Sunday, 24 March 2013

Extreme SfM: underwater archaeology

Hi all,
It is long that Alessandro and me wanted to write this post, but for one reason or another, we could not work on it. 
Today I decided to do it in order to answer two questions that people often asked us during conference or lessons:

1) Is it possible to work with SfM/IBM techniques underwater?

2) Is it possible to extract 3D from a movie?

As regards the first question, I can report that since we (Arc-Team) started to work with SfM and IBM (2009), we did also tests underwater and they gave us positive results. This is one of the main reason why we invested so much time on the research in this filed: SfM and IBM methodology, until now, is one of the best solution in archeology, due to its versatility (it can be used for underwater or aerial documentation, in low light conditions or in precarious situation, during mission abroad, etc...). We already underlined this concept when, with the help of Nicolò Dell'unto (Lund University), we compared different methodology to record 3D documentation of archaeological artifacts. The result of this experiment was presented during the ArcheoFOSS 2012, in Napoli (see the related slides and this post). During the workshop "Low cost 3D: sensori, algoritmi e applicazioni", we had the opportunity to better analyze the use of SfM/IBM in extreme working conditions, strengthening our point of view about this methodology (see the related slides). 
The image below is an example of an aerial 3D documentation done with an open source UAV and Python Photogrammetry ToolboX...

Aerial documentation with a KKopter and ArcheOS (PPT)
... while this other image shows the results we achieved using some pictures that Victor Jansa, of TUWA ("Tauchverein für Unterwasserarchäologie"), sent us to do a test.

Test with Victor Jansa's pictures (done with PPT)

In order to answer the second question, I can say that, facing our experience, it is possible to reconstruct 3D models from videos (and I guess this is one of the aims of SfM itself). We did some tests about this topic, getting acceptable result (at least regarding our primary target, which was to have a fast 3D object for further modeling operations). As an example, I can report one of the last post of Cicero Moraes, who used SfM from a youtube clip to get a 3D skull for forensic facial reconstruction aims. The image below is taken from Cicero's photo album:

3D skull obtained with SfM techniques from a movie

For a better explanation of what I wrote above, I think it is worth to show the results of a project we are undertaking since 2005 (trying to support Prof. Tiziano Camagna on his exploration of Tovel's Lake, in Trentino).  During this project we did several surveys, diving in different parts of the lake and especially in the South-West area, where lies a forest which is now underwater. In 2012 Tiziano Camagna and Andrea Forti, despite the low visibility, where able to record a short movie of some of the threes. We used this video for a fast 3D reconstruction, because it was particularly indicated, due to its characteristics: it was recorded for no SfM aims (as you see the movie sequences are not optimal for a 3D reconstruction), it represents the normal turbidity condition of the lake and it was done with an high lens distortion camera (GoPro Hero 2). For such reasons this material was perfect to hardly test SfM and IBM techniques for underwater archeology. In this animation you can see a short part of the movie (from the 15th second  to the 25th), which we used for the 3D documentation...

From "La foresta sommersa del Lago di Tovel" (T. Camagna, A. Forti)

... and here you can see the result:

I hope this post was useful. Soon, when the season will allow us to start diving again, we will go on with tests and experiment related with underwater archeology. I hope to write soon some new report about it.


  1. Really interesting, congratz!

    1. Thank you Cicero, hopefully soon we will do new test to better develop this methodology for underwater archeology.

  2. Amazing result from that material and interesting blogpost, thank you!

  3. Thank you Anders, I am glad you read this post, I hope it was useful.

    1. Yes, it was useful. And inspiring so I am now doing some tests with the images I showed you in Lund. But during next month (I hope) I will do a new dive and take better photos...

  4. Thank you Luca...I was particularly interested in it, during the lessons at CNR last June: I'll wait for the results of next texts! Congratulations.

  5. hi, luca,
    nice examples! in my diploma thesis i show a workflow to use SfM on the monitoring of underwaterarchaelogical sites by the example of the prehistoric piledwellings, and i find out that it is possible to detect changes in the condition and form of just a few millimeters...

    if you want, i can write something about that, and, especially, about how to extract pics from a movie and how to maniopulate the exif, which is very important...


  6. Hi Viktor and Luca. I created a small video tutorial showing how to auto extract image stills from a video using VLC. The goal is to obtain a large number of images - automatically taken with regular time intervals - suitable to the SfM video process illustrated by Cicero.

    The idea is good, but I didn't like the results. The SfM software require each image EXIF data to obtain good results.

    Thats the reason I didn't post the video. It would be great if Viktor could explain us better this crucial step to create a more detailed tutorial.


  7. Hi Ricardo,
    personally I prefer to use normal pictures when I work with SfM (it is less time consuming and more accurate, because the photos are specifically taken for this issue), but I had good results also with video. The reconstruction of the underwater tree was done by Alessandro and me with SfM directly from a movie (GoPro Hero), just we used ffmpeg to extract the single photo. I think your tutorial is interesting, showing another way to extract images from a clip. If you want to post it, I think it will be useful for ATOR readers.
    PS, if you want, you can send us the picture you get, so that we can also try to reconstruct the model and compare it with your), or, even better, you can send the original clip. Ciao.

  8. Hi Luca. I also prefer photos, but the idea of reconstruct archaeological contexts/objects using video data is attractive. Specially if you can use old film or tv tapes digitalizations. Some films use lot of traveling techniques and this is probably good advantage to SfM processing:

    i.e.) Viaggio in Italia (1954)

    Although I also use ffmpeg, VLC GUI was my choice because it's already included in ArcheOS and don't require command line operations (that may be puzzling for some new GNU/Linux users).

    The video resolution is very crappy but i would send it anyway.



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