IPT色空间及其均匀性研究

IPT色空间在色域映射和图像色貌模型研究中得到应用。介绍了IPT色空间,采用D65光源下的孟塞尔系统和Ebner和Fairchild提出恒常色相数据研究了IPT色空间的均匀性。孟塞尔系统的三刺激值XYZ作为IPT色空间输入值,计算明度、彩度和色相,并与CIELAB色空间计算结果进行均匀性比较。结果表明,在明度均匀性方面,两个色空间结果基本相似,CIELAB色空间比IPT色空间稍好;在彩度均匀性上,CIELAB色空间较好于IPT色空间;在色相均匀上,采用孟塞尔系统数据的计算结果中IPT色空间在对蓝色色相预测和各个色相角的线性关系要比CIELAB色空间好,而对于Ebner和Fairchild恒常感知色相数据,IPT色空间的色相均匀性明显优于CIELAB色空间。     

CIELAB与CIECAM02色空间均匀性比较研究

采用均匀色空间Munsell系统检验和验证了CIELAB和色貌模型CIECAM02色空间的均匀性。利用Mun- sell色卡所对应的三刺激值X,Y,Z作为模型的输入值,预测色空间模型的明度、彩度和色调,并通过预测结果比较和检验了CIELAB色空间和CIECAM02模型色空间的均匀性。研究表明:在预测明度方面,两个色空间的结果相似;在预测彩度时,CIECAM02模型较好于CIELAB色空间;在预测色调时,当色调逐渐从BG变化到B时,CIELAB所预测的色调角与由Munsell色空间计算出来的色调角之差逐渐增大,最大时超过6%,而CIECAM02所预测的色调角与Munsell色空间的色调角之差在2.5%以内,CIECAM02优于CIELAB色空间。     

基于新型色貌模型CAM16的颜色空间均匀性研究

对新型色貌模型CAM16的颜色空间均匀性进行分析评估,发现该模型颜色空间仍旧存在视觉均匀性不够理想的问题,这将会影响跨媒体颜色再现的准确性,导致相同颜色在不同观察环境下产生色差。为此,通过BFD色差数据集对该模型进行优化,以修正视觉均匀性,进而提出更为均匀的色貌模型CAM16-J′a′b′,同时采用国际通用的芒塞尔表色系统对优化后的颜色空间进行均匀性检验。结果表明,优化后的颜色空间均匀性有所提高,为跨媒体颜色再现研究提供了参考。     

基于中国颜色体系的色差公式分析

CIE均匀颜色空间及利用色差公式对色差进行计算对彩色复制品的评价具有很关键的作用。利用中国颜色体系在视觉上为等色差系统以及色调、彩度和明度可以单独改变的特性,分别从色调、彩度、明度三方面对CIEL*a*b*、CMC(l:c)、CIE94、CIEDE2000色差公式进行了比较分析。结果表明:用各色差公式计算出的平均色差值是不同的,它们的均匀性也不同。需要针对不同的明度、彩度、色调选择使用不同的色差公式。     

基于BRDF数据的金属漆颜料制品的色貌预测评估北大核心CSCD

通过双目观察与匹配心理物理学实验,对金属漆颜料制品在两种几何条件下进行了视觉评价,随后将金属漆颜料制品的视觉平均数据分别与基于双向反射分布函数(BRDF)数据和基于PR715分光辐射计测量结果的色貌模型预测结果进行最小二乘法线性拟合。实验结果表明,在不同几何条件下,BRDF数据均可以用来对金属漆颜料制品进行色貌预测;对于三属性预测而言,在明度和彩度方面,基于BRDF数据的色貌模型预测精度不如基于PR715分光辐射计数据的色貌模型预测精度,而在色调角方面,基于BRDF数据的色貌模型预测精度则高于基于PR715分光辐射计数据的色貌模型预测精度。     

色貌模型的人工神经网络方法的研究

色貌模型(CAM)主要解决不同观察条件、不同背景和不同环境下的颜色真实再现问题。采用人工神经网络(ANN)的方法来实现目前最新的色貌模型CIECAM02的预测,包括正向预测(从色度参数到色貌属性参数)和逆向预测(从色貌属性参数到色度参数),应用自然色系统(NCS)中的部分色样作为神经网络的训练和测试样本。由于正向输出色貌属性参数空间不是均匀的,对于网络预测精度用特殊方法评估,而对于逆向模型则可直接利用LAB色差公式评价。测试的结果表明:用神经网络对CIECAM02模型的预测达到了较高的精度。     

低光泽度印刷色谱色貌数据组的建立与评价

色貌预测是现代颜色科学领域的基本理论和技术。为了评价低光泽度彩色印刷色谱的色貌,以丽印胶版纸印制的彩色印刷色谱为研究对象,根据颜色科学和心理物理学相关实验,建立其色貌参数样本数据组。采用相对系数R2和差异系数CV等数理统计方法,评价这种印刷色谱中Y-M内页在三种不同的照明光源下色貌的视觉评估结果。实验数据分析表明,在D65、A和TL84三种光源中,人眼在A光源下对颜色的识别能力明显不同于D65和TL84光源;同样在这三种光源中人眼对明度和色调的评估准确度最高,彩度最差。     

基于视觉容差与色调角相关性的色差公式评价

为了研究CIELAB颜色空间中明度、彩度和色调等视觉容差与色调角之间的相关性对基于CIELAB的典型色差公式(CIELAB、CMC、BFD、CIE94、LCD以及CIEDE2000)预测性能的影响,实验选取了具有不同明度和彩度冰平的5个色调环(L*=30,50,70和Cab*=10,20,30),并采用3个不同视觉尺度(△V=3.02,5.92和8.87CIELAB单位)进行视觉评估实验.8名色觉正常观察者根据心理物理学的恒常刺激法原理,在阴极射线管(CRT)显示器上获取了相应颜色中心的明度、彩度和色调视觉容差.对实验结果的详细分析表明,由于明度容差与色调角无明显相关性,6个色差公式的预测性能均较好,其中CIEDE2000略优于其它色差公式.对于彩度容差,LCD(Leeds color-difference formula)预测性能表现最优,而CIEDE2000和BFD的表现最差,其余色差公式则相差不多.CIEDE2000和LCD的色调权重函数的预测性能明显优于其它色差公式,而CMC预测性能则为最差.     

印刷样品恰可察觉小色差辨色阈值的研究Ⅰ:目视评价实验数据分析

为了研究人眼对反射样品颜色在恰可察觉小色差辨色网值水平上的颜色视觉特性,对印在半光相纸上的17个CIE基本颜色中心区域,约1000对颜色色样,采用心理物理实验方法中的同定刺激法.在CIEl976a*-b*平面上分别测得16名色觉正常的观察者在不同颜色区域的辨色阈值,并以色度椭圆表示辨色阈值实验结果.通过对实验数据的详细处理和分析表明,等明度a*-b*平面非视觉均匀,人眼对一、二象限的色宽容度普遍高于三、四象限的色宽容度,对高彩度的橙色具有最高的宽容度,对蓝色具有最低的宽容度,所有的颜色在红～绿方向的视觉色差尺度均低于黄-蓝方向.实验结果可为均匀颜色空间及其色差公式的改进和发展提供原始数据和参考依据.     

色度学

色度学理论 O432．3 2004020824 一种优化的均匀色貌空间=Optimized uniform colour appearance space[刊,中]／李为(北京理工大学颜色科学与工程国家专业实验室．北京(100081)),肖琨…∥北京理工大学学报．—2003,23(2)．—225-228 依据BFD,RIT-DuPont,Witt999和Leeds等4组色差评估实验数据集。在色貌模型CIECAM97s修订版的基础上建立了一种优化的均匀色貌空间UCAS。用综合统计指标P_F／3评价该色貌空间,并引入优化的明度修正因子。结果表明,其均匀性接近德国1999年的色差标准DIN99,并优于CIE1976年推荐的CIELAB。图4表3参10(杨妹清)     

Lightness and chroma enhancement for food images considering Helmholtz–Kohlrausch effect

The color appearance of a food image is very important for making it look delicious. Many food images acquired in low illumination environments do not give delicious impressions because of the deterioration of their color appearance. We propose an image enhancement method to improve the color appearance of a food image acquired in a low illumination environment. In the proposed method, the lightness of each pixel is first enhanced within a color gamut on an equal hue plane in the CIELAB color space. The boundary of the color gamut is given by a lookup table calculated in advance. Then, when the hue of the pixel is attributed to foods, the chroma is enhanced within the color gamut. In this case, we consider Helmholtz–Kohlrausch (H-K) effect, which indicates the relation of the hue, chroma, and lightness in human visual perception. According to H-K effect, the perceived lightness increases as the chroma is enhanced. Therefore, the perceived lightness is preserved after the chroma enhancement, so that the food image is not too enhanced. To verify the effectiveness of the proposed method, experiments are conducted using several food images.     

Change of Color Appearance in Photopic,Mesopic and Scotopic Vision

Mesopic vision describes a range of light levels where vision is mediated by both cones and rods. The appearance of color in mesopic vision differs drastically from that in photopic vision, where only cones mediate visual information. We used a haploscopic color matching technique to investigate the color appearance under various illuminance levels, ranging from photopic to scotopic via mesopic levels. The observers did color matching between a test color chip under various illuminance levels and a matching color stimulus presented on the Cathode-Ray Tube (CRT) display under the photopic illuminance condition. The results showed that not only chroma and lightness but hue of most color chips changed with illuminance. The manner of the hue changed depended on the color of the test chip, while matching points approached a neutral gray with decrease in illuminance level for all test chips. Chroma reduced continuously with decrease of the illuminance level until 0.1 lx for reddish and yellowish color chips or until 1 lx for greenish and bluish ones. Beyond those illuminance levels, chroma was approximately constant. Lightness decreased with decreasing illuminance level for all test chips except bluish color chips, for which lightness did not decrease much in general and even increased in some cases as predicted by the Purkinje shift. The experimental results obtained in the present study provide critical features that should be considered in predicting the appearance of color at low light levels.     

不同环境参数下CIECAM02的跨媒体颜色复现性能

为研究CIECAM02三种环境参数(dark,dim,average)在三种相应照明环境下的跨媒体颜色复现性能,基于记忆匹配和成对比较的心理物理学实验方法,在灯箱照明打印样本与Dark,Dim,Average三种不同环境照明条件下液晶显示器显示色样之间进行了跨媒体颜色复现实验。实验数据分析表明,采用dim环境参数在三种照明条件下都能达到较优的颜色复现性能,而dark和average参数性能与测试颜色的明度和彩度存在一定相关性,较暗色样在Dark条件下采用dark参数设置复现良好,但在Average实验条件下采用average参数设置时性能很差;对于较亮色样的复现,dark与average参数设置在其相应实验条件下的颜色复现性能无显著差异。     

基于色貌的跨媒体颜色复制

介绍了一种基于视觉匹配的跨媒体颜色复制方法。通过视觉匹配将一个环境下的一些色貌因素"映射"到另一个环境,是一种基于色貌的CRT特性化方法。该方法复制的22个Munsell色卡的平均视觉评价为6分制的5.17分。其中,红色调的复制色块视觉评价较好,蓝色调或蓝色占有较大比例的复制色块误差较大。sRGB作为目前流行的用于颜色通讯的标准色空间,在实验中也进行了比较。实验证明这种基于视觉匹配的特性化方法,已经包含了一些色貌因素,可以满足一般的应用要求,有着广泛的应用前景。     

Color transfer method preserving perceived lightness

Color transfer originally proposed by Reinhard et al. is a method to change the color appearance of an input image by using the color information of a reference image. The purpose of this study is to modify color transfer so that it works well even when the scenes of the input and reference images are not similar. Concretely, a color transfer method with lightness correction and color gamut adjustment is proposed. The lightness correction is applied to preserve the perceived lightness which is explained by the Helmholtz–Kohlrausch (H–K) effect. This effect is the phenomenon that vivid colors are perceived as brighter than dull colors with the same lightness. Hence, when the chroma is changed by image processing, the perceived lightness is also changed even if the physical lightness is preserved after the image processing. In the proposed method, by considering the H–K effect, color transfer that preserves the perceived lightness after processing is realized. Furthermore, color gamut adjustment is introduced to address the color gamut problem, which is caused by color space conversion. The effectiveness of the proposed method is verified by performing some experiments.     

Impacts of appearance parameters on perceived image quality for mobile-phone displays

In order to investigate the perceived image quality (IQ) influenced by different image appearance parameters, a psychophysical experiment was carried out on two mobile-phone displays of different technologies, the in-plane switching (IPS) and the active matrix organic light emitting diode (AMOLED). The test images were generated within a wide range of variations by applying various manipulation methods, involving changes in several kinds of image appearance parameters, i.e. lightness, chroma, hue, and spatial frequency. The perceived attributes of naturalness, colourfulness, brightness, contrast, sharpness, clearness, preference, and overall IQ were visually assessed via categorical judgement method for several application types of test images. The impacts of the manipulation methods on individual attributes have been deeply discussed based on the interrelationships of these attributes, which reveal that the significant parameters towards better overall IQ for mobile-phone displays include high lightness, suitable chroma and lightness frequency distribution, and impartial hue, accordingly better image quality will be accomplished after all these parameters have been optimised at the same time. The achievements of this study would provide constructive instructions on achieving excellent display effects for researchers and manufactories of mobile-phone displays.     

摄像机色度特性的研究

在一个由一些颜色输入输出设备组成的混合系统中,理想情况是输出设备能直接利用输入设备获得的颜色数据,即输入设备是一个标准色度观察者。考察了常用输入设备———摄像机的色度特性,从色差、亮度差、色调差、彩度差等角度计算了摄像机与标准色度观察者的差别,并对结果作出了评价。