变化光照目标图像合成的球调和方法

光照是真实感绘制技术中的一个关键因素。研究了朗伯反射的球调和表示，提出了一种基于球调和函数的任意光照目标图像生成方法。该方法在给定目标基图像及其对应光照属性的基础上，通过基图像数据矩阵的奇异值分解（SVD）分离场景环境光分量、反演计算场景反射率和表面法向量等。新光照条件下绘制图像时，采用四阶球调和函数拟合入射光和朗伯反射核计算直接光照；间接光照采用分离出的基图像环境光分量拟合。实验表明：该方法能够保证绘制精确度，适合复杂光照条件下的目标图像生成。     

基于图形硬件的辐照度环境纹理图的计算方法

辐照度环境纹理图是绘制任意光照环境下漫反射物体表面的一种有效的方法。为了实现动态光照环境下辐照度环境纹理图的实时绘制,基于当前的通用图形硬件,提出了一种采用顶点着色器快速计算辐照度环境纹理图的方法。该方法从分析球面调和函数入手,首先得出环境贴图的二次多项式表达形式;然后用顶点着色程序对多项式系数和球面调和函数进行加速计算,以快速生成辐照度环境纹理图;最后对于动态光照环境,则通过对环境纹理图的分级细化来加快光照系数的计算,进而实现了动态光照环境下辐照度环境纹理图的重新绘制。实验表明,在动态光照条件下,采用该方法在获取真实感光照效果的同时,其运算速度也能满足交互系统的需求。     

一种新的图像合成方法

提出了一种新的图像分割与合成方法。对于图像的分割,提出了新的切割图法,先是手工把一些明显属于物体和明显属于背景的像素分割出来,然后设置了新的能量函数,求它取最小值时的分割结果。对于图像的合成,提出了梯度比较法,在保留物体基本颜色特征的同时,改变了物体的光亮度,使其与新背景的光亮度一致,从而合成图像的光照效果更真实。     

一种基于球面调和函数的柔性体建模方法实现

给出了球面调和函数表达柔性体模型的原理,重点探讨了一种新的基于球面调和函数多尺度性的虚拟柔性体建模方法的实现机理。实验结果表明,该方法可以实现柔性物体的精确建模和实时表达,适合应用于虚拟现实的柔性物体碰撞检测、虚拟手术等对场景实时性要求较高的领域。     

基于GPU的球面深度图实时绘制

提出了一种GPU加速的实时基于图像的绘制算法.该算法利用极坐标系生成对物体全方位均匀采样的球面深度图像;然后根据推导的两个预变换公式将单幅球面深度图像预变换到物体包围球的一个与视点相关的切平面上,以生成中间图像;再利用纹理映射生成最终目标图像.利用现代图形硬件的可编程性和并行性,将预变换移植到Vertex Shader来加快绘制速度;利用硬件的光栅化功能来完成图像的插值,以得到连续无洞的结果图像.此外,还在Pixel Shader上进行逐像素的光照以及环境映射的计算,生成高质量的光照效果.最终,文章解决了算法的视点受限问题,并设计了一种动态LOD(Level of Details)算法,实现了一个实时漫游系统,保持了物体间正确的遮挡关系.     

从单幅高动态范围图像恢复环境中物体的材质

提出一种从单幅高动态范围图像恢复一般环境中物体材质的方法,适用于单一材质物体,对物体形状和光照条件没有任何特殊要求.在一般光照环境中,获取被考察物体的一幅高动态范围图像以及用来近似物体光照的一个或几个高动态范围环境映照,然后用模拟退火算法求解逆向绘制问题.在求解过程中采用了基于图像的光照和光线跟踪技术,充分考虑了物体自身互反射的影响.最后得到了物体表面反射模型的最优参数.若与基于图像的建模技术相结合,可以根据真实物体的照片建立真实感模型.     

基于Lambert-Phong模型的图像盲取证方法

由于现有的图像盲取证方法中所使用的光照模型不能有效地表征物体表面的实际光照效果,提出Lambert-Phong光照模型。该模型同时考虑光照的漫反射和镜面反射,利用该光照模型对无限光源模式下的图像进行蓄意修改检测。实验结果表明,Lambert-Phong光照模型能较准确地计算出图像中不同目标的光照方向,有效地判别出图像是否经过蓄意修改。     

室内环境下基于R-CNN的光照自适应物体检测

物体检测是工作于室内环境的移动机器人必须解决的问题。物体检测受到环境动态变化的影响,其中尤以光照变化的影响最为明显。分析室内环境中光照变化特点,研究如何通过提取图像空间特征快速识别环境中光照状况,并以光照识别结果控制物体检测模式切换,在不同光照状态下,自适应地选择使用图像传感器或者激光传感器数据,结合深度学习的特征选择能力,保证物体检测性能。机器人运行时,首先通过提取图像在CIEXYZ空间Y分量上的统计特征,并结合一些其他特征,实现快速地对图像拍摄时所处环境的光照状态进行估计;在光照适中的情况下,利用R-CNN算法结合移动机器人特点,实现在图像空间下的快速物体检测;在光照不足或过强时,先把三维激光传感器获取的点云转换成深度图像,再利用R-CNN算法实现物体检测。实验结果表明了所提出算法的有效性。     

基于球面调和函数的柔性体力触觉建模研究

为了提高虚拟柔性体力触觉交互中物体形变与力的准确描述及计算效率,提出一种基于球面调和函数表达的柔性体力触觉建模方法。利用球面调和函数的多尺度性进行物体碰撞检测实现,在变形体的密度、杨氏模量、泊松比等参数已知的情况下,根据在力作用下的物体形变使用简化后的波动方程计算变形量和反馈力生成。实验结果表明,采用该方法可以准确描述柔性体的形变过程,其所需的计算成本在可接受的范围之内,可被应用到各种力交互辅助的医学应用中。     

基于图像的光照

基于图像的光照技术是利用现实世界的光照图像来照明现实的或计算机生成的场景及物体的一种方法。介绍了基于图像光照的基本方法和步骤，在LightWave 3D图形软件中建立三维物体模型，加入高动态范围图像作为场景照明环境，通过光通量与高动态范围图像的结合，获得了逼真的光照及环境映射图像。     

Importance Sampling Spherical Harmonics

In this paper we present the first practical method for importance sampling functions represented as spherical harmonics (SH). Given a spherical probability density function (PDF) represented as a vector of SH coefficients, our method warps an input point set to match the target PDF using hierarchical sample warping. Our approach is efficient and produces high quality sample distributions. As a by-product of the sampling procedure we produce a multi-resolution representation of the density function as either a spherical mip-map or Haar wavelet. By exploiting this implicit conversion we can extend the method to distribute samples according to the product of an SH function with a spherical mip-map or Haar wavelet. This generalization has immediate applicability in rendering, e.g., importance sampling the product of a BRDF and an environment map where the lighting is stored as a single high-resolution wavelet and the BRDF is represented in spherical harmonics. Since spherical harmonics can be efficiently rotated, this product can be computed on-the-fly even if the BRDF is stored in local-space. Our sampling approach generates over 6 million samples per second while significantly reducing precomputation time and storage requirements compared to previous techniques.     

Face Relighting from a Single Image under Arbitrary Unknown Lighting Conditions

In this paper, we present a new method to modify the appearance of a face image by manipulating the illumination condition, when the face geometry and albedo information is unknown. This problem is particularly difficult when there is only a single image of the subject available. Recent research demonstrates that the set of images of a convex Lambertian object obtained under a wide variety of lighting conditions can be approximated accurately by a low-dimensional linear subspace using a spherical harmonic representation. Moreover, morphable models are statistical ensembles of facial properties such as shape and texture. In this paper, we integrate spherical harmonics into the morphable model framework by proposing a 3D spherical harmonic basis morphable model (SHBMM). The proposed method can represent a face under arbitrary unknown lighting and pose simply by three low-dimensional vectors, i.e., shape parameters, spherical harmonic basis parameters, and illumination coefficients, which are called the SHBMM parameters. However, when the image was taken under an extreme lighting condition, the approximation error can be large, thus making it difficult to recover albedo information. In order to address this problem, we propose a subregion-based framework that uses a Markov random field to model the statistical distribution and spatial coherence of face texture, which makes our approach not only robust to extreme lighting conditions, but also insensitive to partial occlusions. The performance of our framework is demonstrated through various experimental results, including the improved rates for face recognition under extreme lighting conditions.     

Relighting photographs of tree canopies

We present an image-based approach to relighting photographs of tree canopies. Our goal is to minimize capture overhead; thus the only input required is a set of photographs of the tree taken at a single time of day, while allowing relighting at any other time. We first analyze lighting in a tree canopy both theoretically and using simulations. From this analysis, we observe that tree canopy lighting is similar to volumetric illumination. We assume a single-scattering volumetric lighting model for tree canopies, and diffuse leaf reflectance; we validate our assumptions with synthetic renderings. We create a volumetric representation of the tree from 10-12 images taken at a single time of day and use a single-scattering participating media lighting model. An analytical sun and sky illumination model provides consistent representation of lighting for the captured input and unknown target times. We relight the input image by applying a ratio of the target and input time lighting representations. We compute this representation efficiently by simultaneously coding transmittance from the sky and to the eye in spherical harmonics. We validate our method by relighting images of synthetic trees and comparing to path-traced solutions. We also present results for photographs, validating with time-lapse ground truth sequences.     

Efficient Relighting of RBF-Based Illumination Adjustable Images

An illumination adjustable image (IAI) contains a large number of prerecorded images under various light directions. Relighting a scene under complicated lighting conditions can be achieved from the IAI. Using the radial basis function (RBF) approach to represent an IAI is proven to be more efficient than using the spherical harmonic approach. However, to represent high-frequency lighting effects, we need to use many RBFs. Hence, the relighting speed could be very slow. This brief investigates a partial reconstruction scheme for relighting an IAI based on the locality of RBFs. Compared with the conventional RBF and spherical harmonics (SH) approaches, the proposed scheme has a much faster relighting speed under the similar distortion performance.      

复杂环境光源下虚实融合光照实时计算

在互动电子游戏、增强现实等对实时计算要求很高的交互式图形应用中,大量使用复杂环境光源对虚拟物体进行照明,使其和真实场景的光照一致,虚实融合.提出了用Cook Torrance光照模型进行虚实场景的光照计算;利用球面调和基函数的方法,实时地计算高动态范围环境映射光照系数,得到高动态范围环境映射的二次多项式表达形式,在着色器计算该式得到漫反射分量;通过环境映射技术对镜面反射进行模拟,全部光照计算在GPU中完成.实验结果表明,该方法在动态变化的复杂环境光源下,完成对虚拟物体光照实时计算,绘制速度每秒30帧以上,绘制结果具有较强的真实感.     

Lighting Simulation of Augmented Outdoor Scene Based on a Legacy Photograph

We propose a novel approach to simulate the illumination of augmented outdoor scene based on a legacy photograph. Unlike previous works which only take surface radiosity or lighting related prior information as the basis of illumination estimation, our method integrates both of these two items. By adopting spherical harmonics, we deduce a linear model with only six illumination parameters. The illumination of an outdoor scene is finally calculated by solving a linear least square problem with the color constraint of the sunlight and the skylight. A high quality environment map is then set up, leading to realistic rendering results. We also explore the problem of shadow casting between real and virtual objects without knowing the geometry of objects which cast shadows. An efficient method is proposed to project complex shadows (such as tree's shadows) on the ground of the real scene to the surface of the virtual object with texture mapping. Finally, we present an unified scheme for image composition of a real outdoor scene with virtual objects ensuring their illumination consistency and shadow consistency. Experiments demonstrate the effectiveness and flexibility of our method.     

Geometry-dependent lighting

In this paper, we introduce geometry-dependent lighting that allows lighting parameters to be defined independently and possibly discrepantly over an object or scene based on the local geometry. We present and discuss light collages, a lighting design system with geometry-dependent lights for effective feature-enhanced visualization. Our algorithm segments the objects into local surface patches and places lights that are locally consistent but globally discrepant to enhance the perception of shape. We use spherical harmonics for efficiently storing and computing light placement and assignment. We also outline a method to find the minimal number of light sources sufficient to illuminate an object well with our globally discrepant lighting approach.     

All-Frequency Lighting with Multiscale Spherical Radial Basis Functions

This paper proposes a novel multiscale spherical radial basis function (MSRBF) representation for all-frequency lighting. It supports the illumination of distant environment as well as the local illumination commonly used in practical applications, such as games. The key is to define a multiscale and hierarchical structure of spherical radial basis functions (SRBFs) with basis functions uniformly distributed over the sphere. The basis functions are divided into multiple levels according to their coverage (widths). Within the same level, SRBFs have the same width. Larger width SRBFs are responsible for lower frequency lighting while the smaller width ones are responsible for the higher frequency lighting. Hence, our approach can achieve the true all-frequency lighting that is not achievable by the single-scale SRBF approach. Besides, the MSRBF approach is scalable as coarser rendering quality can be achieved without reestimating the coefficients from the raw data. With the homogeneous form of basis functions, the rendering is highly efficient. The practicability of the proposed method is demonstrated with real-time rendering and effective compression for tractable storage.