Tag Archives: library labs

Automated item data extraction from old documents


The Problem

We have a collection of historic papers from the Scottish Court of Session. These are collected into cases and bound together in large volumes, with no catalogue or item data other than a shelfmark. If you wish to find a particular case within the collection, you are restricted to a manual, physical search of likely volumes (if you’re lucky you might get an index at the start!).

Volumes of Session Papers in the Signet Library, Edinburgh
Volumes of Session Papers in the Signet Library, Edinburgh

The Aim

I am hoping to use computer vision techniques, OCR, and intelligent text analysis to automatically extract and parse case-level data in order to create an indexed, searchable digital resource for these items. The Digital Imaging Unit have digitised a small selection of the papers, which we will use as a pilot to assess the viability of the above aim.

Stage One – Image preparation

Using Python and OpenCV to extract text blocks

I am indebted to Dan Vanderkam‘s work in this area, especially his blog post ‘Finding blocks of text in an image using Python, OpenCV and numpy’ upon which this work is largely based.

The items in the Scottish Session Papers collection differ from the images that Dan was processing, being images of older works, which were printed with a letterpress rather than being typewritten.

The Session Papers images are lacking a delineating border, backing paper, and other features that were used to ease the image processing. In addition, the amount, density and layout of text items is incredibly varied across the corpus, further complicating the task.

The initial task is to find a crop of the image to pass to the OCR engine. We want to give it as much text as possible in as few pixels as possible!

Due to the nature of the images, there is often a small amount of text from the opposite page visible (John’s blog explains why) and so to save some hassle later, we’re going to start by cropping 50px from each horizontal side of the image, hopefully eliminating these bits of page overspill.

A cropped version of the page
A cropped version of the page

Now that we have the base image to work on, I’ve started with the simple steps of converting it to grayscale, and then applying an inverted binary threshold, turning everything above ~75% gray to white, and everything else to black. The inversion is to ease visual understanding of the process. You can view full size versions by clicking each image.

A grayscale version of the page
75% Threshold
75% Threshold

The ideal outcome is that we eliminate smudges and speckles, leaving only the clear printed letters. This entailed some experimenting with the threshold level, as you can see in the image above, a lot of speckling remains. Dropping the threshold to only leave pixels above ~60% gray was a large improvement, and to ~45% even more so:

60% Threshold
60% Threshold
45% Threshold
45% Threshold

At a threshold of 45%, some of the letters are also beginning to fade, but this should not be an issue, as we have successfully eliminated almost all the noise, which was the aim here.

We’re still left with a large block at the top, which was the black backing behind the edge of the original image. To eliminate this, I experimented with several approaches:

  • Also crop 50px from the top and bottom of the images – unfortunately this had too much “collateral damage” as a large amount of the images have text within this region.
  • Dynamic cropping based on removing any segments touching the top and bottom of the image – this was a more effective approach but the logic for determining the crop became a bit convoluted.
  • Using Dan’s technique of  applying Canny edge detection and then use a rank filter to remove ~1px edges – this was the most successful approach, although it still had some issues when the text had a non-standard layout.

I settled on the Canny/Rank filter approach to produce these results:

Result of Canny edge finder
Result of Canny edge finder
With rank filter
With rank filter

Next up, we want to find a set of masks that covers the remaining white pixels on the page. This is achieved by repeatedly dilating the image, until only a few connected components remain:

You can see here that the “faded” letters from the thresholding above have enough presence to be captured by the dilation process. These white blocks now give us a pretty good record of where the text is on the page, so we now move onto cropping the image.

Dan’s blog has a good explanation of solving the Subset Sum problem for a dilated image, so I will apply his technique (start with the largest white block, and add more if they improve the amount of white pixels at a favourable increase in total area size, with some tweaking to the exact ratio):

With final bounding
With final bounding

So finally, we apply this crop to the original image:

Final cropped version
Final cropped version

As you can see, we’ve now managed to accurately crop out the text from the image, helping to significantly reduce the work of the OCR engine.

My final modified version of Dan’s code can be found here: https://github.com/mbennett-uoe/sp-experiments/blob/master/sp_crop.py

In my next blog post, I’ll start to look at some OCR approaches and also go through some of the outliers and problem images and how I will look to tackle this.

Comments and questions are more than welcome 🙂

Mike Bennett – Digital Scholarship Developer


Library Digital Development investigate IIIF

Quick caveat: this post is a partner to the one Claire Knowles has written about our signing up to the IIIF Consortium, so the explanation of the acronym will not be explained here!

The Library Digital Development team decided to investigate the standard due to its appearance at every Cultural Heritage-related conference we’d attended in 2015, and we thought it would be apposite to update everyone with our progress.

First things first: we have managed to make some progress on displaying IIIF formatting to show what it does. Essentially, the standard allows us to display a remotely-served image on a web page, with our choice of size, rotation, mirroring and cropped section without needing to write CSS, HTML, or use Photoshop to manipulate the image; everything is done through the URL. The Digilib IIIF Server was very simple to get up and running (for those that are interested, it is distributed as a Java webapp that runs under Apache Tomcat), so here it is in action, using the standard IIIF URI syntax of [http://[server domain]/[webapp location]/[specific image identifier]/[region]/[size]/[mirror][rotation]/[quality].[format]]!

The URL for the following (image 0070025c.jpg/jp2) would be:



This URL is saying, “give me image 0070025 (in this case an Art Collection poster), at full resolution, uncropped, unmirrored and unrotated: the standard image”.



This URL says, “give me the same image, but this time show me co-ordinates 300px in from the left, 50 down from the top, to 350 in from the left, to 200 down from the top (of the original); return it at a resolution of 200px x 200px, rotate it at an angle of 236 degrees, and mirror it”.

The server software is only one part of the IIIF Image API: the viewer is very important too. There are a number of different viewers around which will serve up high-resolution zooming of IIIF images, and we tried integrating OpenSeaDragon with our Iconics collection to see how it could look when everything is up and running (this is not actually using IIIF interaction at this time, rather Microsoft DeepZoom surrogates, but it shows our intention). We cannot show you the test site, unfortunately, but our plan is that all our collections.ed.ac.uk sites, such as Art and Mimed, which have a link to the LUNA image platform, can have that replaced with an embedded high-res image like this. At that point, we will be able to hide the LUNA collection from the main site, thus saving us from having to maintain metadata in two places.


We have also met, as Claire says, the National Library’s technical department to see how they are doing with IIIF. They have implemented rather a lot using Klokan’s IIIFServer and we have investigated using this, with its integrated viewer on both Windows and Docker. We have only done this locally, so cannot show it here, but it is even easier to set up and configure than Digilib. Here’s a screenshot, to show we’re not lying.


Our plan to implement the IIIF Image API involves LUNA though. We already pay them for support and have a good working relationship with them. They are introducing IIIF in their next release so we intend to use that as a IIIF Server. It makes sense- we use LUNA for all our image management, it saves us having to build new systems, and because the software generates JP2K zoomable images, we don’t need to buy anything to do that (this process is not open, no matter how Open Source the main IIIF software may be!). We expect this to be available in the next month or so, and the above investigation has been really useful, as the experience with other servers will allow us to push back to LUNA to say “we think you need to implement this!”. Here’s a quick prospective screenshot of how to pick up a IIIF URL from the LUNA interface.


We still need to investigate more viewers (for practical use) and servers (for investigation), and we need to find out more about the Presentation API, annotations etc., but we feel we are making good progress nonetheless.

Scott Renton, Digital Developer