I have recently started working on a Freelance project where I need to use text scene recognition based on OpenCV and Tesseract as libraries. I was so motivated to hit the Wolrd of computer vision combined with machine learning and experience developing applications in the field, so I welcomed challenges that come with!
Here I’ll be talking about the first challenge and how I tackled it.

found myself with multiple new things to prepare in order to start coding, without mentioning that it’s been a long time before when I last coded with C++ (back to my university time)!

At first, I was asked to use OpenCV 3.0 and Tesseract 3.02 in order to run the project’s part which is already available. So I installed OpenCV 3.0 and Tesseract 3.02 with Leptonica Library, by following provided documentation about how to build application with OpenCV on Windows Visual Studio in this link. Then, I tried to run the project in Visual Studio 2017. I got more than 800 errors!!! Most of them where LINK errors of type:

mismatch detected for '_MSC_VER': value '1700' doesn't match value '1900' in <filename.obj> 

and

error LNK2038: mismatch detected for 'RuntimeLibrary': value 'MT_StaticDebug' doesn't match value 'MD_DynamicDebug' in file.obj. 

The first error was caused by the fact that objects were compiled by different versions of the compiler which is not supported because different versions of the standard library are binary incompatible. Actually, the numbers 1700 and 1900 meant binary version of  v11 (Visual Studio 2012) is not compatible with the binary version of  v14 (Visual Studio 2015).[1][2]

About the second one, the reason behind is that both the library and your project must be linked with the same settings with regards to the C Runtime Library, and that whether in Release or Debug mode [3]. And here the solution in case you need to use static libs or you can deactivate static lib when building with CMake by setting:

-DBUILD_WITH_STATIC_CRT=OFF

So I decided to change the version of Tesseract so that it will be compatible with newer version of Visual Studio and x64 target project. As compiling Tesseract3.05.1 with cppan client was not possible for me (ccpan crashes just after launching it), I decided to go for VS2015_Tesseract. I followed these steps in addition to using v140 toolset because Visual Studio 2017 was shipped with v141 toolset, but it supports v140.

Besides, there were some other points to consider. Unfortunately, I got again errors after configuring my project exactly like it’s in tesseract\tesseract.vcxproj (extracted from VS2015_Tesseract) and I find out that the reason behind was: Having a “space” in tesseract path directory.

One last thing to consider when using this tesseract version is to the initialization of tesseract: TESSDATA_PREFIX environment variable is required to setup Tesseract data (tesseract/tessdata) folder. Also you can set it via api->Init(<data path here>, <language>)

I wanted to get also a newer version of OpenCV and went for OpenCV 3.02. Fortunately, that was easier than Tesseract installation, but of course not without some difficulties. I followed this Tutorial in order to install it. In these steps, you have to pay attention to the version of visual studio you choose when configuring CMake as there is a version for win64 (or x64) target project and a version for win32 (or x86). I chose “Visual Studio 15 2017 Win64”.

And yes! it worked! I tested opencv3.2 on a sample project and tesseract on a sample project with OpenCV! I was relieved! Finally, I could start coding!…

After launching the project “End to end text recognition“..I discovered that I wasn’t so close! There was a problem with opencv_text module. I got the error: Tesseract not found at runtime. So, I asked for help through OpenCV forum and StackOverflow and got a suggested solution. In the step of OpenCV configuration using CMake, besides providing Lept_LIBRARY Path and Tesseract_INCLUDE_DIR Path and tesseract_LIBRARY, check Build_examples checkbox, uncheck Build_tests and Build_perf_tests (You don’t need tests when you are a beginner and it takes a lot of memory space, as I know). Also, be sure that:

– tesseract305.dll file is in tesseract-3.05.01\build\bin\Release directory;

leptonica-1.74.4.dll file in leptonica-1.74.4/build/bin/Release.

-put them both in opencv\build\bin\Release (if you will use Release mode or bin\Debug if you will use debug mode).

After that, you have to open INSTALL.vcxproj from OpenCV build directory and build it in Debug (if you will need Debug mode later) and then Release. Be sure to select the right mode x64 (if you chose x64 in CMake configure) or x86 (if you chose Win32 in CMake configure), or you can get errors of type:

“fatal error LNK1112: module machine type ‘x64’ conflicts with target machine type ‘X86”  [4]

That’s all! Now you can start creating projects in Visual studio. Don’t forget to configure the project correctly:

To test one of opencv text module sample example, put tesseract.dll and lept.dll in the same directory as your .exe (opencv/build\bin\Release\<projectname.exe>) and copy all files from opencv\opencv_contrib-3.2.0\modules\text\samples to opencv/build\bin\Release\

Note: for me, I didn’t test creating a project because when updating Visual Studio 2017, I had a bug!!! Which I tried to resolve and at the end I switched to Ubuntu 16.4!! 😀 .. and how easy it was because these libraries have a greater community in Ubuntu and there is video series to install latest versions. You just need to follow this Francesco Piscani youtube channel videos on how to install OpenCV, Tesseract and Leptonica. Also, you can choose the option to download Tesseract3.04 and Leptonica1.73, which are older versions that don’t require build from source. But I don’t know how to run the project on Debug mode and if it requires another build condition of opencv library. (like in Windows you have to choose build configuration Release or Debug)

What about your experience with OpenCV, Tesseract and Leptonica? Are you working on any project involving Text recognition in natural images? Share in a comment!

Please don’t hesitate to write me in a comment if you tried the steps and failed! I will do my best to help! 🙂