# 1 finding depth. 2 overview depth from stereo depth from structured light depth from focus / defocus...

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1 Finding depth

Author: tyrone-stephens

Post on 03-Jan-2016

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• Finding depth

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• Depth from stereoP1P2 two cameras with known parameters infer 3D location of point seen in both images sub problem: correspondences for a point seen in the left image, find its projection in the right image

• Depth from stereo: dj-vu mathP1P2 unknowns are w1 and w2 overconstrained system the u2v2 coordinates of a point seen at u1v1 are constrained to an epipolar line

• Epipolar lineP1P2 C1, C2, P1 define a plane P2 will be on that plane P2 is also on the image plane 2 So P2 will be on the line defined by the two planes intersection

• Search for correspondences on epipolar lineP1P2 Reduces the dimensionality of the search space Walk on epipolar segment rather than search in entire image

• Parallel views Preferred stereo configuration epipolar lines are horizontal, easy to search

C1

P

u2

v1

C2

u1

v1

• Parallel views Limit search to epipolar segment from u2 = u1 (P is infinitely far away) to 0 (P is close)

C1

P

u2

v1

C2

u1

v1

u1

• Depth precision analysis 1/z linear with disparity (u1 u2) better depth resolution for nearby objects important to determine correspondences with subpixel accuracy

C1

P

u2

v1

C2

u1

v1

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• Depth from stereo problemCorrespondences are difficult to findStructured light approachreplace one camera with projectorproject easily detectable patternsestablishing correspondences becomes a lot easier

• Depth from structured lightP1P2 C1 is a projector Projects a pattern centered at u1v1 Pattern center hits object scene at P Camera C2 sees pattern at u2v2, easy to find 3D location of P is determined

C1

P

u2

v2

C2

u1

v1

• Depth from structured light challengesAssociated with using projectorsexpensive, cannot be used outdoors, not portableDifficult to identify patternI found a corner, which corner is it?Invasive, change the color of the sceneone could use invisible light, IR

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• Depth of fieldapertureobjectimageCFF Thin lenses rays through lens center (C) do not change direction rays parallel to optical axis go through focal point (F)

• Depth of fieldapertureobjectimage planeCFF For a given focal length, only objects that are at a certain depth are in focus

• Out of focusapertureobjectimage planeCFF When object at different depth One point projects to several locations in the image Out of focus, blurred image

• Focusingapertureobjectimage planeCFF Move lens to focus for new depth Relationship between focus and depth can be exploited to extract depth

• Determine z for points in focusapertureobjectimage planeCFFhihafz

• Depth from defocusTake images of a scene with various camera parametersMeasuring defocus variation, infer range to objectsDoes not need to find the best focusing planes for the various objectsExamples by Shree Nayar, Columbia U

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• OverviewDepth from stereoDepth from structured lightDepth from focus / defocusLaser rangefinders

• Laser range findersSend a laser beam to measure the distancelike RADAR, measures time of flight

• DeltaSphere - depth&color acquisition deviceLars Nyland et al.courtesy 3rd Tech Inc.

• 300o x 300o panoramathis is the reflected light

• 300o x 300o panoramathis is the range light

• courtesy 3rd Tech Inc.spherical range panoramasplanar re-projection

• courtesy 3rd Tech Inc.Jeep one scan

• courtesy 3rd Tech Inc.Jeep one scan

• courtesy 3rd Tech Inc.Complete Jeep model

• Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Using the parameters of the desired view one can re-project, or warp, the sample onto the desired image plane.Here at UNC we are lucky to have Lars Nyland who built the contraption seen here, called the DeltaSphere, that acquires depth images.