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A Large Image Database for Color Constancy Research

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2003-11
Abstract: 

We present a study on various statistics relevant to research on color constancy. Many of these analyses could not have been done before simply because a large database for color constancy was not available. Our image database consists of approximately 11,000 images in which the RGB color of the ambient illuminant in each scene is measured. To build such a large database we used a novel set-up consisting of a digital video camera with a neutral gray sphere attached to the camera so that the sphere always appears in the field of view. Using a gray sphere instead of the standard gray card facilitates measurement of the variation in illumination as a function of incident angle. The study focuses on the analysis of the distribution of various illuminants in the natural scenes and the correlation between the rg-chromaticity of colors recorded by the camera and the rg-chromaticity of the ambient illuminant. We also investigate the possibility of improving the performance of the naïve Gray World algorithm by considering a sequence of consecutive frames instead of a single image. The set of images is publicly available and can also be used as a database for testing color constancy algorithms.

Document type: 
Conference presentation
File(s): 

Estimating Illumination Chromaticity via Support Vector Regression

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2004-11
Abstract: 

The technique of support vector regression is applied to the problem of estimating the chromaticity of the light illuminating a scene from a color histogram of an image of the scene. Illumination estimation is fundamental to white balancing digital color images and to understanding human color constancy. Under controlled experimental conditions, the support vector method is shown to perform better than the neural network and color by correlation methods.

Document type: 
Conference presentation
File(s): 

Optimal Linear RGB-to-XYZ Mapping for Color Display Calibration

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2004-11
Abstract: 

Color display calibration, in part, involves mapping input RGB values to corresponding output values in a standardized color space such as CIE XYZ. A linear model for RGB-to- XYZ mapping is based on a 3-by-3 linear transformation matrix T mapping data from (linearized) RGB to XYZ. Such a mapping is often determined by least squares regression on the difference between predicted and measured XYZ values. However, since displays are calibrated for viewing by human observers, it likely would be better to optimize relative to a perceptually uniform color space. Two new methods are proposed which optimize the total error relative to CIELAB or CIEDE2000. The first method uses weighted least squares with weights based on the rate of change of CIELAB coordinates as a function of change in XYZ. The second method uses Nedler-Mead nonlinear optimization to minimize directly in CIELAB or CIEDE200. Experiments based on calibrating 2 CRT monitors, 3 LCD monitors and 2 LCD projectors show significantly better results than the standard least squares calibration.

Document type: 
Conference presentation
File(s): 

Retinex in MATLAB™

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2004-01
Abstract: 

Many different descriptions of Retinex methods of lightness computation exist. We provide concise MATLAB™ implementations of two of the spatial techniques of making pixel comparisons. The code is presented, along with test results on several images and a discussion of the results. We also discuss the calibration of input images and the postRetinex processing required to display the output images.

Document type: 
Article
File(s): 

Nonlinear RGB-to-XYZ Mapping for Device Calibration

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2005-11
Abstract: 

We introduce a new non-linear method for RGB-to-XYZ color calibration based on the technique of thin plate splines. Originally, thin plate splines were designed for deformable matching between 2-dimensional images for object recognition. We use 3-dimensional thin plate splines to map between sets of RGB device coordinates and corresponding sets of CIE XYZ coordinates. Tests calibrating several displays as well as a camera show thin plate spline calibration to be more accurate than existing linear or non-linear calibration methods.

Document type: 
Conference presentation
File(s): 

Illumination Estimation from Dichromatic Planes

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2005-11
Abstract: 

Adopting the dichromatic reflection model under the assumption of neutral interface reflection, the color of the illuminating light can be estimated by intersecting the planes that the color response of two or more different materials describe. From the color response of any given region, most approaches estimate a single plane on the assumption that only a single material is imaged. This assumption, however, is often violated in cluttered scenes. In this paper, rather than a single planar model, several coexisting planes are used to explain the observed color response. In estimating the illuminant, a set of candidate lights is assessed for goodness of fit given the assumed number of coexisting planes. The candidate light giving the minimum error fit is then chosen as representative of the scene illuminant. The performance of the proposed approach is explored on real images.

Document type: 
Conference presentation
File(s): 

Multispectral Color Constancy: Real Image Tests

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2007-01
Abstract: 

Experiments using real images are conducted on a variety of color constancy algorithms (Chromagenic, Greyworld, Max RGB, and a Maloney-Wandell extension called Subspace Testing) in order to determine whether or not extending the number of channels from 3 to 6 to 9 would enhance the accuracy with which they estimate the scene illuminant color. To create the 6 and 9 channel images, filters where placed over a standard 3-channel color camera. Although some improvement is found with 6 channels, the results indicate that essentially the extra channels do not help as much as might be expected.

Document type: 
Conference presentation
File(s): 

Object Recognition and Pose Estimation across Illumination Changes

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2007-03
Abstract: 

In this paper, we present a new algorithm for color-based object recognition that detects objects and estimates their pose (position and orientation) in cluttered scenes observed under uncontrolled illumination conditions. As with so many other color-based object-recognition algorithms, color histograms are also fundamental to our approach; however, we use histograms obtained from overlapping subwindows, rather than the entire image. Furthermore, each local histogram is normalized using greyworld normalization in order to be as less sensitive to illumination as possible. An object from a database of prototype objects is identified and located in an input image by matching the subwindow contents. The prototype is detected in the input whenever many good histogram matches are found between the subwindows of the input image and those of the prototype. In essence, normalized color histograms of subwindows are the local features being matched. Once an object has been recognized, its 2D pose is found by approximating the geometrical transformation most consistently mapping the locations of prototype’s subwindows to their matched subwindow locations in the input image.

Document type: 
Conference presentation
File(s): 

Automatic White Balancing via Gray Surface Identification

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2007-11
Abstract: 

The key to automatic white balancing of digital imagery is to estimate accurately the color of the overall scene illumination. Many methods for estimating the illumination’s color have been proposed [1-6]. Although not the most accurate, one of the simplest and quite widely used methods is the gray world algorithm [6]. Borrowing on some of the strengths and simplicity of the gray world algorithm, we introduce a modification of it that significantly improves on its performance while adding little to its complexity.

Document type: 
Conference presentation
File(s): 

Skin Colour Imaging that is Insensitive to Lighting Conditions

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2008-06
Abstract: 

In previous human skin models, it has been suggested that the colour of human skin is mostly determined by the concentration of melanin in the epidermal layer combined with the concentration of hemoglobin in the dermal layer. The colour of facial skin changes significantly with changes in the light incident upon it. In this paper we propose a method of normalizing the skin tones of human faces that eliminates the effects of illumination, preserving the skin colour and allowing variations related to melanin concentration only. The method assumes the illumination is reasonably well modelled as blackbody radiation.

Document type: 
Conference presentation
File(s):