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A Method for enforcing integrability in shape from shading algorithms
 IEEE Transactions on Pattern Analysis and Machine Intelligence
, 1988
"... AbstractSeveral recently developed techniques for reconstructing surface shape from shading information estimate surface slopes without ensuring that they are integrable. This paper presents an approach for enforcing integrability, a particular implementation of the approach, an example of its ap ..."
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Cited by 209 (5 self)
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AbstractSeveral recently developed techniques for reconstructing surface shape from shading information estimate surface slopes without ensuring that they are integrable. This paper presents an approach for enforcing integrability, a particular implementation of the approach, an example of its application to extending an existing shapefromshading algorithm, and experimental results showing the improvement that results from enforcing integrability. A possibly nonintegrable estimate of surface slopes is represented by a finite set of basis functions, and integrability is enforced by calculating the orthogonal projection onto a vector subspace spanning the set of integrable slopes. This projection maps closed convex sets into closed convex sets and, hence, is attractive as a constraint in iterative algorithms. The same technique is also useful for noniterative algorithms since it provides a leastsquares fit of integrable slopes to nonintegrable slopes in one pass of the algorithm. The special case of Fou
Height and gradient from shading
 International Journal of Computer Vision
, 1990
"... Abstract: The method described here for recovering the shape of a surface from a shaded image can deal with complex, wrinkled surfaces. Integrability can be enforced easily because both surface height and gradient are represented (A gradient field is integrable if it is the gradient of some surface ..."
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Cited by 107 (1 self)
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Abstract: The method described here for recovering the shape of a surface from a shaded image can deal with complex, wrinkled surfaces. Integrability can be enforced easily because both surface height and gradient are represented (A gradient field is integrable if it is the gradient of some surface height function). The robustness of the method stems in part from linearization of the reflectance map about the current estimate of the surface orientation at each picture cell (The reflectance map gives the dependence of scene radiance on surface orientation). The new scheme can find an exact solution of a given shapefromshading problem even though a regularizing term is included. The reason is that the penalty term is needed only to stabilize the iterative scheme when it is far from the correct solution; it can be turned off as the solution is approached. This is a reflection of the fact that shapefromshading problems are not illposed when boundary conditions are available, or when the image contains singular points. This paper includes a review of previous work on shape from shading and photoclinometry. Novel features of the new scheme are introduced one at a time to make it easier to see what each contributes. Included is a discussion of implementation details that are important if exact algebraic solutions of synthetic shapefromshading problems are to be obtained. The hope is that better performance on synthetic data will lead to better performance on real data.
3D Pose Estimation and Segmentation using Specular Cues
"... We present a system for fast modelbased segmentation and 3D pose estimation of specular objects using appearance based specular features. We use observed (a) specular reflection and (b) specular flow as cues, which are matched against similar cues generated from a CAD model of the object in various ..."
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Cited by 9 (0 self)
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We present a system for fast modelbased segmentation and 3D pose estimation of specular objects using appearance based specular features. We use observed (a) specular reflection and (b) specular flow as cues, which are matched against similar cues generated from a CAD model of the object in various poses. We avoid estimating 3D geometry or depths, which is difficult and unreliable for specular scenes. In the first method, the environment map of the scene is utilized to generate a database containing synthesized specular reflections of the object for densely sampled 3D poses. This database is compared with captured images of the scene at run time to locate and estimate the 3D pose of the object. In the second method, specular flows are generated for dense 3D poses as illumination invariant features and are matched to the specular flow of the scene. We incorporate several practical heuristics such as use of saturated/highlight pixels for fast matching and normal selection to minimize the effects of interreflections and cluttered backgrounds. Despite its simplicity, our approach is effective in scenes with multiple specular objects, partial occlusions, interreflections, cluttered backgrounds and changes in ambient illumination. Experimental results demonstrate the effectiveness of our method for various synthetic and real objects.
Shape from Contour
 MIT, Artificial Intelligence Laboratory, also: AIMTR589
, 1980
"... The problem of using image contours to infer tile shapes and orientations of sur?aces is treated as a problem of statistical estimation. 'l'be basis for soMng this problem lies in an understanding of the geometry of contour fi)rmation, couplect with simple statistical models of the contour ..."
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Cited by 7 (0 self)
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The problem of using image contours to infer tile shapes and orientations of sur?aces is treated as a problem of statistical estimation. 'l'be basis for soMng this problem lies in an understanding of the geometry of contour fi)rmation, couplect with simple statistical models of the contour generating [,rocess. This approach is first applied to the special case of stu'faces known to be planar. The distortion of contour shape imposed by projection is treated as a signal to be estimated, and variations of nonprojcctive origin are treated as noise. The resulting method is then extended to the estimation of curved surfaces, and applied successfidly to natural images. Next, the geometric tr.,atmcnt is furtber extended by relating contour curvature to surface curvature, using cast shadows as a model for contour generation. This geometric relation, combined with a statistical model, prox ides a measure of goodnessoffit between a surface and an image contour. The goodnesoffit measure is applied to the problem of establishing registration between an image and a suffice nodel. Finally, the statistical estimation strategy is experimentally compared to human perception of orienbation: human observers' judgements of tilt correspond closely to the estimates produced by file planar strategy.
Surface Registration with a Helmholtz Reciprocity Image Pair
 In IEEE Workshop on Color and Photometric Methods in Computer Vision
, 2003
"... This paper introduces a novel surface model registration technique based on the Helmholtz reciprocity principle. Initially, a 3D surface model is generated for an object of interest. Two images of the object are then acquired under controlled lighting conditions. Given one of the images and an estim ..."
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Cited by 2 (0 self)
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This paper introduces a novel surface model registration technique based on the Helmholtz reciprocity principle. Initially, a 3D surface model is generated for an object of interest. Two images of the object are then acquired under controlled lighting conditions. Given one of the images and an estimate of the surface pose, Helmholtz reciprocity is used to predict the appearance of the object surface as seen in the other image. This prediction is as accurate as one produced by a complete modeling of the BRDF of the surface, without requiring the BRDF to be explicitly measured. The position and orientation of the model are updated in order to minimize an appropriate metric of the dissimilarity between the predicted image and the observed second image. Experimental results are demonstrated for objects possessing a variety of surface properties. 1.
Video georegistration: Algorithm and quantitative evaluation
"... An algorithm is presented for video georegistration, with a particular concern for aerial video, i.e., video captured from an airborne platform. The algorithm's input is a video stream with telemetry (camera model specification sufficient to define an initial estimate of the view) and geodetica ..."
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An algorithm is presented for video georegistration, with a particular concern for aerial video, i.e., video captured from an airborne platform. The algorithm's input is a video stream with telemetry (camera model specification sufficient to define an initial estimate of the view) and geodetically calibrated reference imagery (coaligned digital orthoimage and elevation map). The output is a spatial registration of the video to the reference so that it inherits the available geodetic coordinates. The video is processed in a continuous fashion to yield a corresponding stream of georegistered results. Quantitative results of evaluating the developed approach with real world aerial video also are presented. The results suggest that the developed approach may provide valuable input to the analysis and interpretation of aerial video.
THE EFFECT OF DATA PRECISION JAMES R CARTER ON THE CALCULATION OF SLOPE Illinois State University/Normal, Illinois AND ASPECT USING GRIDDED United States
"... Abstract Plots of the frequency of occurrences of aspect angles calculated from gridded DEMs showed inordinately large spikes at the cardinal directions. Because such frequencies are not natural, the techniques of calculating slope and aspect were examined. Ultimately, the cause of the spikes was fo ..."
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Abstract Plots of the frequency of occurrences of aspect angles calculated from gridded DEMs showed inordinately large spikes at the cardinal directions. Because such frequencies are not natural, the techniques of calculating slope and aspect were examined. Ultimately, the cause of the spikes was found to be the fact that the elevation values are reported only to the near est meter. Calculations of slope and aspect angles using round ed elevation data from simulated surfaces of known slope and aspect demonstrate the nature of the problem. These simula tions show that the problem of defining aspects is greatest on the most gentle slopes, as well as smaller grid spacings. Consid eration was also given to various techniques of selecting ele vations for deriving the parameters in the slope and aspect equations. At lower slopes, calculated aspects may be off by tens of degrees. Because there is no way to make the data more precise, researchers should exercise caution in making interpre tations of aspect angles. Calculations of slope angles are affect ed by data precision but not as dramatically as in the calcula tions of aspect angles.
Discovering Underlying Similarities in Video
"... The concept of interrogating similarities within a data set has a long history in fields ranging from medicinal chemistry to image analysis. We define a descriptor as an entropic measure of similarity for an image and the neighborhood of images surrounding it. Differential changes in descriptor valu ..."
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The concept of interrogating similarities within a data set has a long history in fields ranging from medicinal chemistry to image analysis. We define a descriptor as an entropic measure of similarity for an image and the neighborhood of images surrounding it. Differential changes in descriptor values imply differential changes in the structure underlying the images. For example, at the location of a zero crossing in the descriptor values, the corresponding image is a watershed image clearly sitting between two dissimilar groups of images. This paper describes a fast algorithm for image sequence clustering based on the above concept. Developed initially for an adaptive system for capture, analysis and storage of lengthy dynamic visual processes, the algorithm uncovers underlying spatiotemporal structures without a priori information or segmentation. 9 The algorithm estimates the average amount of information each image, in an ordered set, conveys about the structure underlying the ordered set. Such signatures enable capture of relevant and salient time periods directly leading to reductions in cost of followup analysis and storage. As a part of the video capture system, the above characterization may provide predictive feedback to an adaptive capture subsystem controlling temporal sampling, framerate and exposure. Details of the algorithm, examples of its application to quantification of biological motion, and video identification and recognition are presented. Prior to the workshop, an efficient implementation will be posted as a web service to generate characterization of unknown videos online.