René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"homography() results depend on order of points","articleBody":"Below is my post from the Google groups page with details on this issue.  It seems that the results of the homography() function depend on the order of the X and Y features passed in.  This should not be the case.\n\n-Eric\n\nI'm trying to use harris() and homography() to do image registration.  I start with an array called imgBase which is a 150x150 array of floats on the range [0,255].  I can plot this in ArrayFire and it looks correct (assuming I normalize it to the range [0,1]).  The image is a portion of a satellite image (part of Matlab's westconcordaerial.png) and has some nice corner features from buildings, roads, etc.\n\nTo try to understand what is going on, I've done the following:\n\naf::features featuresBase = af::harris(imgBase);\naf::features featuresReg = af::harris(imgBase);\naf::homography(H_ctr, inliers, featuresReg.getX(), featuresReg.getY(), featuresBase.getX(), featuresBase.getY(), AF_HOMOGRAPHY_RANSAC, 5.0f, 1000, f32);\n\nThe featuresBase and featuresReg objects each contain 434 features.  These features are identical except that they are ordered differently.  I do not know why they would be ordered differently as they are passed the same image.  I've checked this property of the features by exporting them to Matlab.  When plotted on a 2D plot they overlay exactly.  However, element-wise differences in X or Y coordinates are not identically zero and I can visually inspect that the vectors are not equal.\n\nThe homography() call then reports only 10 inliers.  It returns the homography\n\n```\n0.1973    0.0222    0.0010\n```\n\n   -0.0693   -0.0947    0.0009\n  -20.4484   -5.5688   -0.3293\n\nwhich makes little sense.  Does the ordering of the X, Y points affect the RANSAC result?  It seems like it should not.\n\nIf I change the homography() function call to:\n\naf::homography(H_ctr, inliers, featuresBase.getX(), featuresBase.getY(), featuresBase.getX(), featuresBase.getY(), AF_HOMOGRAPHY_RANSAC, 5.0f, 1000, f32);\n\nthe function behaves as expected.  It returns 434 inliers and a homography array of\n\n```\n0.1970   -0.0000   -0.0000\n0.0000    0.1970    0.0000\n```\n\n   -0.0000   -0.0000    0.1970\n\nI expected this to be the 3x3 identity matrix, but I assume I should normalize by the lower-right-most element.\n","author":{"url":"https://github.com/shie0041","@type":"Person","name":"shie0041"},"datePublished":"2016-02-04T18:16:45.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/1264/arrayfire/issues/1264"}