3D Reconstructing… everything, part quatre: a fully referenced ukulele

In previous posts in this series here and here II and here III: , we’ve worked through a surprising use case in OpenDroneMap: unreferenced imagery for general 3D reconstruction. To get there, we eventually included ground control in the form of a coordinate system set by a builder’s square set in the scene. (Fair warning, I did find a bug in the automated calculation of ground sampling distance documented as an issue here. To replicate this process, you’ll need to choose your orthophoto and elevation model pixel resolution carefully). This was also a test of using 51MP images from a Sigma Quattro H sensor (Foveon X3) to see how it would perform with OpenDroneMap. This is actually the primary purpose of the experiment, but I kept going once I got started… .

By creating a coordinate system using an object of known scale within our scene, we ensure that we have a correctly scaled version of the model and that the Z axis is up.

Random aside, but 5 years ago I wrote a quick set of posts on the Null Archipelago, or finding the 0 origin of all the coordinate systems. As it happens, by hacking the UTM coordinate system to put this unreferenced model into OpenDroneMap, I have effectively placed it on the Null Island of UTM Zone 17 North, which lies on the equator.



Importing the uke into Sketchup lets us see the scale is correct (there are lots of better ways to do this…):


How does our new model look in blender? It looks pretty good, even with really simple lighting.


How well do all these data work geographically? Do we get a good orthophoto, surface model, and can we extract elevation profiles? Yes. yes, and heck yes.


This has been a pretty interesting and odd experiment. As far as I know, this is among a very few photogrammetrically mapped ukuleles. But, it is illustrative of a tool that is somewhat insensitive to scale: use it to map a city, or a musical instrument on your dining room floor. It indicates an interesting market opportunity for OpenDroneMap: how many blender users want easier scanning of objects?

Archiving and sharing

This also has me thinking about other projects. What are the ways in which we can archive and share special projects? Cleveland Museum of Art is thinking along these lines with the release of open access 3D scanned art. The museum sees these pieces as a public good, and as such has released the scans CC0, which is the global equivalent of Public Domain. This is one of the coolest things I have seen a museum do in a long time.

I am a musical instrument nut. I would love to see an online CC0 archive of all the great musical instruments so we can better explore as musical instrument makers what it is that makes a Torres guitar special, or if not what makes it special, what are the starting measurements that might allow us to make something with a similar sound.

Now on to finding out how well I can scan the inside of a guitar… .

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