200,465 research outputs found
A Solution for Multi-Alignment by Transformation Synchronisation
The alignment of a set of objects by means of transformations plays an
important role in computer vision. Whilst the case for only two objects can be
solved globally, when multiple objects are considered usually iterative methods
are used. In practice the iterative methods perform well if the relative
transformations between any pair of objects are free of noise. However, if only
noisy relative transformations are available (e.g. due to missing data or wrong
correspondences) the iterative methods may fail.
Based on the observation that the underlying noise-free transformations can
be retrieved from the null space of a matrix that can directly be obtained from
pairwise alignments, this paper presents a novel method for the synchronisation
of pairwise transformations such that they are transitively consistent.
Simulations demonstrate that for noisy transformations, a large proportion of
missing data and even for wrong correspondence assignments the method delivers
encouraging results.Comment: Accepted for CVPR 2015 (please cite CVPR version
Revisiting Complex Moments For 2D Shape Representation and Image Normalization
When comparing 2D shapes, a key issue is their normalization. Translation and
scale are easily taken care of by removing the mean and normalizing the energy.
However, defining and computing the orientation of a 2D shape is not so simple.
In fact, although for elongated shapes the principal axis can be used to define
one of two possible orientations, there is no such tool for general shapes. As
we show in the paper, previous approaches fail to compute the orientation of
even noiseless observations of simple shapes. We address this problem. In the
paper, we show how to uniquely define the orientation of an arbitrary 2D shape,
in terms of what we call its Principal Moments. We show that a small subset of
these moments suffice to represent the underlying 2D shape and propose a new
method to efficiently compute the shape orientation: Principal Moment Analysis.
Finally, we discuss how this method can further be applied to normalize
grey-level images. Besides the theoretical proof of correctness, we describe
experiments demonstrating robustness to noise and illustrating the method with
real images.Comment: 69 pages, 20 figure
Ball-Scale Based Hierarchical Multi-Object Recognition in 3D Medical Images
This paper investigates, using prior shape models and the concept of ball
scale (b-scale), ways of automatically recognizing objects in 3D images without
performing elaborate searches or optimization. That is, the goal is to place
the model in a single shot close to the right pose (position, orientation, and
scale) in a given image so that the model boundaries fall in the close vicinity
of object boundaries in the image. This is achieved via the following set of
key ideas: (a) A semi-automatic way of constructing a multi-object shape model
assembly. (b) A novel strategy of encoding, via b-scale, the pose relationship
between objects in the training images and their intensity patterns captured in
b-scale images. (c) A hierarchical mechanism of positioning the model, in a
one-shot way, in a given image from a knowledge of the learnt pose relationship
and the b-scale image of the given image to be segmented. The evaluation
results on a set of 20 routine clinical abdominal female and male CT data sets
indicate the following: (1) Incorporating a large number of objects improves
the recognition accuracy dramatically. (2) The recognition algorithm can be
thought as a hierarchical framework such that quick replacement of the model
assembly is defined as coarse recognition and delineation itself is known as
finest recognition. (3) Scale yields useful information about the relationship
between the model assembly and any given image such that the recognition
results in a placement of the model close to the actual pose without doing any
elaborate searches or optimization. (4) Effective object recognition can make
delineation most accurate.Comment: This paper was published and presented in SPIE Medical Imaging 201
Digital design of medical replicas via desktop systems: shape evaluation of colon parts
In this paper, we aim at providing results concerning the application of desktop systems for rapid prototyping of medical replicas
that involve complex shapes, as, for example, folds of a colon. Medical replicas may assist preoperative planning or tutoring in
surgery to better understand the interaction among pathology and organs. Major goals of the paper concern with guiding the
digital design workflow of the replicas and understanding their final performance, according to the requirements asked by the
medics (shape accuracy, capability of seeing both inner and outer details, and support and possible interfacing with other organs).
In particular, after the analysis of these requirements, we apply digital design for colon replicas, adopting two desktop systems. ,e
experimental results confirm that the proposed preprocessing strategy is able to conduct to the manufacturing of colon replicas
divided in self-supporting segments, minimizing the supports during printing. ,is allows also to reach an acceptable level of final
quality, according to the request of having a 3D presurgery overview of the problems. ,ese replicas are compared through reverse
engineering acquisitions made by a structured-light system, to assess the achieved shape and dimensional accuracy. Final results
demonstrate that low-cost desktop systems, coupled with proper strategy of preprocessing, may have shape deviation in the range
of ±1 mm, good for physical manipulations during medical diagnosis and explanation
Galaxy density profiles and shapes -- I. simulation pipeline for lensing by realistic galaxy models
Studies of strong gravitational lensing in current and upcoming wide and deep
photometric surveys, and of stellar kinematics from (integral-field)
spectroscopy at increasing redshifts, promise to provide valuable constraints
on galaxy density profiles and shapes. However, both methods are affected by
various selection and modelling biases, whch we aim to investigate in a
consistent way. In this first paper in a series we develop a flexible but
efficient pipeline to simulate lensing by realistic galaxy models. These galaxy
models have separate stellar and dark matter components, each with a range of
density profiles and shapes representative of early-type, central galaxies
without significant contributions from other nearby galaxies. We use Fourier
methods to calculate the lensing properties of galaxies with arbitrary surface
density distributions, and Monte Carlo methods to compute lensing statistics
such as point-source lensing cross-sections. Incorporating a variety of
magnification bias modes lets us examine different survey limitations in image
resolution and flux. We rigorously test the numerical methods for systematic
errors and sensitivity to basic assumptions. We also determine the minimum
number of viewing angles that must be sampled in order to recover accurate
orientation-averaged lensing quantities. We find that for a range of
non-isothermal stellar and dark matter density profiles typical of elliptical
galaxies, the combined density profile and corresponding lensing properties are
surprisingly close to isothermal around the Einstein radius. The converse
implication is that constraints from strong lensing and/or stellar kinematics,
which are indeed consistent with isothermal models near the Einstein radius,
cannot trivially be extrapolated to smaller and larger radii.Comment: 31 pages, 15 figures; paper II at arXiv:0808.2497; accepted for
publication in MNRAS; PDF file with full resolution figures at
http://www.sns.ias.edu/~glenn/paper1.pd
A flexible geometric model for leaf shape descriptions with high accuracy
Accurate assessment of canopy structure is crucial in studying plant-environment interactions. The advancement of functional-structural plant models (FSPM), which incorporate the 3D structure of individual plants, increases the need for a method for accurate mathematical descriptions of leaf shape. A model was developed as an improvement of an existing leaf shape algorithm to describe a large variety of leaf shapes. Modelling accuracy was evaluated using a spatial segmentation method and shape differences were assessed using principal component analysis (PCA) on the optimised parameters. Furthermore, a method is presented to calculate the mean shape of a dataset, intended for obtaining a representative shape for modelling purposes. The presented model is able to accurately capture a large range of single, entire leaf shapes. PCA illustrated the interpretability of the parameter values and allowed evaluation of shape differences. The model parameters allow straightforward digital reconstruction of leaf shapes for modelling purposes such as FSPMs
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