Wednesday, September 23, 2009

Recognizing Glitches

Working with three-phase scanning a bit recently, I've become very familiar with the kind of glitches that emerge. Watching Radiohead's "House of Cards" video again, I understand the glitches better now than when I first saw it:

  1. At :58-:59, Thom Yorke's neck separates from his head intermittently. This is due to the ambiguity of propagating a wrapped phase across shadows and depth discontinuities (from his jaw) which generally lend to a few possible interpretations of the data.
  2. At 1:29-1:31, someone waves their arms about a bit. If you look at the edges of their arms, you can see "barbs" coming out, something like 3d-interlacing. This is due to the nature of phase-shift scanning as a sequential structured light system rather than a continuous/fixed system. I'm a little amazed that these artifacts exist, as they were probably using a very high framerate camera.
  3. Throughout the video, entire scenes will jump forward and backward. This is due to the unwrapping algorithm again, and that it has to assume an absolute distance at a specific fixed point. If you get noise at that point, the entire image is shifted. I'm actually surprised this was a bug in the system they used.

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