A Fast Display Method of Sky Colour Using Basis Functions

Computer graphics are being used for visual environmental assessment or architectural designs. Displaying the sky as a background is indispensable in generating photorealistic images for such applications. In this paper, we propose a fast display method of the sky colour by expressing the intensity distribution of the sky using basis functions, even if the sun position and/or the camera position are altered. In the proposed method, cosine functions are used as basis functions. The sun altitude is altered at certain intervals and the distributions of the sky colour for each sun altitude are precalculated and stored efficiently using basis functions. The colour of the sky in the view direction of an arbitrary sun position can be obtained from the stored distributions and displayed quickly. © 1997 by John Wiley & Sons, Ltd.     

A Quick Rendering Method Using Basis Functions for Interactive Lighting Design

When designing interior lighting effects, it is desirable to compare a variety of lighting designs involving different lighting devices and directions of light. It is, however, time-consuming to generate images with many different lighting parameters, taking interreflection into account, because all luminances must be calculated and recalculated. This makes it difficult to design lighting effects interactively. To address this problem, this paper proposes a method of quickly generating images of a given scene illustrating an interreflective environment illuminated by sources with arbitrary luminous intensity distributions. In the proposed method, the luminous intensity ditribution is expressed with basis functions. The proposed method uses a series of spherical harmonic functions as basis functions, and calculates in advance each intensity on surfaces lit by the light sources whose luminous intensity distribution are the same as the spherical harmonic functions. The proposed method makes it possible to generate images so quickly that we can change the luminous intensity distribution interactively. Combining the proposed method with an interactive walk-through that employs intensity mapping, an interactive system for lighting design is implemented. The usefulness of the proposed method is demonstrated by its application to interactive lighting design, where many images are generated by altering lighting devices and/or direction of light.     

Sky Based Light Metering for High Dynamic Range Images

Image calibration requires both linearization of pixel values and scaling so that values in the image correspond to real‐world luminances. In this paper we focus on the latter and rather than rely on camera characterization, we calibrate images by analysing their content and metadata, obviating the need for expensive measuring devices or modeling of lens and camera combinations. Our analysis correlates sky pixel values to luminances that would be expected based on geographical metadata. Combined with high dynamic range (HDR) imaging, which gives us linear pixel data, our algorithm allows us to find absolute luminance values for each pixel—effectively turning digital cameras into absolute light meters. To validate our algorithm we have collected and annotated a calibrated set of HDR images and compared our estimation with several other approaches, showing that our approach is able to more accurately recover absolute luminance. We discuss various applications and demonstrate the utility of our method in the context of calibrated color appearance reproduction and lighting design.     

Glare‐limited appearances in HDR images

Abstract— Intraocular glare and simultaneous contrast control appearance in high‐dynamic‐range (HDR) images. This paper describes unique test targets that simulate real images. These targets change the HDR range by 500 times, without significantly changing the veiling glare on the retina. As well, these targets have nearly constant simultaneous contrast. The range of appearances possible from HDR images with different average luminances were measured. The targets displayed a maximum luminance range of 5.4 log units. Using magnitude estimates (MagEst) of appearances, the relationship between luminance and lightness from white to black was measured. With one exception, only small changes in appearance with large changes in dynamic range were found. It was also found that appearance was scene‐dependent. The same dark grays (MagEst = 10) were observed with luminances of 10, 4.2, 1.1, and 0.063, depending on the percentage of white area in the surround. Glare from more white increases the retinal luminance of the test areas. Simultaneous contrast counteracts glare by making the appearance range (white‐black) with a much smaller range of luminances. Appearance is controlled by both the optical scattered light and the spatial processing. A single tone‐scale function of luminance cannot describe appearance controlled by scatter and spatial processing.     

A Deep Residual Network for Geometric Decontouring

Grayscale images are intensively used to construct or represent geometric details infield of computer graphics. In practice, displacement mapping technique often allows an 8-bit grayscale image input to manipulate the position of vertices. Human eyes are insensitive to the change of intensity between consecutive gray levels, so a grayscale image only provides 256 levels of luminances. However, when the luminances are converted into geometric elements, certain artifacts such as false contours become obvious. In this paper, we formulate the geometric decontouring as a constrained optimization problem from a geometric perspective. Instead of directly solving this optimization problem, we propose a data-driven method to learn a residual mapping function. We design a Geometric DeContouring Network (GDCNet) to eliminate the false contours effectively. To this end, we adopt a ResNet-based network structure and a normal-based loss function. Extensive experimental results demonstrate that accurate reconstructions can be achieved effectively. Our method can be used as a relief compressed representation and enhance the traditional displacement mapping technique to augment 3D models with high-quality geometric details using grayscale images efficiently.     

Illumination Morphing: Generating a Smooth Change of Illumination between Two Color Images

The paper proposes a method for generating a sequence of images with smooth change of illumination from two input images with different lighting conditions. The idea of the proposed method is based on image morphing. While conventional image morphing changes object shapes between two input images, here we focus on changing the illumination between two images. The proposed method uses isoluminance curves as a feature primitive. Isoluminance curves acquired from images are warped based on the correspondence of the curves between two images, and transformed luminance distributions are generated from the warped isoluminance curves. The proposed method called "illumination morphing" is able to generate smooth transition of luminance between two color images. The method does not need even the information about the light sources and 3D object models. The proposed method is a promising technique for many applications requiring a scene with variety of lighting effects, such as movies, TV games, and so on.     

基于融合亮度模型和梯度域滤波的图像去雾#br#

为解决暗通道先验算法在处理图像的天空区域时容易出现颜色过饱和、亮度整体偏暗和光晕等问题,提出了一种融合亮度模型和梯度域滤波的图像去雾算法.首先,选择整幅图像中亮度最大的前0.1％ 像素的平均值作为大气光值;然后,利用自适应最小值滤波的改进暗通道模型和亮度模型分别对前景区域和天空区域求解透射率,在将其加权融合得到粗透射率的基础上,使用梯度域导向滤波对透射率进行细化;最后,通过大气散射模型和伽马校正复原出无雾图像.实验结果表明,在包含天空区域的雾图上本文算法能够快速有效地解决天空区域的光晕效应和图像失真问题,复原出来的图像清晰自然,保留了较多的细节信息,在主观和客观2个评价方面均优于其他对比算法.     

基于单幅建筑物图像的太阳光方位角恢复

在建筑规划和历史遗迹重建等领域，需要在重建的场景中添加、删除虚拟物体。能否获得图像拍摄时刻的光照信息，是决定最终合成图像的照片级真实感程度的重要因素之一，本文对基于单幅建筑物图像的光源方向检测技术进行了研究。把户外的太阳光看作为平行光，在建筑物坐标系中其方向可以用方位角和仰角表示。利用图像中建筑物表面阳光强度值的变化来确定方位角。在此基础上，可以利用建筑物表面突出部分与其阴影之间的关系来确定仰角。这项技术可以应用于大多数建筑物的重建系统中。     

Border detection in various levels of surround luminance

In order to provide practical and useful data for display design, border detection with various levels of surround luminance from 0.001 to 500 cd/m² is investigated. The results indicated that luminance difference for border detection exhibits a monotonically increasing curve against the reference luminance on a logarithmic scale. To compare the luminance difference curves for different surround luminances, the results were converted to contrast threshold curves against the reference luminance that are U‐shaped on a logarithmic scale. These contrast thresholds, however, are higher than those of the grayscale standard display function curve. The causes of the difference between the contrast threshold curves of our study and the grayscale standard display function curve are the adaptation conditions and stimulus size, as well as the difference in criteria between the detection of a border and the detection of non‐uniformity. Finally, the necessary number of gradation levels for border detection under various surround luminances is calculated and found to be at least double the current number of levels (256).     

利用线性插值合成图象

室外场景的光照模拟与图象合成的难点在于场景与光照的复杂性。提出了一种利用现有基图象的线性组合来生成不同天候条件下同一场景图象的方法。它根据光照函数的线性关系,估计不同太阳位置和不同天气状况基图象的系数,从而最终通过基图象的组合来产生目标图象。实验结果表明本文的方法用于固定场景下的图象合成是可行的。     

Simulation of natural daylight illumination determined by a high dynamic range image

The use of high dynamic range images as a source of the natural daylight illumination and a background for virtual scenes containing objects with complex optic properties (fabrics, automobile multilayer paints, etc.) is considered. Physically justified algorithms for calculating the luminance of a point, recognition of the sun, generation of shadows, and background specification are considered. An analysis of the correctness of the available high dynamic range images is performed, and their drawbacks are discussed. Modifications of the algorithms are developed that correct these drawbacks in the case of simulating the illumination based on the sky models adopted by the International Commission on Illumination (CIE). The modified illumination simulation algorithms make it possible to produce more accurate images; moreover, they are automatic, and robust with respect to errors in high dynamic range images. These algorithms are used in several computer graphics systems for generating realistic images. A version of this paper with color figures can be found at http://www.keldysh.ru/pages/cgraph/publications/cgd_publ.htm.     

Reliability of computer graphic images for visual assessment

This paper analyzes the use of computer graphics images as an architectural assessment tool. The characteristics of such images as an architectural simulation tool are discussed and their reliability as an assessment tool is evaluated and compared with that of traditional hand-generated perspective drawings. A specimen application, architectural simulation of a city renewal plan, is presented.     

关于天空动态仿真技术研究与实现

天空的建模和渲染是计算机图形学领域的一个研究难点,对其研究有着重要的现实和理论意义。从天空的建模方法、光照模型、绘制效率三个方面对天空仿真系统进行了研究。提出了更加适合在户外大场景仿真中应用的球形天空体的建模方法;改进了Fourier合成自然纹理的方法,并将其应用于云彩模拟中;并采用多层天空盒和雾化技术来增加天空仿真的真实效果。实验结果表明,使用文中的仿真算法得到的仿真图像对于提高用户的沉浸感和真实感效果显著。     

基于重复结构检测的三维建筑物精细模型重建

基于单幅图像的城市建筑物建模是计算机图形学和计算机视觉的一个研究热点.针对具有显著重复结构特征的单幅城市建筑物输入图像,提出了一种通过简单交互方便快速地生成建筑物三维模型中精细细节结构的方法.首先,根据用户的简单交互重建建筑物基本主体部分的三维几何模型;然后,根据输入图像,通过检测水平方向和垂直方向上的边界线自动检测建筑物各个墙面的重复结构,并对重复细节结构按照其包围盒的特征进行分类;其次,通过用户描绘细节结构的局部线画图,根据建筑物细节结构与基本主体部分的射影几何关系,恢复对应凹凸细节结构的几何模型;最后,根据检测得到的建筑物表面重复结构,分别自动生成相同类型的重复细节结构,得到三维建筑物精细模型.实验结果表明,该方法能够简单便捷地重建出具有较强真实感的三维建筑物模型.     

基于OpenGL的飞行物定位探测仪动态模拟

三维动画模拟是计算机图形学发展的一个重要分支;文章阐述了OpenGL的基本原理,运用VC++和OpenGL开发工具,采用纹理映射、光照技术、地形处理、三维图形组合原理,成功地实现了高空飞行物定位探测仪的动态模拟,同时体现了OpenGL在粒子系统方法来模拟水流动画中的重要性;实验结果表明,在天空和地面构成的背景下,该方法模拟的飞行物定位探测仪形态逼真,实时性好。     

结合天空检测与纹理平滑的图像去雾

目的 针对已有图像去雾方法中存在的天空灰暗以及透射率分布与实际情况不一致导致的对比度增强不足等问题,以暗通道先验图像去雾方法为基础,提出结合天空检测与纹理滤波的图像去雾算法。方法 首先,设计了一个基于天空检测的大气光自适应估计策略,以天空区域亮度值较低的像素为依据估计大气光值,能够避免天空色彩失真,获得更明亮且干净的天空恢复结果;其次,对输入图像进行纹理平滑预处理以保持同一平面物体内的像素颜色一致性,并提出一个基于块偏移与导向滤波的透射率精确化计算策略,使透射率估值更符合深度信息的变化趋势,以提升无雾图像的对比度与色彩饱和度;最后,对复原结果进行联合双边滤波后处理,以降低噪声的影响。结果 本文算法得到的大气光估值更为合理,对于不符合暗通道先验的天空区域,能够取得更为自然的天空复原结果;本文算法得到的透射率的变化趋势与实际场景深度之间具有更高的一致性,对于符合暗通道先验的非天空区域,能够取得高对比度与高色彩饱和度的恢复结果。结论 本文算法在大气光与透射率的估值的准确性以及无雾图像的对比度与清晰度增强方面都得到了有效提升,具有较高的鲁棒性,适用于视频监控、交通监管和目标识别等户外获取图像的诸多应用领域。     

一种基于相邻像素一致性的数码照片与计算机图形鉴别方法

愈来愈逼真的计算机图形正在颠覆人们"眼见为实"的传统观念,使人们难以分辨哪些是真实的数码照片,哪些是利用软件合成的图形.然而由于数码照片和计算机图形成像过程的不同,造成两类图像的相邻像素一致性存在较大差异.基于此认识,在HSV颜色空间提取了图像相邻像素一致性直方图特征和共生矩阵特征,对1300幅数码照片和500幅真实感计算机图形进行鉴别,识别率分别可达96%和93.9%.对视觉上难以区分的5幅数码照片和5幅计算机图形也取得了很好的识别效果.     

A Graphics System Distributed across a Local Area Network

Design of a general purpose graphics system in a computer network environment requiresthat the architectural design of the graphics system be suited to such an environment and theadvantages of the network environment be taken for distributed graphics processing and thesharing of resources. An architectural model is designed to meet these requirements. The modelis characterised by the distribution on the machines in the network of graphics facilitiesprovided by several graphics subsystems with various capabilities but compatible functionality,and the distributed processing of graphics across the network. This design structure has beenshown to be viable by using it as the basis for the implementation of the graphics system for theMU6G network at University of Manchester. The design methodology and the structure of thegraphics system are described in the paper.     

Luminance and luminance contrast requirements for legibility of self-luminous displays in aircraft cockpits

A series of studies was conducted to investigate the effects of display background luminance, display contrast ratio and adaptation luminance on the legibility of self-luminous displays in aircraft cockpits. The studies were designed to avoid the typical confounding of these three variables, and employed luminance levels that are representative of operational conditions. The data show that with typical display symbols and luminances under low and moderate ambient illumination, contrast ratio is the major determinant of legibility; that increasing contrast ratios beyond as little as 1·4:1 may provide no significant improvements in legibility; and that, beyond their contribution to contrast ratio, the absolute luminances of the symbol and the background have no significant effect on legibility. Given the high ambient illumination typical of a sunny day, however, the pilot is adapted to luminances of up to 32 264 cd/m² (10 000 fL). Under these conditions, increases in contrast ratio of up to 30:1 are shown to improve legibility. When the adaptation luminance is more than 100 times that of the display background luminance, symbols on the display cannot be immediately identified, and the absolute luminances of the adapting field and the display background contribute to recovery time apart from their ratio relationship. Hyperbolic functions are applied to describe the multiplicative effects of these variables on legibility.