Fixed the GetUTF8Text method to return nullptr instead of asserting

[zdenop]

Description

This PR fixes a crash in the GetUTF8Text method by returning nullptr instead of asserting when best_choice is nullptr.

Background

The issue was originally reported in sirfz/tesserocr#324, where a crash/assertion failure occurred when passing an empty image to tesserocr/Tesseract. The error message was:

best_choice != nullptr:Error:Assert failed: in file F:\Projects\Community\tesseract\src\ccmain\ltrresultiterator.cpp, line 52

While reproducing the issue with a test C++ program, I encountered a similar crash in another location:

it_->word()->best_choice != nullptr:Error:Assert failed: in file F:\Projects\Community\tesseract\src\ccmain\resultiterator.cpp, line 709

Changes

• Modified GetUTF8Text to safely return nullptr if best_choice is nullptr, instead of triggering an assertion failure.

Additional Notes

• I tested this with Warp2, which suggested updating multiple instances of ASSERT_HOST(best_choice != nullptr);. This PR addresses the crash in the context of GetUTF8Text.

• There are still 3 other occurrences of this assertion in the following files:

  • src/ccmain/recogtraining.cpp
  • src/ccstruct/pageres.cpp

  It's unclear if those should also be replaced, as they may be used in different contexts (e.g., training or layout analysis). Further review is recommended before changing them.

Reproduction

Here is a minimal test program that reproduces the crash (when using an empty image):

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

#include <iostream>

int main(int argc, char *argv[]) {
  tesseract::TessBaseAPI *api = new tesseract::TessBaseAPI();

  if (api->Init(NULL, "chi_sim")) {
    fprintf(stderr, "Could not initialize tesseract.\n");
    exit(1);
  }

  // Create a small empty/blank image (1x1 pixel)
  Pix* pix = pixCreate(100, 100, 8);
  if (pix == nullptr) {
    std::cerr << "Could not create empty image." << std::endl;
    return 1;
  }
  api->SetImage(pix);

  api->SetPageSegMode(tesseract::PSM_SINGLE_CHAR);
  api->Recognize(0);
  int out_offset;
  float out_slope;
  if (api->GetTextDirection(&out_offset, &out_slope)) {
    std::cout << "out_offset: " << out_offset << std::endl;
    std::cout << "out_slope: " << out_slope << std::endl;
  } else {
    std::cout << "GetTextDirection failed." << std::endl;
  }
  // Get the iterator and try to imitate relevant part of MapWordConfidences function
  tesseract::ResultIterator* ri = api->GetIterator();
  tesseract::PageIteratorLevel level = tesseract::RIL_WORD;
  if (ri != nullptr) {
    do {
      const char* word = ri->GetUTF8Text(level);
      auto word_confidence = ri->Confidence(level);
      std::cout << "Word confidence(" << word << "): " << word_confidence << std::endl;
    } while (ri->Next(level));
  }

  delete ri;
  api->End();
  delete api;
  pixDestroy(&pix);
  std::cout << "Test completed successfully - no crash!" << std::endl;
  return 0;
}

[stweil]

Is there an example which triggers the assertion here? I am not sure whether the break is the right solution here. Maybe the loop should continue at line 71.

[stweil]

Are there situations where the condition is false?