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Charles Walbridge

RTI or photogrammetry for sarcophagus?

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I want to image this object in our collection here at the Minneapolis Institute of Arts: https://collections.artsmia.org/index.php?page=detail&id=738

It's a low-relief sarcophagus that I think would image well in either RTI or using photogrammetry, but I can't decide which to use.

I know the object would be easy for RTI, and I could do it in sections or its entire sides. But I think I'd like to end up with a file that's shareable outside the RTI Viewer, like an OBJ or an STL. I know we're photographing two spheres in RTIs to make our data ready for 3D modeling, but I haven't heard an update on that since I saw the CHI team in DC in 2012.

With photogrammetry I would use the advice from the BLM publication and either a trial or the $179 version of PhotoScan. I think that technique would let me take the meshes of each of the object's six sides and make them into a beautiful, shareable object.

I'm going to need to figure out PhotoScan in the near future regardless: should this be the object I start with?

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Charles,

 

Like almost everything, there are trade-offs. I think you have to ask questions like, what is the purpose of my imaging project?  For example: interesting graphics for the general public via the web? interesting graphics for the general public via a hi resolution screen in the museum exhibit?, support conservation activity?  Help answer a particular research question?  Collect data so you can monitor change in the object (such as whether a crack is getting longer or deeper?  Etc.  Only when you know what your purpose is can you chose not only the right technique, but also the right level of detail that is needed. Sometimes data collected for one purpose can also serve another purpose.  This isn't always true though.  

 

In terms of photogrammetry, the standard version of photoscan is useful to get a little feedback on your image sets and get started, but if you are serious about wanting to create 3D objects that you can measure, you will want the pro version because it allows you to add scale, and also to refine your calibration.  Calibration data is critical to understand how accurate your model is.  If you are just making models to get the public interested in something this may not be important, but if you want to track change - this s absolutely critical.

 

Carla

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hi Charles,

 

thanks for the post. RTI Vs Photogrammetry. Interesting.

 

Both will give your really interesting results. Dont forget that sharing a '3d' file sometimes isn't easy as pie, well, your viewer will need to know how to navigate the controls in your 3D 'viewer' or 'app', etc etc. I suppose thats the same for the RTI viewer too. Getting '3D' data from the RTI data is possible, its just not quite there yet --- there are still some variables that need to be worked out. We can get results right now, but there's a 'warping' in the mesh that needs more attention. When this gets fixed (outside of a lab environment), a kind of holy grail will be revealed.

 

P-Scan is a killer app. There's a learning curve, but you've already started bc you're a good photographer. The input data is sharp, in focus, properly exposed images. The sequence, image overlap, and camera calibration are critically important to your final result. As Carla mentioned, the Pro version of P-Scan (or the Edu version) allows for automatic measurement / marker detection --- the results being metrics on your final file. If you Optimize your points correctly, the final out come is a very accurate documentation of your object.

 

The "low-relief sarcophagus" is a good candidate for photogrammetry bc it doesn't look that reflective, contains no glass, and has many planar type surfaces (easy to shoot). If you get your lighting just right, the texture on the model will be delightful to view.

 

Maybe you can document detailed areas with RTI and some of the larger parts of the sarcophagus with photogrammetry.(?)

 

Hope this helps to asses.

happy F-stop.

 

Marlin.

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