Textline Detection

Detect textlines in document images

Introduction

This tool performs border, region and textline detection from document image data and returns the results as PAGE-XML. The goal of this project is to extract textlines of a document in order to feed them to an OCR model. This is achieved by four successive stages as follows:

• Border detection
• Layout detection
• Textline detection
• Heuristic methods

The first three stages are based on pixelwise segmentation.

Border detection

For the purpose of text recognition (OCR) and in order to avoid noise being introduced from texts outside the printspace, one first needs to detect the border of the printed frame. This is done by a binary pixelwise-segmentation model trained on a dataset of 2,000 documents where about 1,200 of them come from the dhSegment project (you can download the dataset from here and the remainder having been annotated in SBB. For border detection, the model needs to be fed with the whole image at once rather than separated in patches.

Layout detection

As a next step, text regions need to be identified by means of layout detection. Again a pixelwise segmentation model was trained on 131 labeled images from the SBB digital collections, including some data augmentation. Since the target of this tool are historical documents, we consider as main region types text regions, separators, images, tables and background - each with their own subclasses, e.g. in the case of text regions, subclasses like header/heading, drop capital, main body text etc. While it would be desirable to detect and classify each of these classes in a granular way, there are also limitations due to having a suitably large and balanced training set. Accordingly, the current version of this tool is focussed on the main region types background, text region, image and separator.

Textline detection

In a subsequent step, a binary pixelwise segmentation is used again to classify pixels in a document that constitute textlines. For textline segmentation, a model was initially trained on documents with only one column/block of text and some augmentation with regards to scaling. By fine-tuning parameters also for multi-column documents, additional training data was produced that resulted in a much more robust textline detection model.

Heuristic methods

After training the models, they were used to classify documents according to each step and then tried to use results to extract desirable textlines recatngles.

After applying page extraction model we then found the biggest contour and after fitting a bounding box we were able to crop image inside this box.

By layout detection, it was so important for us to detect textregions clearly separately that is why we have scaled image up. With this trick it was easier for model to detect background spaces between textregions.

We have set a minimum textregion area in respect to area of whole image, so those small textregions which are actullay noises in our prediction are filtered. At the end we have found contours of textregions and corresponding boundin boxes.

Textline segmentation is also done and using bounding boxes from textregions we are now able to get textline segmentation for each individual textregion. The first thing that we face by historical documents is that documents are skewed and even worser that each textregion can be skewed in a differnt manner. So, it was a key feature to deskew each textregion. Actually we have used textline segmengtation in each region to deskew corresponding region. After deskewing , calculating distribution of textlines segmentation result in X-direction helped us to find starting and ending point of every single textline. You can imagine that the hills in mentioned distribution are actully where we have background and no textline. Finally, using this coordinates we were able to find bounding rectangle for each textline.

Installation

pip install .

Models

In order to run this tool you also need trained models. You can download our pretrained models from here:
https://qurator-data.de/sbb_textline_detector/

Usage

sbb_textline_detector -i <image file name> -o <directory to write output xml> -m <directory of models>

Usage with OCR-D

ocrd-example-binarize -I OCR-D-IMG -O OCR-D-IMG-BIN
ocrd-sbb-textline-detector -I OCR-D-IMG-BIN -O OCR-D-SEG-LINE-SBB \
        -p '{ "model": "/path/to/the/models/textline_detection" }'

Segmentation works on raw RGB images, but retains AlternativeImages from binarization steps, so it's OK to perform binarization first followed by textline detection. The used binarization processor must produce an AlternativeImage for the binarized image, rather than replace the original raw RGB image.