Using Tesseract 4 with C#

13/06/2019

3 minutes to read

Recently I built a small tool to read the text of thousands of images.

Introduction

A common technique to extract text from images is know as OCR (Optical character recognition) and the best implementation, that I Know, is called Tesseract.

When a I started to build the tool, I used the most famous Tesseract’s wrapper for .NET.

Although the wrapper worked very well, I was curious about if there was a way to get better peformance results. With a little search I noticed that the .NET wrapper still use Tesseract 3, but there was a version 4 available with a lot of performance improvements:

If you are running Tesseract 4, you can use the “fast” integer models.

Tesseract 4 also uses up to four CPU threads while processing a page, so it will be faster than Tesseract 3

https://github.com/tesseract-ocr/tesseract/wiki/FAQ#can-i-increase-speed-of-ocr

So, I decided to try Tesseract 4 to see how could it impact in the performance of my tool. As at the time there was no .NET wrapper for it, I removed the old wrapper and called Tesseract 4 directly as a process.

The use of Tesseract 4 cut off the time to read the images in almost half

TesseractService

I ended up developing the class below to call the Tesseract 4 command-line (tesseract.exe) directly from the C# code.

	using System;
	using System.Diagnostics;
	using System.IO;
	using System.Linq;
	using System.Text;

	namespace Ocr
	{
	/// <summary>
	/// Service to read texts from images through OCR Tesseract engine.
	/// </summary>
	public class TesseractService
	{
	private readonly string _tesseractExePath;
	private readonly string _language;

	/// <summary>
	/// Initializes a new instance of the <see cref="TesseractService"/> class.
	/// </summary>
	/// <param name="tesseractDir">The path for the Tesseract4 installation folder (C:\Program Files\Tesseract-OCR).</param>
	/// <param name="language">The language used to extract text from images (eng, por, etc)</param>
	/// <param name="dataDir">The data with the trained models (tessdata). Download the models from https://github.com/tesseract-ocr/tessdata_fast</param>
	public TesseractService(string tesseractDir, string language = "en", string dataDir = null)
	{
	// Tesseract configs.
	_tesseractExePath = Path.Combine(tesseractDir, "tesseract.exe");
	_language = language;

	if (String.IsNullOrEmpty(dataDir))
	dataDir = Path.Combine(tesseractDir, "tessdata");

	Environment.SetEnvironmentVariable("TESSDATA_PREFIX", dataDir);
	}

	/// <summary>
	/// Read text from the images streams.
	/// </summary>
	/// <param name="images">The images streams.</param>
	/// <returns>The images text.</returns>
	public string GetText(params Stream[] images)
	{
	var output = string.Empty;

	if (images.Any())
	{
	var tempPath = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
	Directory.CreateDirectory(tempPath);
	var tempInputFile = NewTempFileName(tempPath);
	var tempOutputFile = NewTempFileName(tempPath);

	try
	{
	WriteInputFiles(images, tempPath, tempInputFile);

	var info = new ProcessStartInfo
	{
	FileName = _tesseractExePath,
	Arguments = $"{tempInputFile} {tempOutputFile} -l {_language}",
	RedirectStandardError = true,
	RedirectStandardOutput = true,
	CreateNoWindow = true,
	UseShellExecute = false
	};

	using (var ps = Process.Start(info))
	{
	ps.WaitForExit();

	var exitCode = ps.ExitCode;

	if (exitCode == 0)
	{
	output = File.ReadAllText(tempOutputFile + ".txt");
	}
	else
	{
	var stderr = ps.StandardError.ReadToEnd();
	throw new InvalidOperationException(stderr);
	}
	}
	}
	finally
	{
	Directory.Delete(tempPath, true);
	}
	}

	return output;
	}

	private static void WriteInputFiles(Stream[] inputStreams, string tempPath, string tempInputFile)
	{
	// If there is more thant one image file, so build the list file using the images as input files.
	if (inputStreams.Length > 1)
	{
	var imagesListFileContent = new StringBuilder();

	foreach (var inputStream in inputStreams)
	{
	var imageFile = NewTempFileName(tempPath);

	using (var tempStream = File.OpenWrite(imageFile))
	{
	CopyStream(inputStream, tempStream);
	}

	imagesListFileContent.AppendLine(imageFile);
	}

	File.WriteAllText(tempInputFile, imagesListFileContent.ToString());
	}
	else
	{
	// If is only one image file, than use the image file as input file.
	using (var tempStream = File.OpenWrite(tempInputFile))
	{
	CopyStream(inputStreams.First(), tempStream);
	}
	}
	}

	private static void CopyStream(Stream input, Stream output)
	{
	if (input.CanSeek)
	input.Seek(0, SeekOrigin.Begin);

	input.CopyTo(output);
	input.Close();
	}

	private static string NewTempFileName(string tempPath)
	{
	return Path.Combine(tempPath, Guid.NewGuid().ToString());
	}
	}
	}

view raw TesseractService.cs hosted with ❤ by GitHub

Setup

Just download the gist above and add it to your .NET project.
Install Tesseract 4
- Linux and OSX: https://github.com/tesseract-ocr/tesseract/wiki
- Windows: https://github.com/UB-Mannheim/tesseract/wiki
Download the trained data model for the language you need to read the images
- More accurate, but slower: https://github.com/tesseract-ocr/tessdata_best
- Faster, but less accurate: https://github.com/tesseract-ocr/tessdata_fast.

Usage

	var service = new TesseractService(@"C:\Program Files\Tesseract-OCR", "eng", @"C:\Program Files\Tesseract-OCR\tessdata");

	// var stream = File.OpenRead(string path);
	// var stream = WebRequest.Create(string url).GetResponse().GetResponseStream();
	// var stream = new MemoryStream(byte[] buffer);
	var text = service.GetText(stream);

view raw usage-sample.cs hosted with ❤ by GitHub

If you try to read a image like this one:

You will get this result after call the TesseractService.GetText method: The (quick) [brown] {fox} jumps! Over the $43,456.78 <lazy> #90 dog & duck/goose, as 12.5% of E-mail from aspammer@website.com is spam. Der ,.schnelle" braune Fuchs springt iiber den faulen Hund. Le renard brun «rapide» saute par-dessus le chien paresseux. La volpe marrone rapida salta sopra il cane pigro. El zorro marron rapido salta sobre el perro perezoso. A raposa marrom rapida salta sobre o céo preguicoso.

Using Tesseract 4 with C#

Introduction

TesseractService

Setup

Usage

Further reading