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  1. Stereo

  2. Why do we have two eyes? Cyclope vs. Odysseus

  3. 1. Two is better than one

  4. 2. Depth from Convergence Human performance: up to 6-8 feet

  5. 3. Depth from binocular disparity P: converging point C: object nearer projects to the outside of the P, disparity = + F: object farther projects to the inside of the P, disparity = - Sign and magnitude of disparity

  6. Stereogram

  7. Stereo scene point image plane optical center

  8. Stereo • Basic Principle: Triangulation • Gives reconstruction as intersection of two rays • Requires • calibration • point correspondence

  9. Stereo image rectification

  10. Stereo image rectification • Image Reprojection • reproject image planes onto common plane parallel to line between optical centers • a homography (3x3 transform)applied to both input images • pixel motion is horizontal after this transformation • C. Loop and Z. Zhang. Computing Rectifying Homographies for Stereo Vision. IEEE Conf. Computer Vision and Pattern Recognition, 1999.

  11. Stereo Rectification

  12. Your basic stereo algorithm For each epipolar line For each pixel in the left image • Improvement: match windows • This should look familar... • Can use Lukas-Kanade or discrete search (latter more common) • compare with every pixel on same epipolar line in right image • pick pixel with minimum match cost

  13. Window size W = 3 W = 20 • Smaller window • Larger window Effect of window size

  14. Stereo results • Data from University of Tsukuba • Similar results on other images without ground truth Scene Ground truth

  15. Results with window search Window-based matching (best window size) Ground truth

  16. Better methods exist... • State of the art method • Boykov et al., Fast Approximate Energy Minimization via Graph Cuts, • International Conference on Computer Vision, September 1999. Ground truth

  17. Stereo Algorithm Competition • http://vision.middlebury.edu/stereo/

  18. Depth from disparity depth map 3D rendering [Szeliski & Kang ‘95] X z x x’ f f baseline C C’ input image (1 of 2)

  19. Stereo reconstruction pipeline • Steps • Calibrate cameras • Rectify images • Compute disparity • Estimate depth What will cause errors? • Camera calibration errors • Poor image resolution • Occlusions • Violations of brightness constancy (specular reflections) • Large motions • Low-contrast image regions

  20. Stereo matching • Need texture for matching Julesz-style Random Dot Stereogram

  21. Active stereo with structured light camera 1 camera 1 projector projector camera 2 • Project “structured” light patterns onto the object • simplifies the correspondence problem Li Zhang’s one-shot stereo

  22. Active stereo with structured light

  23. Spacetime Faces: High-Resolution Capture for Modeling and Animation • http://grail.cs.washington.edu/projects/stfaces/ • Watch the video !

  24. Laser scanning • Optical triangulation • Project a single stripe of laser light • Scan it across the surface of the object • This is a very precise version of structured light scanning • Digital Michelangelo Project • http://graphics.stanford.edu/projects/mich/

  25. Portable 3D laser scanner (this one by Minolta)

  26. Real-time stereo • Used for robot navigation (and other tasks) • Several software-based real-time stereo techniques have been developed (most based on simple discrete search) • Nomad robot searches for meteorites in Antartica • http://www.frc.ri.cmu.edu/projects/meteorobot/index.html

  27. Summary • Things to take away from this lecture • Epipolargeometry • Stereo image rectification • Stereo matching • window-based epipolar search • effect of window size • sources of error • Active stereo • structured light • laser scanning