Emphasising Inscriptions Using 123D Catch

One of the most interesting projects I have been working on over the past few months has been trying to emphasise inscriptions on artefacts using Photogrammetry. The theory is that if the model is accurate enough it should be possible for a program to determine the different bumps in a surface and exaggerate them in order to make them easier to identify.

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My first attempt was with a lead stopper (of which I have posted before), which appeared to have some form of writing inscribed on its surface. Having made a model using 123D Catch, I run it through Meshlab and tested many different filters to see if any of them gave me different results. One in particular seemed to do exactly what I wanted, changing the colour of the texture based on the form of the surface. This is the Colourise curvature (APSS) filter, with a very high MLS (I went for 50000) and using Approximate mean as curvature type. he results were some form of inscription, clearer than the photographs, but still quite murky.

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In addition to this scarce results, other tests with different artefacts pointed towards a lucky coincidence rather than an actual method.

One of the main issues I was also having was that Meshlab kept on crashing when using certain filters or shaders, which meant I could only test out only some of the possible options. So when I made a model of a series of modern graffiti at Cardiff Castle the results were also deluding.

The other day though I so happened to update my Meshlab program, as well as bumping into this interesting article, which gave me the exact solution I was looking for http://www.dayofarchaeology.com/enhancing-worn-inscriptions-and-the-day-of-archaeology-2012/

One of the shaders that I had tried using but that crashed every time was the Radiance Scaling tool, which does exactly what I was aiming to do. I run the graffiti and the results are amazing. The original model stripped of textures is a very unclear blurry surface, but after Radiance Scaling the graffiti come to life. The exact mechanics behind this tool, and hence the accuracy, are currently something I am working on, but at least visually the results are there.

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If this proves to be an effective method, it would suggest that Photogrammetry is not simply technology for technology’s sake, but it can actually be used to interpret archaeology in ways not available before.

Edit: I’d also like to draw attention to the fact that this model was made entirely using my iPhone.

Textures in Photogrammetry: How They Can Deceive Us

One of the advantages I find in Photogrammetry is that unlike other methods such as laser scanning and regular 3D recording, the results are photorealistic. The models seem natural and precise due to the textures that are associated with the points, and aesthetically it is much more pleasing. In addition to this it is amazing for archaeological recording, as all colours and shades are preserved as they were originally.

However the more I experimented with this great technology, the more I realised that as useful as this tool is, the textures sometime give us a wrong idea of what is going on. Generally this causes no problems at all, but in some situations I have found myself trying to get things to work and failing due to a false perception I had of the model.

A good example of this are these graffiti from Cardiff castle.

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The model itself is not great. It was made with my Iphone camera, and I didn’t even use the maximum quality on 123D Catch, however it does help me prove the point.

I spent quite a lot of time trying to test a theory of mine, which I’ve written about before. Theoretically, using a filter or renderer on a photogrammetric model, it should be possible to emphasise inscriptions in order to make them clearer. I was originally trying the APSS filter, but I recently read articles suggesting the use of Radiance Scaling in Meshlab (I’m still experimenting with this, but results seem positive for now). Therefore I decided to test out a number of filters on the graffiti model, as the ridges of the letters appeared to have been handled very well by the program. Even when rotating the textures caused me the illusion that quite a few points had been found around each letter, points that I could manipulate to my advantage.

Having played around for some time with no real results, I decided to try removing the textures to manipulate the shadow, but having done that I noticed that the beautiful ridges that had appeared previously, disappeared when the textures were removed, like so:

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Looking at it even better I noticed that the points that had been found were basically evenly distributed, rather than surrounding the letters as I first thought. As a result the reason the filters were not working was because there was nothing to work on.

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So even if the textures were providing a much needed aesthetic relief, and helping with the recording, for more specific actions they only got in the way, creating illusions.

This however does not in any way limit the technology. The case shown was that of an extreme case, in which the circumstances caused a lack of points. Most models have a much larger number of points and a greater accuracy, which makes it suited for most situations. However when pushing the limits of the technology it is important to remember that in the end the model is just a collection of points, and that these are where the potential is.