From: Xiaowei Li <nemesis@bi...>  20041014 01:52:53

Hi,Everyone=A3=A1 =09I'm a new comer to VXL, and I wanna use it in my multiple view= geometry applications, since VXL seems having more expansibility= and easier implementation of many prevailing algorithms. I write= this mail to see whether you can give some ideas on my= experiments. =09One key issue in my experiment is to track a planar target in= image sequences. The plane would be identified at the first= frame, by drawing on that image a region inside which is rightly= the main portion of the plane. After this, the tracking is= absolutely automatic. Homography(H) between consecutive images= would be computed with RANSAC. According to the Multiple View= Geometry theory and the RANSAC method, this H's computation= would result in a robust estimation of H and a set of 2D point= correspondences as inliers, and of course these corresondences= are on the plane. "Extending" the region a bit more in the= previous image,making correspondences from the extended region= of the previous image to the consecutive image, and now using= the computed H to test these new correspondences, we could= verify which are the ones on the same plane, and furtherly a= rough boundary of that plane is obtained in the latter image.= Doing this in every frame, thus the plane is tracked robustly. =09That's the method. I think when we assume that no too large= occlusions enter that plane, this tracking would be sufficiently= robust. Do you think so? =09But I meet some problems in the implementation, because of my= unfamiliarness to the VXL environment. =09The first thing is to identify the plane by drawing a geometric= region in the 1st frame. The common geometric objects which= could represent the ROI in VXL are always simple rectangles= ,circles, or ellipses. How to represent more general objects in= VXL, such as regions needed in our experiment with free curves= as boundary, or polygons ? =09The second thing is to make correspondences. The first step to= use RANSAC is to get putative correspondences based on proximaty= and similiarity of their intensity neighbourhood. When we've= already got two sets of Harris points, one set for each image of= a image pair, how to match them ? Does there exist an= offtheshelf class to achieve this in VXL ? =09Next thing is the robust estimation. I found that in the Robust= Estimation Library, the minimization of some objective functions= is also based on RANSAC sampling, not based on traditional= derivative analysis, Isn't it? Can this offer the same accuracy= ? =09The final thing is about the XCV. I think this is very useful= for me. I don't know whether anyone is still working on its= updation. If you do this work, can you mail me a newly version= of XCV, with source code,please ? =09Thank you very very much for your help ! =09=09 Best wishes, Shawrie Lee(Xiaowei Li) AR Lab, BIT, P.R. China nemesis@... TEL:+86 10 6891 2565 20041013 
From: Amitha Perera <perera@cs...>  20041014 17:34:02

On Wed 13 Oct 2004, Xiaowei Li wrote: > Next thing is the robust estimation. I found that in the Robust > Estimation Library, the minimization of some objective functions is > also based on RANSAC sampling, not based on traditional derivative > analysis, Isn't it? Can this offer the same accuracy ? Some of the objective functions, such as RANSAC and MUSE, can only be minimised using random sampling. Others, such as the BeatonTukey biweight, can be minimised using IRLS or by random sampling. rrel supports all of these. Simply choose a estimation problem (e.g. rrel_homography2D_est), an objective function (e.g. rrel_ransac_obj), and a compatible minimization technique (e.g. rrel_ransam_search). Mix, and get the answer. rrel/examples/homography2d_fit.cxx has examples of estimating a homography using different objective functions. Note that while RANSAC is commonly used, and appears effective for homography estimation, other objective functions have better statistical properties. You may want to experiment with those. rrel_msac_obj and rrel_tukey_obj are good starting points. > The final thing is about the XCV. I think this is very useful for > me. I don't know whether anyone is still working on its updation. If > you do this work, can you mail me a newly version of XCV, with > source code,please ? Noone is actively working on xcv, unfortunately. The source code you have is probably what everyone has. Amitha. 