

Please note that while this example gets 100% accuracy after user_patterns are applied, that may not always be the case. User patterns (like user dictionaries) are merely applied as a hint while decoding, but not exclusively. Pre-processing the image usually improves the quality of recognition and is recommended.

Requirements

In order to apply user patterns for improving recognition, the following are required.

user patterns file

The user patterns file should contain one pattern per line in UTF-8 format. In choosing which patterns to include please be aware of the fact that providing very generic patterns will make tesseract run slower. Best results may be obtained by having a single pattern in the file.

Details of type of patterns that can be used are given in trie.h.

Example of a user patterns file

Make a text file, and write each pattern on a separate line, with UNIX line endings (line-feed character) and a blank line at the end, e.g.

\A\A\d\d\d\d\A

In the following, let’s assume you named that pattern file path/to/my.patterns.

config file

For the API, the information about the user patterns file needs to be specified in a config file.

(For the CLI, this works as well, but there is also a direct option for the user patterns file alone.)

Example of a config file

Make a text file, and write user_patterns_file into it verbatim, followed by the path name in one line, with UNIX line endings (line-feed character) and a blank line at the end, e.g.

user_patterns_file path/to/my.patterns

In the following, let’s assume you named that config file path/to/my.patterns.config.

CLI Example

From the command line, user patterns can be invoked as follows:

tesseract input.tif output --user-patterns path/to/my.patterns

API Example

Take the following image file (Arial.png) as input:

The following code uses the above user patterns file and config file on that image file:

#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>

int main()
{
    Pix *image;
    char *outText;
    char *configs[]={"path/to/my.patterns.config"};
    int configs_size = 1;
    tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
    if (api->Init(NULL, "eng", tesseract::OEM_LSTM_ONLY, configs, configs_size, NULL, NULL, false)) {
      fprintf(stderr, "Could not initialize tesseract.\n");
      exit(1);
    }
    image = pixRead("Arial.png");
    api->SetImage(image);
    outText = api->GetUTF8Text();
    printf(outText);
    api->End();
    delete [] outText;
    pixDestroy(&image);
    return 0;
}

Build and run script

#!/bin/bash

export CPLUS_INCLUDE_PATH=$CPLUS_INCLUDE_PATH:/usr/local/include
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib

g++ -std=c++17  -o my.patterns.api my.patterns.api.cpp   -llept -ltesseract

export TESSDATA_PREFIX=~/tessdata_best

./my.patterns.api > Arial-patterns.txt

diff -u  Arial-patterns.txt  Arial-gt.txt

With the user patterns file for this image, the recognition is 100% correct. Without it, there are a number of errors:

--- Arial-patterns-no.txt       2019-07-05 04:21:04.367188492 +0000
+++ Arial-gt.txt        2019-07-05 04:05:11.000000000 +0000
@@ -1,20 +1,20 @@
 DQ2679M
 LO6217I
 QK2101G
-JBO363H
+JB0363H
 KN2873M
-Z2B0929J
+ZB0929J
 JF3829W
-YNO0584J
-SVv8400Q
+YN0584J
+SV8400Q
 FY4523X
 KS0016J
 OB3016R
 VA4335P
-QHO205V
-UH20932
+QH0205V
+UH2093Z
 GW3760Y
-S02306T
+SO2306T
 XT8204F
-MR6804|
-0OX5866M
+MR6804I
+OX5866M