Color constancy at a pixel

In computational terms we can solve the color constancy problem if device red, green, and blue sensor responses, or RGB's, for surfaces seen under an unknown illuminant can be mapped to corresponding RGB's under a known reference light. In recent years almost all authors have argued that this three-dimensional problem is too hard. It is argued that because a bright light striking a dark surface results in the same physical spectra as those of a dim light incident on a light surface, the magnitude of RGB's cannot be recovered. Consequently, modern color constancy algorithms attempt only to recover image chromaticities under the reference light: They solve a two-dimensional problem. While significant progress has been made toward achieving chromaticity constancy, recent work has shown that the most advanced algorithms are unable to render chromaticity stable enough so that it can be used as a cue for object recognition [B. V. Funt, K. Bernard, and L. Martin, in Proceedings of the Fifth European Conference on Computer Vision (European Vision Society, Springer-Verlag, Berlin, 1998), Vol. II, p. 445.] We take this reductionist approach a little further and look at the one-dimensional color constancy problem. We ask, Is there a single color coordinate, a function of image chromaticities, for which the color constancy problem can be solved? Our answer is an emphatic yes. We show that there exists a single invariant color coordinate, a function of R, G, and B, that depends only on surface reflectance. Two corollaries follow. First, given an RGB image of a scene viewed under any illuminant, we can trivially synthesize the same gray-scale image (we simply code the invariant coordinate as a gray scale). Second, this result implies that we can solve the one-dimensional color constancy problem at a pixel (in scenes with no color diversity whatsoever). We present experiments that show that invariant gray-scale histograms are a stable feature for object recognition. Indexing on invariant distributions supports almost perfect recognition for a dataset of 11 objects viewed under five colored lights. In contrast, object recognition based on chromaticity histograms (post-color constancy preprocessing) delivers much poorer recognition.     

Color constancy from blackbody illumination

We present a theoretical analysis of what we believe to be a new color constancy method that inputs two chromaticities of an identical surface taken under two blackbody illuminations. By using the Planck formula for modeling spectra of outdoor illumination and by assuming that a narrowband camera sensitivity function is sufficiently narrow, surface colors can be estimated mathematically. Experiments with simulation and real data have been conducted to evaluate the effectiveness of the method. The results showed that although this method is a perfect vehicle for simulation data, it produces significant errors with real data. A thorough investigation of the cause of errors indicates how important the assumptions on both blackbody illuminations and narrowband camera sensitivities are to the method. Finally, we discuss the robustness of our method and the limitation of solving color constancy using the illumination constraint.     

Color constancy based on texture pyramid matching and regularized local regression

Considering that no single algorithm available is universal in color constancy, we propose an effective combination approach using a texture-based matching strategy and a local regression with prior-knowledge regularization. To represent the images, we construct a texture pyramid using an integrated Weibull distribution. Then we define an image similarity measure to search for the K most similar images of the test image. To combine the single algorithms, we integrate prior knowledge into a regularized local regression in a decorrelated color space. Regression weights are obtained on these similar images, and the regularization is implemented by the frequency ratio of the best single algorithm. Experiments on two real world datasets show our approach outperforms the state-of-the-art single algorithms and popular combination approaches with a performance increase of at least 29% compared to the best-performing single algorithm w.r.t median angular error.     

Design space dimensionality reduction through physics-based geometry re-parameterization

The effective control of the extent of the design space is the sine qua non of successful geometry-based optimization. Generous bounds run the risk of including physically and/or geometrically nonsensical regions, where much search time may be wasted, while excessively strict bounds will often exclude potentially promising regions. A related ogre is the pernicious increase in the number of design variables, driven by a desire for geometry flexibility—this can, once again, make design search a prohibitively time-consuming exercise. Here we discuss an instance-based alternative, where the design space is defined in terms of a set of representative bases (design instances), which are then transformed, via a concise, parametric mapping into a new, generic geometry. We demonstrate this approach via the specific example of the design of supercritical wing sections. We construct the mapping on the generic template of the Kulfan class-shape function transformation and we show how patterns in the coefficients of this transformation can be exploited to capture, within the parametric mapping, some of the physics of the design problem.     

彩色亮度计光源色度的不确定度分析

JJG 211-2005亮度计检定规程已颁布实施,它同时囊括了亮度计和彩色亮度计的检定,增加了光源色度参数的校准.在实施实践中,校准彩色亮度计所用的标准反射色板采用了色差计校准用的一套标准反射色板,校准时的标准值取标准反射色板溯源证书中的A/2,0/45的数据.从彩色亮度计中光源色度参数的实验数据,以及对其测量结果的不确定度分析,均证明新规程校准方法正确,而共用标准反射色板亦可行,校准效果良好.     

Linear models in color constancy theory

Linear models in color constancy theory face, at least, five major problems different in nature, to be solved as a prerequisite for a satisfactory theory of the phenomenon. Solutions are proposed.     

Increments and decrements in color constancy

The present study examines whether increment-decrement asymmetries reported in a number of recent center-surround situations occur in more complex images as well. Subjects saw the CRT simulation of a whole uniformly illuminated array of foreground surfaces presented against a large background surface and, for a number of different viewing contexts, made achromatic settings over a wide range of luminance values. Three results emerged. First, subjects' achromatic loci did not fall on a single straight line in color space but rather fell on two separate lines intersecting at some point in this space. Second, the intersection points were not identical to but dependent largely on background color and showed only small effects of foreground colors. Third, cone signals that were decremental relative to the intersection point were more responsive to illuminant changes than cone signals that were incremental, the latter additionally showing some variation with foreground colors. The results are interpreted in terms of increment-decrement asymmetries. They suggest that these asymmetries occur in more complex images as well.     

Laser Induced Coffee-Ring Structure through Solid-Liquid Transition for Color Printing

Metallic micro/nano structures with special physicochemical properties have undergone rapid development owing to their broad applications in micromachines and microdevices. Ultrafast laser processing is generally accepted as an effective technology for functional structures manufacture, however, the controllable fabrication of specific metallic micro/nano structures remains a challenge. Here, this work proposes a novel strategy of laser induced transient solid-liquid transition to fabricate unique structures. Through modulating the transient state of metal from solid to liquid phase using the initial pulse excitation, the subsequent ultrafast pulse-induced recoil pressure can suppress the plasma emission and removal of liquid phase metals, resulting in the controllable fabrication of coffee-ring structures. The solid-liquid transition dynamics, which related with the transient reflectivity and plasma intensity, are revealed by established two temperature model coupled with molecular dynamics model. The coffee-ring structure exhibits tunable structure color owing to various optical response, which can be used for color printing with large scale and high resolution. This work provides a promising strategy for fabricating functional micro/nano structures, which can greatly broaden the potential applications.     

Color space transformations for digital photography exploiting information about the illuminant estimation process

The color reproduction accuracy is a key factor to the overall perceived image quality in digital photography. In this framework, both the illuminant estimation process and the color correction matrix concur in the formation of the overall perceived image quality. To the best of our knowledge, the two processes have always been studied separately, thus ignoring the interactions between them. We investigate here these interactions, showing how the color correction transform amplifies the illuminant estimation errors. We demonstrate that incorporating knowledge about the illuminant estimation behavior in the optimization of the color correction matrix makes it possible to alleviate the error amplification. Different strategies to improve color accuracy under both perfect and imperfect white point estimations are investigated, and the experimental results obtained with a digital camera simulator are reported.     

Reevaluation of color constancy algorithm performance

The relative performance of color constancy algorithms is evaluated. We highlight some problems with previous algorithm evaluation and define more appropriate testing procedures. We discuss how best to measure algorithm accuracy on a single image as well as suitable methods for summarizing errors over a set of images. We also discuss how the relative performance of two or more algorithms should best be compared, and we define an experimental framework for testing algorithms. We reevaluate the performance of six color constancy algorithms using the procedures that we set out and show that this leads to a significant change in the conclusions that we draw about relative algorithm performance as compared with those from previous work.     

彩色亮度计光源色度的不确定度分析

JJG211—2005亮度计检定规程已颁布实施,它同时囊括了亮度计和彩色亮度计的检定,增加了光源色度参数的校准。在实施实践中,校准彩色亮度计所用的标准反射色板采用了色差计校准用的一套标准反射色板,校准时的标准值取标准反射色板溯源证书中的A／2,0／45的数据。从彩色亮度计中光源色度参数的实验数据,以及对其测量结果的不确定度分析,均证明新规程校准方法正确,而共用标准反射色板亦可行,校准效果良好。     

Rare-earth doped solid-state phosphor with temperature induced variable chromaticity

Temperature induced variable chromaticity lanthanide multidoped solid-state phosphors are presented. The phosphors are composed of ytterbium-sensitized multiple-doped (Tm, Er, Ho) PbGeO₃-PbF₂-CdF₂ glass excited at 1.064 µm. The temperature induced color variation exploits the heat enhanced effective absorption cross-section of the ytterbium sensitizer under quasi-resonance excitation. The temperature enhancement of the energy-transfer mechanism between the sensitizer and the appropriate active light emitter allows the selective intensity control of the RGB emissions due to distinct excitation routes. The suitable combination of active ions generated variable chromaticity light with CIE-1931 coordinates changing from CIE-X = 0.283; Y = 0.288 at 20 °C to CIE-X = 0.349;Y = 0.412 at 190 °C, and CIE-X = 0.285; Y = 0.361 at 25 °C to CIE-X = 0.367; Y = 0.434 at 180 °C in Yb³⁺/Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺/Er³⁺ samples, respectively.     

Sensor sharpening for computational color constancy.

Sensor sharpening [J. Opt. Soc. Am. A 11, 1553 (1994)] has been proposed as a method for improving computational color constancy, but it has not been thoroughly tested in practice with existing color constancy algorithms. In this paper we study sensor sharpening in the context of viable color constancy processing, both theoretically and empirically, and on four different cameras. Our experimental findings lead us to propose a new sharpening method that optimizes an objective function that includes terms that minimize negative sensor responses as well as the sharpening error for multiple illuminants instead of a single illuminant. Further experiments suggest that this method is more effective for use with several known color constancy algorithms.