运动图像序列中遮挡现象的频谱分析

本文在一维图像信号遮挡现象分析的基础上，通过对信号频率分量复杂程度的分析，指出当图像信号由较多复杂频率所构成的情况下（如自然图像序列），遮挡现象的频谱中失真项因为不同频率分量能量的叠加而不能用采区分遮挡运动速度和被遮挡运动速度。     

The frequency structure of one-dimensional occluding image signals

We present a theoretical investigation of the frequency structure of 1D occluding image signals. We show that image signal occlusion contains relevant information which is most easily extractable from its representation in the frequency domain. For instance, the occluding and occluded signal velocities may be identified as such and translucency phenomena may be understood in the terms of this theoretical investigation. In addition, it is found that the structure of occluding 1D signals is invariant under constant and linear models of signal velocity. This theoretical framework can be used to describe the exact frequency structure of non-Fourier motion and bridges the gap between such visual phenomena and their understanding in the frequency domain     

Shape from regular patterns

This paper describes an algorithm to obtain local surface orientation from the apparent surface-pattern distortion in an image.We propose a spherical projection to model perspective imaging. A mapping is defined based on the measurement of the local distortions of a repeated known texture pattern due to the image projection. This mapping maps an apparent shape on the image sphere to a locus of possible surface orientations on the Gaussian sphere.An iterative constraint propagation algorithm with the orientations at occluding boundaries reduces possible surface orientations to a unique orientation. This algorithm can recover local surface orientation as well as interpolate surface orientations where no information is available. This algorithm is applied to a real image to demonstrate its performance.     

Energy-optimal trajectory planning for car-like robots

When a battery-powered robot needs to operate for a long period of time, optimizing its energy consumption becomes critical. Driving motors are a major source of power consumption for mobile robots. In this paper, we study the problem of finding optimal paths and velocity profiles for car-like robots so as to minimize the energy consumed during motion. We start with an established model for energy consumption of DC motors. We first study the problem of finding the energy optimal velocity profiles, given a path for the robot. We present closed form solutions for the unconstrained case and for the case where there is a bound on maximum velocity. We then study a general problem of finding an energy optimal path along with a velocity profile, given a starting and goal position and orientation for the robot. Along the path, the instantaneous velocity of the robot may be bounded as a function of its turning radius, which in turn affects the energy consumption. Unlike minimum length paths, minimum energy paths may contain circular segments of varying radii. We show how to efficiently construct a graph which generalizes Dubins’ paths by including segments with arbitrary radii. Our algorithm uses the closed-form solution for the optimal velocity profiles as a subroutine to find the minimum energy trajectories, up to a fine discretization. We investigate the structure of energy-optimal paths and highlight instances where these paths deviate from the minimum length Dubins’ curves. In addition, we present a calibration method to find energy model parameters. Finally, we present results from experiments conducted on a custom-built robot for following optimal velocity profiles.     

LFM信号限幅失真对侧扫声纳声图的影响

本文分情况讨论了LFM信号限幅失真对脉冲压缩及侧扫声图的影响并进行了湖试验证。结果表明:限幅失真对于脉压波形的主瓣几乎没有影响,即不会影响到声图的分辨率,但是会使脉压波形的幅度产生失真。当限幅程度较大时,由于信号的叠加会进一步恶化脉压波形,使脉压信号的幅度明显降低,甚至产生假目标,信号的动态变化范围被严重压缩;即使当单个信号不限幅时,而由于多目标回波相互叠加也可能会引起信号限幅,此时也可能对处理结果造成较为严重的失真,使信号动态范围被压缩。     

Camera calibration: a quick and easy way to determine the scalefactor

The author presents a quick and easy solution to the camera calibration problem of finding the scale factor that arises because of a difference between the scanning frequency of the camera sensor plane and the scanning frequency of the image capture board frame buffer. The method presented is based on the observation that the scale factor is related to the distortion in an image of a circle: this is a distortion that can easily be measured by considering the occluding boundary of a precisely fabricated sphere, such as a ball bearing. Implementation results are presented     

A generative sketch model for human hair analysis and synthesis

In this paper, we present a generative sketch model for human hair analysis and synthesis. We treat hair images as 2D piecewise smooth vector (flow) fields and, thus, our representation is view-based in contrast to the physically-based 3D hair models in graphics. The generative model has three levels. The bottom level is the high-frequency band of the hair image. The middle level is a piecewise smooth vector field for the hair orientation, gradient strength, and growth directions. The top level is an attribute sketch graph for representing the discontinuities in the vector field. A sketch graph typically has a number of sketch curves which are divided into 11 types of directed primitives. Each primitive is a small window (say 5 /spl times/ 7 pixels) where the orientations and growth directions are defined in parametric forms, for example, hair boundaries, occluding lines between hair strands, dividing lines on top of the hair, etc. In addition to the three level representation, we model the shading effects, i.e., the low-frequency band of the hair image, by a linear superposition of some Gaussian image bases and we encode the hair color by a color map. The inference algorithm is divided into two stages: 1) We compute the undirected orientation field and sketch graph from an input image and 2) we compute the hair growth direction for the sketch curves and the orientation field using a Swendsen-Wang cut algorithm. Both steps maximize a joint Bayesian posterior probability. The generative model provides a straightforward way for synthesizing realistic hair images and stylistic drawings (rendering) from a sketch graph and a few Gaussian bases. The latter can be either inferred from a real hair image or input (edited) manually using a simple sketching interface. We test our algorithm on a large data set of hair images with diverse hair styles. Analysis, synthesis, and rendering results are reported in the experiments.     

On the Fourier Properties of Discontinuous Motion

Retinal image motion and optical flow as its approximation are fundamental concepts in the field of vision, perceptual and computational. However, the computation of optical flow remains a challenging problem as image motion includes discontinuities and multiple values mostly due to scene geometry, surface translucency and various photometric effects such as reflectance. In this contribution, we analyze image motion in the frequency space with respect to motion discontinuities and translucence. We derive the frequency structure of motion discontinuities due to occlusion and we demonstrate its various geometrical properties. The aperture problem is investigated and we show that the information content of an occlusion almost always disambiguates the velocity of an occluding signal suffering from the aperture problem. In addition, the theoretical framework can describe the exact frequency structure of Non-Fourier motion and bridges the gap between Non-Fourier visual phenomena and their understanding in the frequency domain.     

A Shape‐Preserving Approach to Image Resizing

We present a novel image resizing method which attempts to ensure that important local regions undergo a geometric similarity transformation, and at the same time, to preserve image edge structure. To accomplish this, we define handles to describe both local regions and image edges, and assign a weight for each handle based on an importance map for the source image. Inspired by conformal energy, which is widely used in geometry processing, we construct a novel quadratic distortion energy to measure the shape distortion for each handle. The resizing result is obtained by minimizing the weighted sum of the quadratic distortion energies of all handles. Compared to previous methods, our method allows distortion to be diffused better in all directions, and important image edges are well‐preserved. The method is efficient, and offers a closed form solution.     

Analysis of photometric properties of occluding edges by thereversed projection blurring model

This paper analyzes photometric properties of occluding edges and proves that an object surface behind a nearer object is partially observable beyond the occluding edges. We first discuss a limitation of the image blurring model using the convolution, and then present an optical flux based blurring model named the reversed projection blurring (RPB) model. Unlike the multicomponent blurring model proposed by Nguyen et al., the RPB model enables us to explore the optical phenomena caused by a shift-variant point spread function that appears at a depth discontinuity. Using the RPB model, theoretical analysis of occluding edge properties are given and two characteristic phenomena are shown: (1) a blurred occluding edge produces the same brightness profiles as would be predicted for a surface edge on the occluding object when the occluded surface radiance is uniform and (2) a nonmonotonic brightness transition would be observed in blurred occluding edge profiles when the occluded object has a surface edge. Experimental results using real images have demonstrated the validity of the RPB model as well as the observability of the characteristic phenomena of blurred occluding edges