Does RTI give repeatable and reliable normals of objects taken at different times and positions to facilitate detection of changes?

On the Linked-In discussion group Cultural Heritage Conservation Science. Research and practice’s discussion on 3-D digital imaging and photogrammetry for scientific documentation of heritage sites and collections , Greg Bearman wrote the following question:

“Does RTI give repeatable and quantitative set of normals good enough for looking for change? If I take an RTI set, rotate the object, let it warp a bit (flexible substrate), what do I get the second time? How do I align the datasets for comparison?

what is the system uncertainty? ie if I just take repeated images of the same object without moving anything, how well does the RTI data line up. Second, suppose I take something with some topography but is totally inflexible and cannot distort(make up test object here!) and I do repeated RTI on it in different orientations? Can I make the data all the same? If you are going to use an imaging method to determine changes in an object, the first thing to do is understand what is in inherent noise and uncertainty in the measuring system. It could be some combination of software, camera or inherent issues with the method itself”

I wrote back: “Hey Greg – tried sending response earlier last week but I do not see it!? Sorry. I’m on vacation until the 22nd – trying to recover and recharge. It is going well but I wanted to jot down my initial thoughts. One of my interns – Greg Williamson – is working on aberration recognition software that can recognize and highlight changes in condition captured by different H-RTI computational image assemblies – obviously taken at different times, but also with different equipment and with randomly different highlight flash positions. It seems, initally, that normal reflection is normal reflection, regardless of object or flash position and that the software correctly interpolates 3D positions of surface characteristics regardless of the precise position of the flash, because it is accustomed to calculating the highlights both the capture points and everywhere in between! Likewise, we have had promise with photogrammetry when the resolution of the images used to create the mesh and solids are similar. What may turn out to be key is a calibration set that will allow correction of the various lens distortions that would naturally come from different lenses. I know Mark Mudge at Cultural Heritage Imaging has suggested that we begin taking a calibration set before RTI capture, as we had before Photogrammetry. He may be working on incorporating a calibration correction into the highlight RTI Builder that CHI has made available. I’m sending this discussion along to the CHI forum at http://forums.cultur… to see what others might have to add. When I return to work, I’ll ask Greg to give this some additional thought”

Whadaya think, Greg?


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