Tesseract OCR: Interactive Debugging Continued. Baseline Viewer

Here I'll describe a method of viewing baselines in Tesseract's interactive debug environment.

Those who use Tesseract 3.02 should first read my former post called Tesseract OCR: Setting Up Interactive Debug Environment On Windows and complete all steps from it. However instead of the installation suite mentioned there you would need another which contains updated Tess config files as Tesseract developers had renamed/removed a number of internal debug parameters since version 3.01 used in that tutorial. Download the updated suite at http://www.4shared.com/zip/FnP8RSu0/tess_debug_3_02.html. Version 3.01 users can still use the old installation suite.

So now that you've completed the step 5 from the former tutorial and the debug window has appeared, do the following:

1. In the main menu choose Modes->Show BL Norm Word. No apparent reaction from the UI should follow. This is normal.
2. Now click on any word you're interested in. A new window titled BlnWords should appear.
3. At first sight the BlnWords window is empty. But in fact this is not true. Nothing is visible only because of the quirky scaling logic used by ScrollView. To find something inside the window you need to use window scrollbars to pan and mouse scroll wheel to scale up/down. I suggest the following sequence for initial setting of the view:
• slowly drag down vertical scrollbar thumb until you see baselines and/or outlines,
• move horizontal scrollbar thumb approximately to the center,
• use mouse wheel to scale the window contents properly,
• you may also resize the window to your taste.
4. While you click other words in the main window the contents of the BlnWords window updates. You can adjust the view as needed using the methods described above.

What is displayed inside the BlnWords window are so called baseline normalized words. In this type of view words are shown as if their baselines (which can be curved and/or inclined in the source image) get straightened and positioned strictly horizontally. In addition to the baseline the window shows also x-height, ascender and descender lines. See more at Wikipedia: x-height. Using this view you can clearly see if a baseline found by Tesseract is right or wrong: incorrect baselines cause characters to "jump" or "fall."

Baseline finding greatly influences character classification. Various baseline-relative positions of the same character can lead to completely different recognition results. That's why incorrect baselines often serve as sources of errors in Tesseract recognition.

A few examples. Let's take the "conventional" phototest.tif file:

The main debug window should look like this:

All baselines seem to be found perfectly:

For more complex images things go worse. Here I've taken an photographic image of a restaurant receipt. In the image the receipt appears to be inclined and perspectively distorted. The paper is a bit curved, just like it usually happens with receipts. The image is precooked by my image processor (only done binarization and noise cleanup) so that Tesseract is able process it, at some degree of success.

The main debug window already shows several segmentation failures. Some characters are grayed out and some are missing completely:

In BlnWords one can see that many baselines are good but some are determined incorrectly, for instance:

Also there are some epic failures, like these (meaning that characters from adjacent rows get segmented into a single word):

So why would you want to use this debugging method? It can be of use when you're investigating the reasons of some Tesseract failure. Baseline viewer can help you to see that an additional preprocessing is required to cope with the image or a set of images, either programmatic or by means of 3rd party software such as ImageMagick. Passing image block by block (i.e. full or partial pre-segmentation) might also help. Another approach is tweaking internal Tesseract segmentation and baseline finding parameters via config files. Yet another approach is source code changes.

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Tesseract OCR: Setting Up Interactive Debug Environment On Windows

The following are the step-by-step instructions for setting up and running Tesseract’s internal state viewer (called "ScrollView") on Windows.

Although there already exists a dedicated wiki article (and the instructions herein are based upon it), it can cause some confusion for Tesseract newbies and those who don’t feel comfortable with the technology mixture required for the setup.

1. First off, you need to make sure you have Java Runtime Environment (or simply “Java”) installed. If you haven’t, then go to http://www.java.com/en/download/manual.jsp and download it. Most likely, an offline version for Windows will suit you well. After the download completes, run the downloaded executable, follow several wizard steps and wait until the installation is finished.
   
2. Tesseract’s viewer requires a few JAR files which hadn’t been changed for years and are a bit of hassle to get. So I decided to pack them all into a single archived installation suite along with the Tesseract 3.01 executable and other required minimal infrastructure. You can grab it here: http://www.4shared.com/get/Z4gnbJdP/tess_debug.html
   
3. Then create some folder say C:\tess_debug and extract into it all the files from the downloaded installation suite preserving the folder structure.
4. Launch the Windows Command Prompt and change the current directory to your folder by running the command
   
   cd C:\tess_debug
5. Now you are ready to launch the Tesseract debug environment. My installation suite contains the test file phototest.tif so the command to display segmentation data for it would be
   
   tesseract phototest.tif test1 segdemo inter
   Type the above command in the Windows Command Prompt. The viewer window containing letter outlines should appear shortly.
   A few words on the command-line parameters used:
   
   • test1 indicates the name of the txt file which will be created as a result of Tesseract’s work. It will contain the recognized text.
   • segdemo and inter are config files required to run Tesseract in this kind of debug mode (segmentation debugging); you can see these within installation suite’s folder.
   • To run segmentation debugging with your file, indicate its name instead of phototest.tif. If your file is located outside installation suite’s folder then you’ll need to prefix the filename with the path.
   • The above command runs recognition using the default language file eng.traineddata. To use your own language file, specify it using the -l command-line argument e.g.
     
     tesseract image.tif test1 -l yourlang segdemo inter
     In order for this command to run successfully, the language file called yourlang.traneddata should be placed into the tessdata subfolder of the installation suite folder.
6. The above paragraph describes how to debug the segmentation. Nearly the same technique is used to debug the classifier. One thing you need in order to change the debugging mode is to replace in the command line segdemo with matdemo, like this:
   
   tesseract phototest.tif test1 matdemo inter
   NOTE: The matdemo config file can also be found in the installation suite folder.

This is all that can be said about installation of and launching the Tesseract viewer. For information on how to use Tesseract viewer’s user interface please refer to http://code.google.com/p/tesseract-ocr/wiki/ViewerDebugging

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