Using Iphone Camera for Photogrammetry

I mentioned before I recently received an Iphone 4s, and having been a strong supporter of Windows against Apple, I am slowly being converted over. Apart from the great thing of being able to carry my models around and show fellow archaeologists without risking the life of my laptop, I have started exploring the advantages of having a pretty good camera on me at all times.

By using the 123D Catch app, it is possible to instantly create amazing models wherever you are, but how accurate are 3D models made using the Iphone camera? I don’t have the app itself due to a lack of 3G, but I have been going around site the last week or so and have taken a number of photographs and then processed them once got home.

Once again I experimented with larger and smaller objects and features, comparing the results I got with those done with regular SLR cameras. I can’t actually upload the images due to site restrictions, but I created a model of a toy car as an example. I followed the usual methods of recording, so to not alter the results in any way.

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These are some of the points I have found:

Image stitching: Comparing the number of images that stitched in normal models and those done with the Iphone’s camera there is a bit of a difference between the two. Especially with similarly coloured images only some of the images are stitched together. This however happened only on a few occasions and as such doesn’t constitute a major flaw.

Point cloud: The number of points within the models done with the Iphone seem to be equal, if not more than those in the normal photographs. I believe this is because the Iphone seems to adjust the colours and lighting automatically and digitally, which makes the photographs seem more consistent. On the other hand this also seems to have the negative effect of artificially changing the images, thus playing with the contrast and colour balance, which affects the accuracy of the model.

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Textures: The textures in the Iphone models seem to be extremely good, probably due to the digital adjustment mentioned above. In this case I wouldn’t say this is a problem, and the results are quite bright and distinct, which is a good thing when analysing the models.

General look: This is the point I have the greatest issues with. The number of keypoints the program finds made me expect extremely crisp models, but they look to me much more murky than they should. The digital altering of the images, and the fact that the size of the images is below the 2 MBs makes the model much less accurate, and the results suffer greatly.

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Overall though I am happy with this method. If the models were of extreme importance, I wouldn’t even consider using the Iphone camera, but for simple and less important models it is perfect. The practical nature of being able to capture images in a matter of minutes and have them upload directly to my Dropbox is great, and on more than one occasion I’ve been caught without my camera, so it is a great alternative.

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About Rob Barratt

Mphil in Archaeological Research at Cambridge Univerity, BA in archaeology from Cardiff University, field archaeologist, technology enthusiast and computer geek. I like writing codes and making fancy models of old stuff.

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