利用数据场和欧氏距离的图像边缘提取

图像边缘是图像分析和识别的基础,图像边缘信息的准确性和完整性对后续图像分析和识别有重要影响。为实现图像边缘有效提取,提出一种利用数据场和图像欧氏距离的图像边缘提取方法。首先,该方法利用数据场理论构建图像数据场,实现图像灰度值特征空间到数据场势值空间的转换。然后,在对图像数据场的势值计算时引入图像欧氏距离,利用图像区域欧氏距离扩大像素差异,抑制微小细节和噪声,得到"背景"和"目标"相对分离的势值图。最后,用改进Canny算法对势值图进行边缘提取。实验表明,用本文方法可以有效提高边缘提取的准确性,减少伪边缘,抑制冗余细节和噪声。     

密度场图像演化分析

短裂纹活动引起的密度场图像演化分析是应用工业CT研究疲劳损伤中的热点。在仿真环境下,应用亚体素模型和局部离散化完成了短裂纹群建模,实现了非均匀密度场的建模。通过高、低两种不同分辨率CT分别观看细节特征和宏观行为,初步开展了短裂纹活动效应的2维、3维信号分析。引入了图像隶属度,提出了针对裂纹细节变化的模糊增强算法,增强了密度场演化的3维分析信号,有助于监测短裂纹群活动效应。     

基于主色检测与灰度传播的彩色图像灰度化方法

针对已有图像灰度化方法不能同时体现原始图像的全局和局部对比度,且不能保持灰度像素的灰度值等问题,提出一种基于主色检测与灰度传播的彩色图像灰度化方法.首先提出一种基于直方图峰值的主色检测方法,用于提取输入图像中具有代表性的少量主要颜色特征;然后采用基于梯度域的主色灰度化方法将图像主色映射到灰度空间,最大程度地在灰度空间保持主色在彩色空间的视觉对比度;再利用基于局部线性嵌入的灰度传播方法,将主色灰度值和原有灰度像素的灰度值扩散到与其颜色相似的像素上;最后检查图像中是否存在没有被灰度化的彩色像素,如果存在,则使用基于梯度域的局部灰度化方法将其灰度化,得到最终的灰度图像.大量实验结果表明,该方法将全局和局部对比度保持有机结合,得到的灰度图像较好地保持了彩色图像中色彩之间的差异性,并且保持了灰度像素的灰度值.     

结合小波变换的梯度场图像渐进融合算法

基于梯度场的图像融合算法只适用于尺度差异不大于1∶4的多光谱图像与全色图像。针对尺度差异为1∶8的北京一号卫星多光谱图像及高分辨率全色图像的融合问题,提出一种结合小波变换的梯度场图像尺度渐进融合算法。利用小波变换方法将多光谱图像与高分辨率全色图像尺度差异倍数缩小,得到基于小波变换的初级融合,再进行基于梯度场的Poisson图像融合。实验结果表明,渐进融合图像与多光谱图像的平均颜色差异值为23.5,与高分辨率全色图像的平均梯度差异值为2.1,多尺度纹理特征值差异值分别为3.98、10.2、18.9,渐进融合图像与高分辨率全色图像的空间细节和纹理细节吻合程度更好。     

基于自适应粒子群算法和数据场的图像二维阈值分割

针对图像分割中最优阈值选择的问题,将粒子群优化算法和数据场理论相结合,提出一种图像二维阈值分割算法.首先把数据场的理论引入到图像处理中,将图像的灰度值空间映射到数据场的势空间;然后通过自适应的粒子群优化算法寻找数据场中最大势值,该势值对应最优阈值;最后根据找到的阈值进行图像分割.在进行空间映射的过程中,将二维直方图中的序偶?p,q?视作数据对象,其中p代表像素的灰度值,q代表邻域的灰度值,选用拟核力场高斯势函数计算各数据对象之间的相互作用,生成了二维直方图的三维数据场.文中亦对数据场的各个参数进行了详尽的探讨.实验结果表明,文中算法不仅合理、有效,而且大大降低了计算的复杂性,能够适应大多数图像的分割.     

基于蚁群优化的图像边缘检测算法

为提高图像边缘检测的精度与抗噪性能,提出一种基于蚁群优化的图像边缘检测算法.将图像像素梯度值和像素圆形邻域统计均值的相对差共同作为蚁群的启发信息,引导蚁群搜索图像边缘.实验结果表明,该算法能最大限度地保留边缘细节,并能抑制噪声和纹理,具有较好的鲁棒性.     

利用云模型和数据场的图像分割方法

针对图像自动分割中的最优阈值选择问题,提出一种基于云模型和数据场的图像分割方法。 该方法引入数据场实现图像灰度值特征空间到数据场势值空间的非线性映射,设定两个不同的质量函数分别形成相对数据场和绝对数据场。利用两类数据场的特点,结合全局和局部统计特征获得自适应势阈值对图像像素进行划分,产生图像潜在的背景或目标像素集合。进一步由逆向云发生器算法产生图像背景和目标的云模型表示,根据图像像素隶属于背景、目标云模型的程度,采用极大判定法则得到最终的分割结果。 实验结果表明,该方法的分割效果较好、性能稳定,具有合理性和有效性。     

一种融入局部信息的直觉模糊C-均值聚类图像分割算法

模糊C-均值(FCM)算法对图像噪声敏感,聚类过程中只考虑图像的数值特征信息而忽略像素间空间约束关系,同时单一隶属度并不能充分描述图像的不确定性,这使得基于FCM的图像分割不够准确.融入局部信息的改进FCM算法虽然对图像噪声有一定鲁棒性,但对图像细节保持不够,难以分割微小区域.针对上述问题,提出一种基于直觉模糊集的改进模糊C-均值(IFS_FCM)图像分割算法.该方法将直觉模糊集理论融入到FCM中,充分考虑图像的不确定性,同时在目标函数中引入空间邻域信息,使得该分割算法对噪声鲁棒性增强的同时还能保持图像细节信息.实验结果表明,IFS_FCM能获得更加理想的图像分割效果.     

虚平面映射:深度图像内部视点的绘制技术

首先推导与归纳了图像三维变换中像素深度场的变换规律,同时提出了基于深度场和极线原则的像素可见性别方法,根据上述理论和方法,提出一种基于深度图像的建模与绘制(image-based modeling and rendering,简称IBMR)技术,称为虚平面映射.该技术可以基于图像空间内任意视点对场景进行绘制.绘制时,先在场景中根据视线建立若干虚拟平面,将源深度图像中的像素转换到虚平面上,然后通过对虚平面上像素的中间变换,将虚平面转换成平面纹理,再利用虚平面的相互拼接,将视点的成像以平面纹理映射的方式完成.新方法还能在深度图像内侧,基于当前视点快速获得该视点的全景图,从而实现视点的实时漫游.新方法视点运动空间大、存储需求小,且可以发挥图形硬件的纹理映射功能,并能表现物体表面的三维凹凸细节和成像视差效果,克服了此前类似算法的局限和不足.     

色调保持的自适应图像视频细节增强算法

针对已有方法难以保持输入图像的色调分布且过于经验化的问题,文中提出了一种色调保持的自适应图像与视频细节增强方法.首先,采用保持边缘的多尺度图像分解算法,快速地将输入帧分解成含有大尺度边缘信息的基图像和含有小尺度细节信息的细节层.然后,在细节增强与合成过程中,根据边缘梯度响应构造一个自适应的细节增强函数,减少对强边缘的放大倍数,提升对小细节的增强幅度.最后,在保持输入帧的颜色值和增强图像的梯度值的期望下,基于能量优化的色调校正算法避免细节增强图像与输入帧之间的显著色调差异.实验结果表明:对任意的输入图像和视频,通过统一的参数设置,均可获得色调保持的良好细节增强效果.     

Image-based modeling of inhomogeneous single-scattering participating media

We propose a method to reconstruct inhomogeneous single-scattering participating media, which could preserve fine high-frequency details of density field.Volumetric data and the ratio of absorption coefficient to scattering coefficient are used to describe the spatial-varying density distribution and optical properties of certain participating media, and a function between the above parameters and captured pixel values is built.Thus the problem of how to find solutions to these parameters is formulated into...     

Interactive rendering of globally illuminated scenes including anisotropic and inhomogeneous participating media

Although new graphics hardware has accelerated the rendering process, the realistic simulation of scenes including participating media remains a difficult problem. Interactive results have been achieved for isotropic media as well as for single scattering. In this paper, we present an interactive global illumination algorithm for the simulation of scenes that include participating media, even anisotropic and/or inhomogeneous media. The position of the observer is important in order to render inhomogeneous media according to the transport equation. Previous work normally needed to be ray-based in order to compute this equation properly. Our approach is capable of achieving real time using two 3D textures on a simple desktop PC. For anisotropic participating media we combine density estimation techniques and graphics hardware capabilities.     

Adaptive records for volume irradiance caching

In this paper, we present a new irradiance caching scheme using Monte Carlo ray tracing for efficiently rendering participating media. The irradiance cache algorithm is extended to participating media. Our method allows to adjust the density of cached records depending on illumination changes. Direct and indirect contributions can be stored in the records but also multiple scattering. An adaptive shape of the influence zone of records, depending on geometrical features and irradiance variations, is introduced. To avoid a high density of cached records in low interest areas, a new method controls the density of the cache when adding new records. This record density control depends on the interpolation quality and on the photometric characteristics of the medium. Reducing the number of records accelerates both the computation pass and the rendering pass by decreasing the number of queries to the cache data structure (Kd-tree). Finally, instead of using an expensive ray marching to find records that cover the ray, we gather all the contributive records along the ray. With our method, pre-computing and rendering passes are significantly speeded-up.     

非经典参与介质的渲染方法研究综述

参与介质在自然界中广泛存在,也是包括影视特效、电子游戏、仿真系统等大多数图形应用中的主要场景元素之一,对其外观的模拟和再现,可以极大地提升场景的真实感和沉浸感.但是由于参与介质本身结构以及光线在参与介质中的传播过程都非常复杂,所以迄今为止,对参与介质渲染的研究都一直是图形领域的热点和难点.为了处理的方便和计算的高效,传统的参与介质渲染方法都基于两点假设:独立散射假设和局部连续假设.这两点假设也是经典的辐射传输方程成立的关键.但实际上,自然界中的很多参与介质都不满足这两点假设,因此导致现有的参与介质渲染方法生成的图片效果和真实世界的效果存在一定的差异.近年来,研究者们提出了各种非经典参与介质渲染方法,试图打破上述的两点假设,用更符合物理客观规律的方式来处理参与介质,从而进一步提升参与介质渲染的物理真实感.从相干介质渲染技术和离散介质渲染技术两方面展开对现有的面向非经典参与介质的渲染方法进行分析和讨论,重点阐述经典和非经典参与介质渲染方法的区别,以及现有非经典参与介质渲染方法的原理、优势和不足.最后,展望一些开放性问题并进行总结.本综述希望能启发相关领域的研究人员进一步攻克非经典参与介质渲...     

Real‐time Indirect Illumination of Emissive Inhomogeneous Volumes using Layered Polygonal Area Lights

Indirect illumination involving with visually rich participating media such as turbulent smoke and loud explosions contributes significantly to the appearances of other objects in a rendering scene. However, previous real‐time techniques have focused only on the appearances of the media directly visible from the viewer. Specifically, appearances that can be indirectly seen over reflective surfaces have not attracted much attention. In this paper, we present a real‐time rendering technique for such indirect views that involves the participating media. To achieve real‐time performance for computing indirect views, we leverage layered polygonal area lights (LPALs) that can be obtained by slicing the media into multiple flat layers. Using this representation, radiance entering each surface point from each slice of the volume is analytically evaluated to achieve instant calculation. The analytic solution can be derived for standard bidirectional reflectance distribution functions (BRDFs) based on the microfacet theory. Accordingly, our method is sufficiently robust to work on surfaces with arbitrary shapes and roughness values. In addition, we propose a quadrature method for more accurate rendering of scenes with dense volumes, and a transformation of the domain of volumes to simplify the calculation and implementation of the proposed method. By taking advantage of these computation techniques, the proposed method achieves real‐time rendering of indirect illumination for emissive volumes.     

Unbiased Light Transport Estimators for Inhomogeneous Participating Media

This paper presents a new stochastic particle model for efficient and unbiased Monte Carlo rendering of heterogeneous participating media. We randomly add and remove material particles to obtain a density with which free flight sampling and transmittance estimation are simple, while material particle properties are simultaneously modified to maintain the true expectation of the radiance. We show that meeting this requirement may need the introduction of light particles with negative energy and materials with negative extinction, and provide an intuitive interpretation for such phenomena. Unlike previous unbiased methods, the proposed approach does not require a‐priori knowledge of the maximum medium density that is typically difficult to obtain for procedural models. However, the method can benefit from an approximate knowledge of the density, which can usually be acquired on‐the‐fly at little extra cost and can greatly reduce the variance of the proposed estimators. The introduced mechanism can be integrated in participating media renderers where transmittance estimation and free flight sampling are building blocks. We demonstrate its application in a multiple scattering particle tracer, in transmittance computation, and in the estimation of the inhomogeneous air‐light integral.     

均匀参与介质的渲染方法研究

参与介质在现实世界中广泛存在,光线在参与介质中的传播过程比在表面上的传播过程更加复杂,比如在高度散射参与介质中会发生成千上万次反射、在低散射参与介质中由于表面聚集出现体焦散效果,从而使得光线的模拟过程非常耗时。目前常用的方法包括点、光束和路径统一模型法（unifying points,beams and paths,UPBP）以及流型探索梅特罗波利斯光线传递方法（manifold exploration Metropolis light transport,MEMLT）等,这些方法在一定程度上改进了原有方法,但是在一些特殊情况下仍然需要很长时间才能收敛。本文介绍几种针对均匀参与介质的高效渲染方法。1）基于点的参与介质渲染方法,主要通过在参与介质内分布一些点来分别加速单次、二次和多次散射的计算,在GPU （graphics processing unit）实现的基础上,最终达到可交互的效率,并且支持对任意的均匀参与介质的编辑。2）基于多次反射的预计算模型,预计算出无限参与介质中的多次散射分布,通过分析光照分布的对称性,将该分布的维度从4维减低为3维,并且将该分布应用到多种蒙特卡洛渲染方法中,比如MEMLT、UPBP等,从而提高效率。3）参与介质中的路径指导方法,通过学习光线在参与介质中的分布,该分布用SD-tree （spatial-directional tree）来表示,与相位函数进行重采样来产生出射方向。以上3种方法分别从不同角度加快了参与介质的渲染效率。     

Irradiance Gradients in the Presence of Participating Media and Occlusions

In this paper we present a technique for computing translational gradients of indirect surface reflectance in scenes containing participating media and significant occlusions. These gradients describe how the incident radiance field changes with respect to translation on surfaces. Previous techniques for computing gradients ignore the effects of volume scattering and attenuation and assume that radiance is constant along rays connecting surfaces. We present a novel gradient formulation that correctly captures the influence of participating media. Our formulation accurately accounts for changes of occlusion, including the effect of surfaces occluding scattering media. We show how the proposed gradients can be used within an irradiance caching framework to more accurately handle scenes with participating media, providing significant improvements in interpolation quality.     

Approximate Bias Compensation for Rendering Scenes with Heterogeneous Participating Media

In this paper we present a novel method for high‐quality rendering of scenes with participating media. Our technique is based on instant radiosity, which is used to approximate indirect illumination between surfaces by gathering light from a set of virtual point lights (VPLs). It has been shown that this principle can be applied to participating media as well, so that the combined single scattering contribution of VPLs within the medium yields full multiple scattering. As in the surface case, VPL methods for participating media are prone to singularities, which appear as bright “splotches” in the image. These artifacts are usually countered by clamping the VPLs' contribution, but this leads to energy loss within the short‐distance light transport. Bias compensation recovers the missing energy, but previous approaches are prohibitively costly. We investigate VPL‐based methods for rendering scenes with participating media, and propose a novel and efficient approximate bias compensation technique. We evaluate our technique using various test scenes, showing it to be visually indistinguishable from ground truth.     

Adaptive lattice‐based light rendering of participating media

The visual world around us displays a rich set of light effects because of translucent and participating media. It is hard and time consuming to render these effects with scattering, caustic, and shaft because of the complex interaction between light and different media. This paper presents a new rendering method based on adaptive lattice for lighting participating media of translucent materials such as marble, wax, and shaft light. Firstly, on the basis of the lattice‐based photon tracing model, multi‐scale hierarchical lattice was constructed by mixed lattice types sampling combined cubic Cartesian and face‐centered cubic with view‐dependent adaptive resolution. Then, an adaptive method to trace diffuse photons and marked specular photons with different phase functions was suggested. Multiple lights and heterogeneous materials were also considered here. Further, the mixed rendering method and GPU accelerate technology were introduced to render different light effects under different participating media. Copyright © 2011 John Wiley & Sons, Ltd.