1. Plenoptic Stitching: A Scalable Method for Reconstructing 3D Interactive Walkthroughs Daniel G. Aliaga Aliaga@bell-labs.com Ingrid Carlbom Carlbom@bell-labs.com Presented by Matthew McCrory

2. Introduction Interactive walkthroughs require detailed 3D models of their environments Traditional approaches: time consuming, still don’t achieve high detail, subtle lighting effects Plenoptic functions allow capture of high detail, subtle lighting effects, in small amount of time

3. Plenoptic Functions 7D plenoptic function describes light intensity passing through every viewpoint, in every direction, for all time and wavelengths Currently, all IBR techniques generate lower-dimensional plenoptic functions using a set of images as their input Aliaga’s and Carlbom’s technique reconstructs a 4D plenoptic function

4. Overview of the process Givens: Viewer moving in open area in a large, complex environment. Camera motion restricted to a plane at eye-height Omnidirectional environment images taken along paths of irregular grid Closed image loops “stitched together” Arbitrary views generated at runtime

5. Benefits of this Approach Ease of capture – Complex environments captured in a matter of minutes Automated processing – Except camera pose estimations, everything’s automatic Scalability – Method is easily scaled to large environments Support for arbitrarily shaped environments – Because image loops may be irregular, arbitrarily shaped environments can be acquired

6. Walkthrough Parameterization One possibility: parameterize all potential light rays by their intersection with two perpendicular planes Creates a 4D plenoptic function Vertical field-of-view limited, straight up/straight down ignored

7. Walkthrough Parameterization To reconstruct a continuous function, the entire open space should be sampled densely, which just isn’t practical! A solution: sample observer plane using irregular grid of omnidirectional image sequences forming image loops in the observer plane

8. Capture Relatively inexpensive camera system used, built from off-the-shelf components. Camera placed on motorized cart with battery, computer, frame grabber, and fast RAID disk Camera uses a convex paraboloidal mirror w/an orthographic projection

9. Camera Pose Estimation Calibration scheme developed by Aliaga and Carlbom that uses beacons for calibration In 2 corners of a region, beacons are placed. Before recording, user initializes pose estimation by identifying the projections of the beacons in the first captured image As camera moves, beacon is tracked and, using triangulation, derives camera position and orientation

10. Reconstruction Given a set of image loops, create novel planar views of environment from arbitrary viewpoints inside a loop Combine pixels from omnidirectional images in the forward-looking view frustum with pixels in the omnidirectional images in the rear-looking frustum

11. Reconstruction New view created via column-by-column reconstruction of pixels from the omnidirectional images If viewing direction for a column intersects the COP of 2 complimentary omnidirectional images, then that direction maps directly to radial lines in the images.

12. Reconstruction Corresponding segments of each radial line warped to the column in reconstructed image Geometry of omnidirectional camera must be considered Using similar triangles…

13. Reconstruction Because pixels from the image behind the viewpoint are generally stretched during the warp, pixels are drawn using fixed sized splats Vertical disocclusions are filled in using longer than expected splats Usually, viewing direction will not intersect exactly with COP of an omnidirectional image Radial is generated then by blending 2 parallel radials

14. Reconstruction Difficult to reliably identify corresponding features in the radial lines from omnidirectional images on opposite sides of a loop Temporal coherence of the entire image loop is used to identify the required features Start with an arbitrary omni image, track features all the way around the loop

15. Optimization In ideal conditions, radial lines differ only by a radial displacement and should be easy to recover In practice, feature tracking and camera pose estimation introduce errors into mapping

16. Optimization: Rotational Correction This optimization accounts for incorrect pairing of radial lines Features should move along same set of circles Error represented by sum of squares of distances between corresponding features Errors reduced by searching for better aligned rotations

17. Optimization: Column Correlation Ideally, pairs of warped columns should be vertically aligned Feature drifting, lack of sufficient features, and inaccurate distance estimation between the images cause misalignment To correct this, one column is scaled prior to blending the two columns together

18. Implementation To achieve real-time performance, reconstruction is divided into a preprocessing and a runtime phase A modified JPEG compression and a 3 cache system help to quickly and dynamically load images and data

19. Results Reconstruction time varies slightly depending on # of features per column Reconstructions of 320x160 and 640x320 pixels take between 5 to 10 frames/sec with a pixel splat of 1x4 pixels Maximum capture time for the four environments recorded was 25 minutes

20. Results Top image: Reconstructed using described method for viewpoint near middle of a loop Bottom image: Planar re-projection of an image captured from approximately the same viewpoint

21. Results Example reconstructions for viewpoints near the middle of image loops

22. Results Environmental statistics:

23. That’s it! Questions?