Tesseract always missing a text line in picture

Question

I am trying to extract data from a picture with OCR. I use Tesseract API in C++ to achieve this.

The original picture is this:

Now the for me important data is this:

However the marked blue line is never recognized does not matter what I try.

The code to analyze the picture with tesseract looks like this:

std::string readFromFile(const std::string& filename)
{
    tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
    api->SetPageSegMode(tesseract::PSM_AUTO);
    if (api->Init("folder_to_tessdata", "deu+eng")) {
        fprintf(stderr, "Could not initialize tesseract.\n");
        exit(1);
    }

    // Open input image with leptonica library
    Pix *image = pixRead(filename.c_str());
    api->SetImage(image);
    // Get OCR result

    char *outText = api->GetUTF8Text();

    std::string result{ outText };

    api->End();
    delete[] outText;
    pixDestroy(&image);

    return result;
}

I tryed to improve the accuracy by preprocessing the image like it is suggested in this question: image processing to improve tesseract OCR accuracy

The Code for the preprocessing:

cv::Mat image;
image = cv::imread(filename, cv::IMREAD_COLOR);
cv::resize(image, image, cv::Size{}, 1.2, 1.2, cv::INTER_CUBIC);

cv::cvtColor(image, image, cv::COLOR_BGR2GRAY);

auto kernel = cv::Mat(1, 1, CV_8UC1, cv::Scalar(1));
cv::dilate(image, image, kernel);
cv::erode(image, image, kernel);

cv::Mat filter;
cv::bilateralFilter(image, filter, 5, 75, 75);

cv::threshold(filter, image, 0, 255, cv::THRESH_BINARY + cv::THRESH_OTSU);

Am I missing something? Can I tweak Tesseract itself more or should I change the preprocessing of the image?

Answer 1

My reference is here.

Note: You don't need to deal with preprocess steps because it seems you already have a pure image. It doesn't have noises much.

My environment information:

Operating system: Ubuntu 16.04

Tesseract version by the command of tesseract --version:

tesseract 4.1.1-rc2-21-gf4ef
 leptonica-1.78.0
  libgif 5.1.4 : libjpeg 8d (libjpeg-turbo 1.4.2) : libpng 1.2.54 : libtiff 4.0.6 : zlib 1.2.8 : libwebp 0.4.4 : libopenjp2 2.1.2
 Found AVX
 Found SSE
 Found libarchive 3.1.2

OpenCV Version by the command of pkg-config --modversion opencv:

3.4.3

Difference: When I checked your code, I have only seen the clear difference with this one. You are opening the image with leptonica library one more time instead of opencv.

Here is the code and resulted output:

Input:

Output texts:

Al AQ A3 Ad AS A6 Al A8

| 2 3 4 5 6 7 8

WH GN YE GY PK Bu RD VT
K101 K102 K103 K104 K105 K107 K109 K110
Q30,0 Q30.1 Q30.2 Q30.3 Q30.4 Q30.5 Q30.6 Q30.7
=13/L.2 =13/2.2 =13/4.2 =13/6.2 =13/7.2 =13/10.2 FIBL.2 = 1312.2

AS AlO All Al2 AL3 Al4 ALS AL6

9 10 ll 12 13 14 15 16
GY /PK RD/BU WH/GN BN/GN WH/YE YE/BN WH/GY GY/BN
Kl1l K112 y114 K115 K117 K118 K124
Q31,0 Q31.1 Q31.2 Q31.3 Q31.4 Q31.5 Q31.6 Q31.7
=13/13.2 =13/14.2 =13/15.2 =13/16.2 =1B7.2 PIB. =13/21.2

Beckhoff KL 2809

Code:

#include <string>
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main(int argc, char* argv[])
{
    string outText;


    // Create Tesseract object
    tesseract::TessBaseAPI *ocr = new tesseract::TessBaseAPI();

    ocr->Init(NULL, "eng", tesseract::OEM_LSTM_ONLY);


    // Set Page segmentation mode to PSM_AUTO (3)
    ocr->SetPageSegMode(tesseract::PSM_AUTO);


    // Open input image using OpenCV
    Mat im = cv::imread("/ur/image/directory/tessatest.png", IMREAD_COLOR);


    // Set image data
    ocr->SetImage(im.data, im.cols, im.rows, 3, im.step);

    // Run Tesseract OCR on image
    outText = string(ocr->GetUTF8Text());

    // print recognized text
    cout << outText << endl;

    // Destroy used object and release memory
    ocr->End();

    return EXIT_SUCCESS;
}

The compilation of the code:

g++ -O3 -std=c++11 test.cpp -o output `pkg-config --cflags --libs tesseract opencv`