Computing Science, School of

Receive updates for this collection

Spectral Compression: Weighted Principal Component Analysis versus Weighted Least Squares

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-02
Abstract: 

Two weighted compression schemes, Weighted Least Squares (wLS) and Weighted Principal Component Analysis (wPCA), are compared by considering their performance in minimizing both spectral and colorimetric errors of reconstructed reflectance spectra. A comparison is also made among seven different weighting functions incorporated into ordinary PCA/LS to give selectively more importance to the wavelengths that correspond to higher sensitivity in the human visual system. Weighted compression is performed on reflectance spectra of 3219 colored samples (including Munsell and NCS data) and spectral and colorimetric errors are calculated in terms of CIEDE2000 and root mean square errors. The results obtained indicate that wLS outperforms wPCA in weighted compression with more than three basis vectors. Weighting functions based on the diagonal of Cohen’s R matrix lead to the best reproduction of color information under both A and D65 illuminants particularly when using a low number of basis vectors.

Document type: 
Conference presentation
File(s): 

Subspace-Clustering-Based Multispectral Image Compression

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-11
Abstract: 

This paper describes a subspace clustering strategy for the spectral compression of multispectral images. Unlike standard PCA, this approach finds clusters in different subspaces of different dimension. Consequently, instead of representing all spectra in a single low-dimensional subspace of a fixed dimension, spectral data are assigned to multiple subspaces having a range of dimensions from one to eight. For a given compression ratio, this tradeoff reduces the maximum reconstruction error dramatically. In the case of compressing multispectral images, this initial compression step is followed by lossless JPEG2000 compression in order to remove the spatial redundancy in the data as well.

Document type: 
Conference presentation
File(s): 

Gaussian Illuminants and Reflectances for Colour Signal Prediction

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-11
Abstract: 

An alternative to the von Kries scaling underlying the chromatic adaptation transforms found in colour appearance models such as CIECAM02 is suggested for predicting what the colour signal (e.g., XYZ) reflected from a surface under a first illuminant is likely to become when lit instead by a second illuminant. The proposed method, G2M, employs metameric Gaussian-like functions to model the illuminant and reflectance spectra. The method’s prediction is based on relighting the Gaussian-like reflectance spectrum with the second Gaussian-like illuminant. Tests show that the proposed G2M method significantly outperforms CIECAT02.

Document type: 
Conference presentation
File(s): 

Metamer Mismatch Volumes of Flat Grey

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-11
Abstract: 

Metamer mismatching refers to the fact that two objects reflecting light causing identical colour signals (i.e., cone response or XYZ) under one illumination may reflect light causing nonidentical colour signals under a second illumination. As a consequence of metamer mismatching, two objects appearing the same under the first illuminant can be expected to appear different under the second illuminant. Metamers of the flat grey reflectance (i.e., 50% across the visible spectrum) are of particular interest since they show the potential seriousness of metamer mismatching. Metamer mismatching of flat grey is very significant for some lights and includes the possibility of 20 objects having the same colour signal as flat grey under red light dispersing into a whole hue circle under a neutral (“white”) light. Flat grey under LED illumination is also shown to have a significant metamer mismatch volume when the light is changed to D65.

Document type: 
Conference presentation
File(s): 

How Multi-Illuminant Scenes Affect Automatic Colour Balancing

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-05
Abstract: 

Many illumination-estimation methods are based on the assumption that the imaged scene is lit by a single course of illumination; however, this assumption is often violated in practice. We investigate the effect this has on a suite of illumination-estimation methods by manually sorting the Gehler et al. ColorChecker set of 568 images into the 310 of them that are approximately single-illuminant and the 258 that are clearly multiple-illuminant and comparing the performance of the various methods on the two sets. The Grayworld, Spatio-Spectral-Statistics and Thin-Plate-Spline methods are relatively unaffected, but the other methods are all affected to varying degrees.

Document type: 
Conference presentation
File(s): 

Metamer Mismatching as a Measure of the Color Rendering of Lights

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-05
Abstract: 

We propose a new method for evaluating the colour rendering properties of lights. The new method uses the degree of metamer mismatching for the CIE XYZ corresponding to flat grey (constant reflectance of 0.5) quantified in terms of the metamer mismatch volume index proposed by Logvinenko et al. (Logvinenko 2014). A major advantage of this method is that unlike many previous color rendering indices it does not depend on the properties of a chosen set of representative test objects.

Document type: 
Conference presentation
File(s): 

Comparing Colour Camera Sensors Using Metamer Mismatch Indices

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-05
Abstract: 

A new method of evaluating the colorimetric accuracy of a color camera is proposed that is based on the size (appropriately normalized) of the metamer mismatch volume induced by a change of 'observer' from camera to human eye and vice-versa. The degree of metamer mismatching indicates the range in the discrepancy of the colour signals that can arise and as such is a more well-founded measure of colorimetric accuracy than traditional spectral-based measures such as the root mean squared difference in fit between the camera and eye's sensitivity functions.

Document type: 
Conference presentation
File(s): 

Robust Chroma and Lightness Descriptors

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-10
Abstract: 

New descriptors for lightness and chroma are presented that are based on properties of a wraparound Gaussian metameric to the given XYZ tristimulus coordinates. For the 1600 samples of the Munsell glossy set, both descriptors are found to correlate to Munsell value and chroma at least as well as the corresponding CIECAM02 descriptors when the Munsell samples are under the CIE C illuminant. However, when the illuminant is changed the new descriptors were found to be considerably more consistent under the second illuminant than those of CIECAM02.

Document type: 
Conference presentation
File(s): 

Gaussian-Based Hue Descriptors

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-04
Abstract: 

A robust and accurate hue descriptor that is useful in modeling human color perception and for computer vision applications is explored. The hue descriptor is based on the peak wavelength of a Gaussian-like function (called a wraparound Gaussian) and is shown to correlate as well as CIECAM02 hue to the hue designators of papers from the Munsell and Natural Color System color atlases and to the hue names found in Moroney’s Color Thesaurus. The new hue descriptor is also shown to be significantly more stable under a variety of illuminants than CIECAM02. The use of wraparound Gaussians as a hue model is similar in spirit to the use of subtractive Gaussians proposed by Mizokami et al., but overcomes many of their limitations.

Document type: 
Article
File(s): 

Rethinking Colour Constancy

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-09
Abstract: 

Colour constancy needs to be reconsidered in light of the limits imposed by metamer mismatching. Metamer mismatching refers to the fact that two objects reflecting metameric light under one illumination may reflect non-metameric light under a second; so two objects appearing as having the same colour under one illuminant can appear as having different colours under a second. Yet since Helmholtz, object colour has generally been believed to remain relatively constant. The deviations from colour constancy registered in experiments are usually thought to be small enough that they do not contradict the notion of colour constancy. However, it is important to determine how the deviations from colour constancy relate to the limits metamer mismatching imposes on constancy. Hence, we calculated metamer mismatching’s effect for the 20 Munsell papers and 8 pairs of illuminants employed in the colour constancy study by Logvinenko and Tokunaga and found it to be so extensive that the two notions—metamer mismatching and colour constancy—must be mutually exclusive. In particular, the notion of colour constancy leads to some paradoxical phenomena such as the possibility of 20 objects having the same colour under chromatic light dispersing into a hue circle of colours under neutral light. Thus, colour constancy refers to a phenomenon, which because of metamer mismatching, simply cannot exist. Moreover, it obscures the really important visual phenomenon; namely, the alteration of object colours induced by illumination change. We show that colour is not an independent, intrinsic attribute of an object, but rather an attribute of an object/light pair, and then define a concept of material colour in terms of equivalence classes of such object/light pairs. We suggest that studying the shift in material colour under a change in illuminant will be more fruitful than pursuing colour constancy’s false premise that colour is an intrinsic attribute of an object.

Document type: 
Article
File(s):