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RTI and Photometric Stereo


Alessandro

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Dear CHI developers,

I did two experiments with the RTI technique on the Valcamonica engravings (Italy) and I got pretty impressive results with your RTI Builder and Visualizer.

I am aware that the Photometric Stereo tecnique can produce the same kind of image output plus very accurate 3D models (see the papers by Paolo Bagnoli on Academia.edu website).  Photometric Stereo basically works with a similar acquisition setting but with just four pictures. I wonder if the same algorithms described by Bagnoli could be implemented in the RTI viewer.

 

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  • 2 weeks later...

Photometric Stereo on its own introduces ambiguities when dealing with surface discontinuities. This is true regardless of how many lights were used in the determination of surface normals, although 3 is the minimum and 4 is often used allowing one to discard a single sample per pixel that introduces specular highlights or shadows. In addition, assumptions of simplified (typically diffuse) material properties also lead to normal estimation errors.

 

When we wrote the original PTM viewer, we just found the maximum of the reflectance function, which we had in analytic form as a polynomial, to compute surface normals. As the RTI viewer was built on the PTM viewer, I'm assuming the same computations are performed there, but I'm not 100% positive. There are literally hundreds of papers that have been written on other variants of photometric stereo for surface normal estimation and subsequent surface reconstruction, but they all suffer from the two issues mentioned above, until combined with some sort of triangulation, photogrametry, SLAM or other camera pose/geometry estimation. In short, much has been already published on improving surface normals within RTI, and more could be done., its a fruitful topic.

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