# Image Mosaics. Mosaics. How to do it? Aligning images = Goal. Today s Readings Szeliski, Ch 5.1, 8.1. Basic Procedure

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1 Mosaics Image Mosaics = StreetView Today s Readings Szeliski, Ch 5.1, 8.1 Goal Stitch together several images into a seamless composite How to do it? Aligning images Basic Procedure Take a sequence of images from the same position Rotate the camera about its optical center Compute transformation between second image and first Shift the second image to overlap with the first Blend the two together to create a mosaic If there are more images, repeat How to account for warping? Translations are not enough to align the images

2 Alignment Demo Image reprojection mosaic PP The mosaic has a natural interpretation in 3D The images are reprojected onto a common plane The mosaic is formed on this plane Image reprojection Image reprojection Basic question How to relate two images from the same camera center? how to map a pixel from PP1 to PP2 Answer Cast a ray through each pixel in PP1 Draw the pixel where that ray intersects PP2 PP2 PP1 Don t need to know what s in the scene! Observation Rather than thinking of this as a 3D reprojection, think of it as a 2D image warp from one image to another

3 Homographies each image is warped with a homography mosaic PP Homographies Image warping with homographies Perspective projection of a plane Lots of names for this: homography, texture-map, colineation, planar projective map Modeled as a 2D warp using homogeneous coordinates p H p To apply a homography H Compute p = Hp (regular matrix multiply) Convert p from homogeneous to image coordinates divide by w (third) coordinate image plane in front black area where no pixel maps to image plane below

4 Panoramas Spherical projection systems What if you want a 36 field of view? mosaic Projection Sphere CAVE (UI Chicago) Omnimax Spherical projection Map 3D point (X,Y,Z) onto sphere Spherical reprojection How to map sphere onto a flat image? to Y Z X Convert to spherical coordinates Y Z X unit sphere Convert to spherical image coordinates s defines size of the final image» often convenient to set s = camera focal length side view unwrapped sphere Spherical image top-down view

5 Spherical reprojection Spherical reprojection Y Z X How to map sphere onto a flat image? to Use image projection matrix! or use the version of projection that properly accounts for radial distortion, as discussed in projection slides. This is what you ll do for project 2. side view input f = 2 (pixels) f = 4 f = 8 Map image to spherical coordinates need to know the focal length top-down view Aligning spherical images Aligning spherical images Suppose we rotate the camera by about the vertical axis How does this change the spherical image? Suppose we rotate the camera by about the vertical axis How does this change the spherical image? Translation by This means we can align spherical images by translating them

6 Spherical image stitching Computing transformations Given a set of matches between images A and B How can we compute the transform T from A to B? What if you don t know the camera rotation? Solve for the camera rotations Note that a pan (rotation) of the camera is a translation of the sphere! Use feature matching to solve for translations of spherical-warped images Simple case: translations Find transform T that best agrees with the matches But not all matches are good How do we solve for? What do we do about the bad matches?

7 RAndom SAmple Consensus RAndom SAmple Consensus Select one match, count inliers (in this case, only one) Select one match, count inliers (4 inliers) Least squares fit RANSAC Same basic approach works for any transformation Translation, rotation, homographies, etc. Very useful tool Find average translation vector for largest set of inliers General version Randomly choose a set of K correspondences Typically K is the minimum size that lets you fit a model Fit a model (e.g., homography) to those correspondences Count the number of inliers that approximately fit the model Need a threshold on the error Repeat as many times as you can Choose the model that has the largest set of inliers Refine the model by doing a least squares fit using ALL of the inliers

8 Assembling the panorama Problem: Drift Stitch pairs together, blend, then crop Error accumulation small errors accumulate over time Problem: Drift Full-view Panorama (x 1,y 1 ) (x n,y n ) + + copy of first image + + Solution add another copy of first image at the end this gives a constraint: y n = y 1 there are a bunch of ways to solve this problem add displacement of (y 1 y n )/(n -1) to each image after the first compute a global warp: y = y + ax run a big optimization problem, incorporating this constraint» best solution, but more complicated» known as bundle adjustment

9 Different projections are possible Project 2 Take pictures with your phone (or on a tripod) Warp to spherical coordinates Extract features Align neighboring pairs using RANSAC Write out list of neighboring translations Correct for drift Read in warped images and blend them Crop the result and import into a viewer Roughly based on Autostitch By Matthew Brown and David Lowe Image Blending Feathering =

10 Effect of window (ramp-width) size Effect of window size 1 left right Good window size Pyramid blending 1 Optimal window: smooth but not ghosted Doesn t always work... Create a Laplacian pyramid, blend each level Burt, P. J. and Adelson, E. H., A multiresolution spline with applications to image mosaics, ACM Transactions on Graphics, 42(4), October 1983,

11 Poisson Image Editing Alpha Blending I 3 p For more info: Perez et al, SIGGRAPH 23 I 1 I 2 Encoding blend weights: I(x,y) = ( R, G, B, ) color at p = Implement this in two steps: 1. accumulate: add up the ( premultiplied) RGB values at each pixel 2. normalize: divide each pixel s accumulated RGB by its value Q: what if =? Optional: see Blinn (CGA, 1994) for details: =7531&prod=JNL&arnumber=3174&arSt=83&ared=87&arAu hor=blinn%2c+j.f. Image warping Forward warping h(x,y) y y x x f(x,y) g(x,y ) h(x,y) y y x x f(x,y) g(x,y ) Given a coordinate transform (x,y ) = h(x,y) and a source image f(x,y), how do we compute a transformed image g(x,y ) = f(h(x,y))? Send each pixel f(x,y) to its corresponding location (x,y ) = h(x,y) in the second image Q: what if pixel lands between two pixels?

12 Forward warping Inverse warping h(x,y) y y x x f(x,y) g(x,y ) y h -1 (x,y) y x x f(x,y) g(x,y ) Send each pixel f(x,y) to its corresponding location (x,y ) = h(x,y) in the second image Q: what if pixel lands between two pixels? A: distribute color among neighboring pixels (x,y ) Known as splatting Get each pixel g(x,y ) from its corresponding location (x,y) =h -1 (x,y ) in the first image Q: what if pixel comes from between two pixels? Inverse warping Forward vs. inverse warping Q: which is better? y h -1 (x,y) y x x f(x,y) g(x,y ) A: usually inverse eliminates holes however, it requires an invertible warp function not always possible... Get each pixel g(x,y ) from its corresponding location (x,y) =h -1 (x,y ) in the first image Q: what if pixel comes from between two pixels? A: resample color value We discussed resampling techniques before nearest neighbor, bilinear, Gaussian, bicubic

13 Other types of mosaics Can mosaic onto any surface if you know the geometry See NASA s Visible Earth project for some stunning earth mosaics Click for images

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