# Perceiving 3D from 2D Images - University of Washington

Perceiving 3D from 2D Images How can we derive 3D information from one or more 2D images? There have been 2 approaches: 1. intrinsic images: a 2D representation that stores some 3D properties of the scene 2. 3D shape from X: methods of inferring 3D depth information from various sources 1 What can you determine about 1. the sizes of objects 2. the distances of objects from the camera? What knowledge do you use to analyze this image? 2

What objects are shown in this image? How can you estimate distance from the camera? What feature changes with distance? 3 Intrinsic Images: 2.5 D The idea of intrinsic images is to label features of a 2D image with information that tells us something about the 3D structure of the scene. occluding edge convex edge 4 Contour Labels for Intrinsic Images convex crease (+)

concave crease (-) blade (>) M + + + limb (>>) shadow (S) illumination boundary (I) 5 reflectance boundary (M) S

I Labeling Simple Line Drawings Huffman and Clowes showed that blocks world drawings could be labeled (with +, -, >) based on real world constraints. Labeling a simple blocks world image is a consistent labeling problem! Waltz extended the work to cracks and shadows and developed one of the first discrete relaxation algorithms, known as Waltz filtering. 6 Simple Blocks World Constraints for Objects with Trihedral Junctions There are only 16 topologically possible junctions for this class of images. Huffman/Clowes categorized these. +

Ls + + + + + + + + arrows forks Ts

+ 7 2 Interpretations floating 8 glued to the wall Line Drawing Labeling Given a line drawing extracted from an image, find the correct labeling(s). + + + + +

+ 9 impossible junction L junctions cannot have + and - Automatic Labeling Pi Pj Finding a legal labeling can be done by: 1. tree search with backtracking when a node is inconsistent 2. Waltz filtering or discrete relaxation Initialize the label set for each line segment to {+,-,>,<}

At each iteration, remove inconsistent labels as follows If L is a label for edge Pi and there is another edge Pj connected to Pi that has no label consistent with L, then remove L from the label set of Pi. 10 Problems with this Approach Research on how to do these labelings was confined to perfect blocks world images There was no way to extend it to real images with missing segments, broken segments, nonconnected junctions, etc. It led some groups down the wrong path for a while. 11 3D Shape from X

12 shading silhouette texture mainly research stereo light striping motion used in practice Perspective Imaging Model: 1D real image point E xi f

O This is the axis of the real image plane. camera lens D zc image of point B in front image xf xc B 3D object point 13

O is the center of projection. This is the axis of the front image plane, which we use. xi = xc f zc Perspective in 2D Yc (Simplified) P=(xi,yi,f) 3D object point P=(xc,yc,zc) =(xw,yw,zw) yc

ray yi F f xi optical axis zw=zc Zc xc

Here camera coordinates equal world coordinates. 14 xi xc = f zc yi = yc f zc xi = (f/zc)xc yi = (f/zc)yc camera Xc

3D from Stereo left image 3D point right image disparity: the difference in image location of the same 3D point when projected under perspective to two different cameras. d = xleft - xright 15 Depth Perception from Stereo Simple Model: Parallel Optic Axes image plane Z

f camera L baseline camera R z xl b f xr x-b X z f 16

= x xl z f P=(x,z) x-b = xr z f =

y = yl y yr y-axis is perpendicular to the page. Resultant Depth Calculation For stereo cameras with parallel optical axes, focal length f, baseline b, corresponding image points (xl,yl) and (xr,yr) with disparity d: z = f*b / (xl - xr) = f*b/d x = xl*z/f or b + xr*z/f y = yl*z/f 17

or yr*z/f This method of determining depth from disparity is called triangulation. Finding Correspondences If the correspondence is correct, triangulation works VERY well. But correspondence finding is not perfectly solved. for the general stereo problem. For some very specific applications, it can be solved for those specific kind of images, e.g. windshield of a car. 18

2 Main Matching Methods 1. Cross correlation using small windows. dense 2. Symbolic feature matching, usually using segments/corners. sparse 19 Epipolar Geometry Constraint: 1. Normal Pair of Images The epipolar plane cuts through the image plane(s) forming 2 epipolar lines. z1

y1 z2 y2 P1 C1 b P2 epipolar plane x C2

The match for P1 (or P2) in the other image, must lie on the same epipolar line. 20 P Epipolar Geometry: General Case P y1 y2 P1 e2 e1 C1

P2 x2 x1 C2 21 1. Epipolar Constraint: Matching points lie on corresponding epipolar lines. Constraints P 2. Ordering Constraint:

Usually in the same order across the lines. Q e2 e1 C1 C2 22 Structured Light light stripe 3D data can also be derived using a single camera a light source that can produce stripe(s) on the 3D object

light source 23 camera Structured Light 3D Computation 3D data can also be derived using 3D point (x, y, z) a single camera a light source that can produce stripe(s) on the 3D object b

[x y z] = --------------- [x y f] f cot - x image 3D 24 light source b (0,0,0) f (x,y,f) x axis Depth from Multiple Light Stripes What are these objects?

25 Our (former) System 4-camera light-striping stereo cameras projector rotation table 3D object 26

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