Processing dark text images for tesseract ocr - ocr

Question

I have noticed that whenever I use dark background images for tesseract I am getting a segmentation fault. I was trying to extract symbols using this code
#include <tesseract/baseapi.h>
#include <leptonica/allheaders.h>
#include <iostream>
#include <map>
#include <bits/stdc++.h>
using namespace std;
int main()
{
char *outText;
map<pair<char*,char*>,float> matrix;
set<char> allChars;
tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();
// Initialize tesseract-ocr with English, without specifying tessdata path
if (api->Init(NULL, "eng")) {
fprintf(stderr, "Could not initialize tesseract.\n");
exit(1);
}
int a[256][256];
for(int i=0;i<256;i++){
for(int j=0;j<256;j++){
a[i][j]=0;
}
}
// Open input image with leptonica library
string images[] = {List of images};
for (int ii=0;ii<7;ii++){
Pix *image = pixRead((const char*) images[ii].c_str());
cout << images[ii] << endl;
api->Init(NULL, "eng");
api->SetImage(image);
string valid_set = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890~`!##$%^&*()_-+=,./<>/:;'[]{}|";
api->SetVariable("tessedit_char_whitelist", valid_set.c_str());
api->SetVariable("save_blob_choices", "T");
//api->SetRectangle(37, 128,648, 523);
//api->SetRectangle(30, 86, 590, 100);
//api->SetRectangle(30,100,430,30);
api->Recognize(NULL);
tesseract::ResultIterator* ri = api->GetIterator();
tesseract::PageIteratorLevel level = tesseract::RIL_SYMBOL;
if(ri != 0) {
do {
const char* symbol = ri->GetUTF8Text(level);
//cout << symbol << endl;
if(ri!=0){
float conf = ri->Confidence(level);
}
//cout << "err" << endl;
if(symbol != 0) {
//printf("symbol %s, conf: %f", symbol, conf);
bool indent = false;
tesseract::ChoiceIterator ci(*ri);
do {
const char* choice = ci.GetUTF8Text();
if (indent) //printf("\t\t ");
// printf("\t- ");
//cout << symbol<<" Look up "<<choice << endl;
matrix[make_pair(strdup(symbol), strdup(choice))]=ci.Confidence();
//printf("%s conf: %f\n", choice, ci.Confidence());
indent = true;
} while(ci.Next());
}
//printf("---------------------------------------------\n");
delete[] symbol;
} while((ri->Next(level)));
}
int count = 0;
for(map<pair<char*,char*>,float>::iterator it = matrix.begin();it!=matrix.end();it++){
allChars.insert((strdup)(it->first.first)[0]);
allChars.insert((strdup)(it->first.second)[0]);
//cout<<it->first.first<<" "<<it->first.second<<endl;
//cout << (strdup)(it->first.first)[0]<<" "<<(strdup)(it->first.second)[0]<<endl;
a[(strdup)(it->first.first)[0]][(strdup)(it->first.second)[0]]+=it->second;
count++;
}
// cout << count << endl;
for(set<char>::iterator it = allChars.begin();it!=allChars.end();it++){
//cout << *it << endl;
}
for(int i=0;i<256;i++){
for(int j=0;j<256;j++){
if(a[i][j]!=0){
///cout << i << " " <<j<<endl;
//cout << a[i][j]<<endl;
}
//cout << a[i][j] << endl;
}
}
api->End();
pixDestroy(&image);
}
return 0;
}`
Specifically in the code where
float conf = ri->Confidence(level);
So what can be the solution to this problem? Should we train with more dark images?
Edit:
Sample Image