Summary: We have generalized the F¨orstner-Harris detector [7,9,15] to select key-points that provide repeatable scale and orientation, as well as repeatable position, over re-detections, even in the face of simple local illumination changes. Keypoints are se-lected to maximize a minimum-eigenvalue-basedlocal stability criterion obtained from a second order analysis of patch self-matching precision under affine image deformations, compensated for linear illumination changes.
Future work: The approach given here ensures accurate re-localizability (by inter-image template matching) of keypoint inter-image patches under various transformations, but it does not always provide accurate ‘centres’ for them. To improve this, we would like to characterize the stability and localization accuracy of the local maxima of the saliency measure (minimum eigenvalue) under the given transformations. In other words, just as we derived the local transformational-stability matrixCred(x)formatchingfrom the scalar matching metricQ(x), we need to derive a local transformational-stability matrix forsaliencyfrom the scalar saliency metric. Only here, the saliency measure is already based on matching stability, so a second level of analysis will be needed.
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Bill Triggs
CNRS-INRIA, 655 avenue de l’Europe, 38330 Montbonnot, France.
Bill.Triggs@inrialpes.fr http://www.inrialpes.fr/movi/people/Triggs
Abstract
We introduce ‘Joint Feature Distributions’, a general statistical framework for feature based multi-image matching that explic-itly models the joint probability distributions of corresponding features across several images. Conditioning on feature posi-tions in some of the images gives well-localized distribuposi-tions for their correspondents in the others, and hence tight likelihood re-gions for correspondence search. We apply the framework in the simplest case of Gaussian-like distributions over the direct sum (affine images) and tensor product (projective images) of the image coordinates. This produces probabilistic correspondence models that generalize the geometric multi-image matching con-straints, roughly speaking by a form of model-averaging over them. These very simple methods predict accurate correspon-dence likelihood regions for any scene geometry including pla-nar and near-plapla-nar scenes, without ill-conditioning or explicit model selection. Small amounts of distortion and non-rigidity are also tolerated. We develop the theory for any number of affine or projective images, explain its relationship to matching tensors, and give results for an initial implementation.
Keywords: Joint Feature Distributions, matching constraints, multi-image geometry, feature correspondence, statistical mod-elling.
1 Introduction
This paper introduces a natural statistical framework for multi-image feature correspondence,Joint Feature Dis-tributions (JFD’s), and uses them to “probabilize” the entire range of affine and projective geometric matching constraints [12,15,2,3,5,7,21,20]. JFD’s are simply joint probability distributions over the positions of correspond-ing features inm > 1different images, used as a sum-mary of some population of interesting correspondences (all valid ones, those near a particular surface or object, background ones...). Conditioning on some of the features gives tight probabilistic correspondence search regions for the remaining ones. Although we will choose parametric forms that reproduce and generalize the standard match-ing constraints, JFD’s are in essence descriptive statistical models rather than normative geometric ones: they aim to summarize the observed behaviour of the given train-ing correspondences, not to rigidly constrain them to an
Extended version of a paper appearing in the 2001 IEEE Int. Conf. Com-puter Vision. c2001 IEEE Computer Society Press. This work was supported by European Union FET project VIBES.
ideal predefined geometry. We believe that JFD’s will become the standard method for many correspondence search problems. Their benefits over matching constraints include: more precise search focusing; built-in handling of noise and distortion (small non-rigidities, lens distor-tion...); and globally stable estimation, even for geome-tries that are degenerate for classical matching constraints.
The projective two image model is perhaps the most use-ful. It generalizes the epipolar constraint, but instead of searching along the full length of epipolar lines, it searches ellipses (Gaussians) whose centre, axis, length and width are determined by the point being matched, its epipolar line, the range of disparities seen in the training data, and the noise level. JFD’s stably and accurately adapt to any scene geometry from deep 3D through to copla-nar: as the depth range of the training data decreases, the search ellipses progressively shorten until ultimately the model becomes essentially homographic. There is no ill-conditioning for near-coplanar scenes, no need to choose between epipolar and homographic correspondence mod-els, and no under- or over-estimation of the plausible cor-respondence regions. In contrast, epipolar models typi-cally search entire (and perhaps inaccurate) epipolar lines, wasting effort and greatly increasing the probability of false correspondences.
The idea of using JFD’s for image correspondence is very natural and obvious in retrospect. As far as we know it has not appeared before, but there are many related threads in the literature. JFD’s react against explicit model selection for matching constraints [9,10,17] by incorpo-rating the well-known statistical rule that you can only predict events similar to the ones you trained on — extrap-olating a full epipolar geometry from near-coplanar data is unstable, but irrelevant for predicting near-coplanar cor-respondences,c.f.e.g. [13]. JFD’s have analogies with Bayesian model averaging [18] but are much simpler and more direct. Plane+parallax [11,14,8,23,1,22] offers sta-bler geometric parametrizations than matching tensors for near-planar scenes. These could no doubt be “probabi-lized” in much the same way as we do here.
Another aspect of this work is a new theoretical frame-work for studying multi-image geometry and especially matching constraints, based on the notion of thetensor joint image. We will use some isolated results from this, 159
x2 exact projections of 3D features,
noise gives JFD cross−section population
projection distributions
matching geometry gives ‘‘backbone’’
Figure 1: The basic principle of JFD feature correspon-dence.
but the full development had to be omitted for lack of space.
§2 sketches the general principles of JFD matching,§3 develops some tools, then we focus on Gaussian-like JFD models for affine (§4) and perspective (§5) camera geome-tries. §6 briefly discusses the implementation and some preliminary experiments and§7 concludes.
Notation: We assume familiarity with affine and pro-jective matching constraints at the level of,e.g. [6], and the ability to think tensorially at need [21,20]. Slanted fonts denote inhomogeneousx,yand homogeneousx,y image vectors, upright fonts inhomogeneousx,yand ho-mogeneousx,y3D ones.Pdenotes3×4image projection matrices, p()probability distributions, [·]× 3×3cross product matrices ([x]×y=x∧y),h−ixexpectation over the distribution ofx.