Building an Automatic RTI Dome: Wrapping It Up

This is the third post in a series, written by  Kevin Falcetano.  See the first post for an introduction and the second post for the build process.

The RTI project was not without its share of problems, mistakes, and general annoyances, but it was ultimately both interesting and rewarding to go through. I’ve separated my concluding remarks into three distinct semi-chronological sections under which the trials, tribulations, and triumphs of this project are detailed.

Building

The construction of this project required many time-intensive repeated steps, but the hardest part, the planning, had already been worked out to a detailed part list and the very placement of the circuit components. Once again, a great many thanks to Leszek Pawlowicz for all of this work. In all of the soldering, striping, crimping, and wiring, I had made only a few easily correctable mistakes. That success is attributed to those thorough instructions and guiding images.

A few lessons were learned, still:

  • Helping hands are wonderful. Use them.
  • Lead free solder may or may not kill you more slowly, but it has a higher melting point. What that really means is it requires a delicate balance between “hot enough to heavily oxidize your soldering tip” and “too cool to flow the solder quickly.” Throw the balance too close to one side or the other and you get the same result: the heat spreads further over the extended heating period (by way of either too low a temperature or restricted conductivity through an oxide layer too thick to be affected by the flux core), melting plastics easily and possibly damaging sensitive components. I killed two switches and probably a piezo speaker this way. Be careful or use leaded solder.
  • Paint drips are fine if all you need is coverage.
  • 3M chalkboard tape may be matte black, which is wonderful for the inside of our domes, but it doesn’t stick. At all.
  • Hot glue is your friend. It’s not pretty or perfect but it’s accessable. What use is a build if you can’t repair it?
  • If one LED isn’t working in your matrix of LEDs, it is, logically, the LED that’s the problem. And by the LED being the problem I mean you are the problem. You put it in backwards but were too proud to check. The D in LED stands for diode – one way.
  • Be sure to have access to a dremel when trying to put large square holes in a hobby box.  Or have low standards.
  • When taking on a wiring project of this caliber, have an auto stripper handy. I may have lost my mind if I hadn’t used one.
  • Leave yourself some slack in the wires in case you break a dupont pin.
  • Don’t break dupont pins.

Calibrating and Processing

The calibration process was initially a bit of a headache, but I still must be grateful to Leszek and Cultural Heritage Imaging for their open software, without which more frustration would have ensued.

CHI provides a java applet that generates lp files. The file extension stands for light positions (or light points) and is pretty self explanatory: An lp file is a plain text file with the angles of the lights attributed to each of the lighting positions in order of first to last picture taken. This is done automatically with some clever math and dodgy code. CHI has told us they have a more stable version in the works, but it’s not quite done yet. Anyway, calibration is done using a black sphere (in this case also provided by CHI) that reflects each LED in the dome as a bright point. The sphere gets centered under the dome and a pass is run taking a photo for each light angle.  The rotational orientation of the dome relative to the camera is marked so as to be consistent for future use. This is because the dome is consistent so long as it it rotated to the correct position over the object, which means the lp may be reused.

The program uses an edge detection algorithm to figure out where the radius, and in turn, the center of the sphere is. After that, it uses the known geometry of the sphere and a highlight detection algorithm to figure out where the reflection point of each LED is for each picture, and then uses its position relative to the center and radius of the sphere to calculate the angle of each light point. For H-RTI, this is supposed to be done every time with a reference sphere next to every subject, but as I said before, we can reuse the lp because everything is fixed. This applet is glitchy and cumbersome, but it works when treated properly. It will run out of memory if we use full resolution photos from the Canon 5D, but downscaling them does not introduce any significant error into the calibration process. It also has very strict guidelines for filenames and a clunky default user interface, the indicators of purely functional software. It’s useful, but tedious.

Leszek has his own software that strips the lp file of its association with the calibration sphere image files and stores it for future use as a general lps file. This can then be used in his program to create a new lp for each RTI run associated with the new image files, and then it and the photos get run through various command line scripts for different RTI and PTM (polynomial texture map) processing methods. This ensures that, after initial calibration, subsequent workflow is very fast and efficient, with minimal fiddling required.

Customizing

Of course, there are some things specific to our use cases that a third party couldn’t anticipate, so some changes were made. The biggest change in functionality, but arguably the easiest change to make programming-wise, was the addition of a mode switch to toggle which dome is in use. This allows the automatic, manual, and preview functions of the control box to change depending on which dome is being used. There are more details on how I changed the hardware and code in the build blog post, but all in all it was relatively painless. Leszek’s code is very clean and manageable, so I was able to easily navigate and change what I needed (which was quite a bit). It worked perfectly from the beginning, but not for exactly what we needed it for.

Final Thoughts

My rough estimate puts a first-time build of this project from start to finish at around 80 hours. It was fun overall, and not terribly frustrating save a few parts. We are currently thinking through how to make a custom shield PCB to optimize the build process of this RTI dome’s control unit, since there may be demand for a pre-built rig from others at the University. It would be very difficult to streamline the dome building process considering it is such a custom endeavor. I could see a template being made for mounting the LEDs and placing them in the domes, possibly with ribbon cables, but it still seems pretty far off.

Although there are certain significant limitations, RTI appears, in our testing, to be a very versatile tool, especially when automated in the way we have it. We are working with researchers from across campus to explore the possibilities, and the speed and precision of these domes is a huge benefit to that work.  In the future, we hope to be able to combine RTI and Photogrammetry to produce models with surface detail we have yet to come close to being able to capture.

If you would like to take advantage of our ever-expanding imaging resources, or would simply like to chat with us about what we’re doing and how we’re doing it, feel free to contact aisos@umn.edu.

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