Presentation on theme: "Multi-view Stereo via Volumetric Graph-cuts"— Presentation transcript:
1 Multi-view Stereo via Volumetric Graph-cuts George VogiatzisRoberto Cipolla Cambridge Univ. Engineering Dept.Philip H. S. TorrDepartment of Computing Oxford Brookes University
2 Multi-view Dense Stereo Calibrated images of Lambertian scene3D model of scene
3 Multi-view Dense Stereo Two main approachesVolumetricDisparity (depth) mapVolumetric
4 Dense Stereo reconstruction problem: Two main approachesVolumetricDisparity (depth) mapDisparity-map
5 Shape representation Disparity-maps MRF formulation – good optimisation techniques exist (Graph-cuts, Loopy BP)MRF smoothness is viewpoint dependentDisparity is unique per pixel – only functions represented
6 Shape representation Volumetric – e.g. Level-sets, Space carving etc. Able to cope with non-functionsLevelsets: Local optimizationSpace carving: no simple way to impose surface smoothness
7 Our approach Cast volumetric methods in MRF framework Use approximate surface containing the real scene surfaceE.g. visual hullBenefits:General surfaces can be representedNo depth map merging requiredOptimisation is tractable (MRF solvers)Smoothness is viewpoint independent
8 Volumetric Graph cuts for segmentation Boykov and Jolly ICCV 2001Volume is discretizedA binary MRF is defined on the voxelsVoxels are labelled as OBJECT and BACKGROUNDLabelling cost set by OBJECT / BACKGROUND intensity statisticsCompatibility cost set by intensity gradient
9 Volumetric Graph cuts for stereo Challenges:What do the two labels representHow to define cost of setting themHow to deal with occlusionInteractions between distant voxels
25 Protrusion problem ‘Balooning’ force favouring bigger volumes that fill the visual hullL.D. Cohen and I. Cohen. Finite-element methods for active contour models and balloons for 2-d and 3-d images. PAMI, 15(11):1131–1147, November 1993.
26 (x) dS - dV Protrusion problem ‘Balooning’ force favouring bigger volumes that fill the visual hullL.D. Cohen and I. Cohen. Finite-element methods for active contour models and balloons for 2-d and 3-d images. PAMI, 15(11):1131–1147, November 1993.
35 Summary Questions ? Novel formulation for multiview stereo Volumetric scene representationComputationally tractable global optimisation using Graph-cuts.Visual hull for occlusions and geometric constraintOcclusions approximately modelledQuestions ?