Colour cluster analysis for pigment identification

This paper presents image processing algorithms designed to analyse the colour CIE Lab histogram of high resolution images of paintings. Three algorithms are illustrated which attempt to identify colour clusters, cluster shapes due to shading and finally to identify pigments. Using the image collection and pigment list of the National Gallery London large numbers of images within a restricted period have been classified with a variety of algorithms. The image descriptors produced were also used with suitable comparison metrics to obtain content-based retrieval of the images

Spectral modeling of a six-color inkjet printer

After customizing an Epson Stylus Photo 1200 by adding a continuous-feed ink system and a cyan, magenta, yellow, black, orange and green ink set, a series of research tasks were carried out to build a full spectral model of the printers output. First, various forward printer models were tested using the fifteen two color combinations of the printer. Yule- Nielsen-spectral-Neugebauer (YNSN) was selected as the forward model and its accuracy tested throughout the colorant space. It was found to be highly accurate, performing as well as a more complex local, cellular version. Next, the performance of nonlinear optimization-routine algorithms were evaluated for their ability to efficiently invert the YNSN model. A quasi-Newton based algorithm designed by Davidon, Fletcher and Powell (DFP) was found to give the best performance when combined with starting values produced from the non-negative least squares fit of single-constant Kubelka- Munk. The accuracy of the inverse model was tested and different optimization objective functions were evaluated. A multistage objective function based on minimizing spectral RMS error and then colorimetric error was found to give highly accurate matches with low metameric potential. Finally, the relationship between the number of printing inks and the ability to eliminate metamerism was explored

Colorimetria hiperespectral de pinturas artísticas

Tese de doutoramento em Ciências (área de conhecimento em Física)Os museus e galerias de arte recorrem a uma grande variedade de fontes de luz para iluminação de pinturas artísticas. A preocupação principal dos responsáveis na escolha das fontes de luz neste contexto é a prevenção dos danos provocados pela radiação nas obras de arte. No entanto, a impressão visual de uma pintura artística é fortemente influenciada pela intensidade e perfil espectral do iluminante. Consequentemente, este é também um aspecto importante a considerar na selecção da iluminação mais adequada. Apesar da óbvia importância dos aspectos visuais não existe neste momento uma metodologia segura de avaliação da qualidade global de reprodução cromática de uma fonte de luz nem estudos sistemáticos da relação entre os efeitos visuais e a distribuição espectral da iluminação. Esta relação, entre o perfil espectral da fonte de luz e a aparência visual dos cenários iluminados, assume particular relevância no desenvolvimento de novas fontes de luz, que podem hoje ter um espectro quase arbitrário, como, por exemplo, nas novas fontes de luz baseadas em LEDs. Os objectivos gerais deste trabalho foram determinar de forma empírica, através de experiências psicofísicas, o perfil espectral da iluminação preferida por observadores para apreciar pinturas artísticas e estudar, de forma sistemática, a relação entre perfil espectral e os efeitos cromáticos da fonte de luz em pinturas. A parte experimental e a parte computacional do trabalho basearam-se na análise dos espectros de reflectância de um conjunto de pinturas obtidos com um sistema de imagiografia hiperespectral parcialmente desenvolvido no âmbito do trabalho desta tese. Das experiências psicofísicas foi concluído que os observadores preferem uma luz do dia com uma temperatura correlacionada de cor de cerca de 5000 K e que essa preferência pode ser, em parte, determinada pela diversidade cromática percebida nas pinturas. Da análise computacional foi concluído que a diversidade cromática máxima não é obtida com espectros de iluminação com variações suaves mas sim com espectros muito estruturados e com um número de bandas espectrais reduzido e localizadas em zonas específicas do espectro visível. Além disso, à máxima diversidade cromática correspondem valores baixos do índice de reprodução cromático, sugerindo que cores naturais e boa discriminação de cores são, de alguma forma, incompatíveis. Estes resultados no seu conjunto põem em causas algumas práticas correntes em iluminação de pinturas artísticas e constituem um avanço significativo no conhecimento sobre os efeitos visuais da iluminação.The museums and art galleries use a large variety of light sources for lighting art paintings. The main concern of the curators when selecting a light source is the prevention of damage caused by light irradiation. However, the impression of an artistic painting is strongly influenced by the intensity and spectral profile of the illumination, and this is also an important issue to consider. Although the visual aspects are of obvious importance there is still no firm methodology to assess the global colour quality of lighting or systematic studies of the relationships between the visual effects and the spectral profile of the illumination. The relationship between illuminant spectral structure and visual appearance assumes particular relevance with the advent of light sources with almost arbitrary spectral distribution, like modern LED based lighting. The general goals of this work were to determine psychophysically the spectral profile of the illumination preferred by observers when seeing artistic paintings and to study systematically the relationships between spectral profile of the illumination and its chromatic effects on paintings. The experimental and the computational parts of this work were based on the analysis of the reflectance spectra of a set of paintings obtained with a spectral imaging system partly developed in the context of the work of this thesis. From the psychophysical experiments it was concluded that observers prefer daylight with a correlated colour temperature of about 5000 K and that the chromatic diversity perceived may influence the preference for a specific illuminant. From the computational analysis it was concluded that the maximum chromatic diversity is not obtained with smooth spectral variations but with spectrally structure spectra and with a small number of spectral bands located in specific regions of the visible spectrum. In addition, when the maximum chromatic diversity is obtained the colour rendering index is low suggesting that natural colours and large discrimination are incompatible. These results challenge some of the current practices in illumination of artistic paintings and represent a considerable advancement on the knowledge about the visual effects of illumination.Fundação para a Ciência e a Tecnologia (FCT) - Bolsa de Doutoramento SFRH BD 49112 2008

Image segmentation and pigment mapping of cultural heritage based on spectral imaging

The goal of the work reported in this dissertation is to develop methods for image segmentation and pigment mapping of paintings based on spectral imaging. To reach this goal it is necessary to achieve sufficient spectral and colorimetric accuracies of both the spectral imaging system and pigment mapping. The output is a series of spatial distributions of pigments (or pigment maps) composing a painting. With these pigment maps, the change of the color appearance of the painting can be simulated when the optical properties of one or more pigments are altered. These pigment maps will also be beneficial for enriching the historical knowledge of the painting and aiding conservators in determining the best course for retouching damaged areas of the painting when metamerism is a factor. First, a new spectral reconstruction algorithm was developed based on Wyszecki’s hypothesis and the matrix R theory developed by Cohen and Kappauf. The method achieved both high spectral and colorimetric accuracies for a certain combination of illuminant and observer. The method was successfully tested with a practical spectral imaging system that included a traditional color-filter-array camera coupled with two optimized filters, developed in the Munsell Color Science Laboratory. The spectral imaging system was used to image test paintings, and the method was used to retrieve spectral reflectance factors for these paintings. Next, pigment mapping methods were brought forth, and these methods were based on Kubelka-Munk (K-M) turbid media theory that can predict spectral reflectance factor for a specimen from the optical properties of the specimen’s constituent pigments. The K-M theory has achieved practical success for opaque materials by reduction in mathematical complexity and elimination of controlling thickness. The use of the general K-M theory for the translucent samples was extensively studied, including determination of optical properties of pigments as functions of film thickness, and prediction of spectral reflectance factor of a specimen by selecting the right pigment combination. After that, an investigation was carried out to evaluate the impact of opacity and layer configuration of a specimen on pigment mapping. The conclusions were drawn from the comparisons of prediction accuracies of pigment mapping between opaque and translucent assumption, and between single and bi-layer assumptions. Finally, spectral imaging and pigment mapping were applied to three paintings. Large images were first partitioned into several small images, and each small image was segmented into different clusters based on either an unsupervised or supervised classification method. For each cluster, pigment mapping was done pixel-wise with a limited number of pigments, or with a limited number of pixels and then extended to other pixels based on a similarity calculation. For the masterpiece The Starry Night, these pigment maps can provide historical knowledge about the painting, aid conservators for inpainting damaged areas, and digitally rejuvenate the original color appearance of the painting (e.g. when the lead white was not noticeably darkened)