Peter Morovic

Colour constancy using the chromagenic constraint

In this paper we propose that two images are captured of every scene: a normal image and an image captured where a coloured filter is placed in front of the camera. This additional information is then used in solving for colour constancy. The novelty of our approach is not that we add a colour filter (this is an old idea) but in how we use the additional information. In contradistinction to previous work we propose that the dimensionality of the 6 measurements per image pixel remains at 3 (not 6): we do not add a filter to increase the number of degrees of freedom but rather as a way of estimating the illuminant. We say that a filter is chromagenic if the relationship between filtered and unfiltered RGBs varies with and depends strongly on illumination. The canonical chromagenic algorithm works by testing the applicability of pre-computed relations in situ in an image. We extend the chromagenic approach to incorporate knowledge of the gamut of colours we expect to see under a given light and so in effect we make a hybrid gamut mapping + chromagenic algorithm. Experiments validate our approach with chromagenic gamut mapping shown to deliver significantly better constancy than all other algorithms tested.

Metamer-set-based approach to estimating surface reflectance from camera RGB

We present an approach to estimating the reflectance of a surface given its camera response. In this approach we first solve the general form of this problem: we calculate the set of all possible surface reflectances, called the metamer set, and then choose a member from this set. Three possibilities in choosing a single reflectance are described here. First, we assume that all reflectances are equally likely and minimize worst-case error. Second, we adopt the assumption that reflectances follow a normal probability distribution and maximize this probability. Finally, we assume that reflectances are smooth and maximize this property. The results of our experiments show that there is significant benefit from the proposed approach in terms of the accuracy of the estimation compared with that of standard estimation methods. Moreover, the present approach introduces a notion of robustness of estimates in the form of error bars.

A multi-spectral image database and its application to image rendering across illumination

We present a database of full spatial and spectral resolution images of everyday objects captured with a novel multi-spectral device: the applied spectral imaging Spectracube camera. We summarise the operation of the device and present a study of its spectral accuracy before demonstrating the usefulness of the data set by using it to evaluate different white point correction transforms: transforms which map RGBs under one illuminant to a corresponding set of RGBs under a different light. We find that while the best transforms can often give results close to the results obtained using spectral techniques, the results are both image and illuminant dependent and in a number of cases all transforms tested introduce significant errors.

Large color gamut displays with diffraction gratings

The ability to display a broad variety of colors has great benefits not only in the context of entertainment but also as a means to streamline design in prototyping and manufacturing processes. Displays that use RGB filters or backlights cannot span all colors that occur in nature. To improve the accuracy of color reproduction, there have been attempts to include additional color primaries in displays. Existing solutions, however, have an impact on cost, scalability, and spatial resolution and are predominantly applicable to projection systems. We propose an approach based on combining diffraction grating extractors and the HANS imaging pipeline initially developed for printing. This combination offers unprecedented potential to attain large color gamuts with the same backlights commercially used today.

Gamuts of input and output color imaging media

The color gamuts of color imaging media are important parameters in the reproduction of color images between them and their assumed magnitudes directly influence the degree to which colors are modified. In spite of this, the determination of gamut boundaries is often done in a way that ignores some basic implications that follow from the definition of color gamuts. This is partly due to the fact that some of these implications are not understood and partly due to the fact that if they are understood their magnitude is underestimated. Hence, the approach that is taken in this paper is to first discuss the theoretical implications of what color gamuts are and subsequently to illustrate them by experimental means..

Direct Pattern Control Halftoning of Neugebauer Primaries

Halftoning is a key stage of any printing image processing pipeline. With colorant-channel approaches, a key challenge for matrix-based halftoning is the co-optimization of the matrices used for individual colorants, which becomes increasingly complex and over-constrained as the number of colorants increases. Both choices of screen angles (in clustered-dot cases) or structures, and control over how individual matrices relate to each other and result in over-versus side-by-side printing of the colorants, impose challenging restrictions. The solution presented in this paper relies on the benefits of a halftone area Neugebauer separation pipeline, where local Neugebauer Primary use is specified at each pixel and where halftoning can be performed using a single matrix, regardless of the number of colorants. The provably complete plane dependence of the resulting halftones will be presented among the solution’s benefits.

Computational Print Control

Printing may seem like a dinosaur among today’s imaging technologies, since its roots stretch back to Becquerel’s work that lead to the first color photographs and the first mechanical color reproduction at the end of the nineteenth century. Ten years ago, we then made the fundamental discovery of a new print control domain, where instead of choices about colorant amounts that are akin to the effect of color filters used since the beginning of color printing, print can be specified by the probabilities of colorant combinations, the Neugebauer Primaries. This has led to the ability to print patterns that were previously inaccessible and consequently, by using large-scale computational optimization, to delivering more color gamut, greater ink use efficiency and greater sharpness and detail in print, while using the same materials and printing system as before. This keynote will present the basic principles of the HANS print control paradigm, review the highlights of results obtained using it to date and indicate its potential future developments.