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For inhomogeneous materials, the standard reflectance model suggests that under all viewing geometries surface reflectance functions can be described as the sum of a constant function of wavelength (specular) and a diffuse function that is characteristic of the material. As the viewing geometry varies, the relative contribution of these two terms v...
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... Shoji Tominaga et al ont aussi montré que l'expression de la réflexion spéculaire met en exergue la géométrie de la substance, par contre celle de la réflexion diffuse fait ressortir les caractéristiques chimiques et optiques du corps [39]. ...
... Geostatistical re-sampling Table 1) [39], [40]. Those Image Quality Assessment parameters are defined by: ( 2 + 2 )( ̅ 2 + ̅ 2 ) (26) where ...
Une surveillance efficace par un diagnostic précoce et exact est cruciale pour la lutte contre la pandémie du paludisme. En effet, le paludisme est une maladie infectieuse qui touche près de la moitié de la population mondiale et tue une personne toutes les 30 secondes dans les zones endémiques.Dans ce travail, nous présentons une nouvelle méthodologie de diagnostic basée sur l’imagerie multispectrale. La première contribution permet de réduire le temps d’acquisition des images de cellules sanguines tout en conservant le processus de pré-traitement de normalisation. L’objectif de ce pré-traitement est de soustraire les bruits optiques et électriques, d’assurer l’homogénéité de la luminosité. Nous nous appuyons, pour cela sur des méthodes de statistique spatiale (géostatistique) pour la reconstitution de l’image de référence basée sur la seule image de l’échantillon. Une deuxième approche de reconstruction est le ré-échantillonnage par Bootstrap. Aussi, avons-nous adressé les bruits électroniques par la méthode de détection et d’imputation des données aberrantes d’Hampel. La solution proposée est très rapide et s’exécute en moins de 20 secondes. Nous avons ensuite mis en place un processus de détection des érythrocytes, en exploitant la standardisation statistique des images multispectrales en transmission, des algorithmes de segmentation adaptative, de ligne de partage des eaux et la fermeture de contour par hystérésis.Afin d’isoler des cellules parasitées, une procédure de classification est proposée sur 12 descripteurs de textures de cellules segmentées pour plusieurs longueurs d’onde et trois géométries (diffusion, réflexion et transmission). L’analyse en composantes principales pour données fonctionnelles multivariées a été effectuée sur les données ainsi constituées avant une classification non supervisée, par les algorithmes de partitionnement K-moyennes et classification ascendante hiérarchique (CAH) permettant d’isoler les classes de cellules parasitées et saines. Lesquelles sont ensuite utilisées pour une classification supervisée des cellules.En définitive, nous avons réduit au tiers le temps d’acquisition qui est passé de 12 à 4 minutes. De plus, les deux processus de segmentation et de classification combinés s’exécutent à environ 8 minutes, ceci pourrait donner lieu à un diagnostic en temps réel.En outre, le processus de segmentation ne nécessite aucun prétraitement. Il présente ainsi un avantage dans la prise en compte des images de mauvaise qualité (faible contraste).Par ailleurs, nos méthodes surpassent celles de l'état de l’art en termes d’amélioration du contraste, d’indice de similarité et erreur quadratique moyenne pour ce qui de la normalisation. La segmentation quant à elle a enregistré une précision de 98,47%, rappel de 98,23% et un degré de précision de 98,34%.
... In the real world, however, most objects are non-Lambertian, and so have some glossy or highlight component. The combination of matte reflectance together with a geometry dependent highlight component is modeled by the dichromatic reflectance model [16,19,20,12,17]. ...
The dichromatic reflectance model introduced by Shafer [16] predicts that the colour signals of most materials fall on a plane spanned by a vector due to the material and a vector that represents the scene illuminant. Since the illuminant is in the span of all dichromatic planes, colour constancy can be achieved by finding the intersection of two or more planes. Unfortunately, this approach has proven to be hard to get to work in practice. First, segmentation needs to be carried out and second, the actual intersection computation is quite unstable: small changes in the orientation of a dichromatic plane can significantly alter the location of the intersection point.
... All of these approaches apparently solve colour constancy for colour deficient scenes yet none work well in practice (in images of natural scenes). Perhaps the most studied physics-based colour constancy algorithms, and the ones which show the most (though still limited) functionality, are based on the dichromatic reflectance model for inhomogeneous dielectrics (proposed by Shafer (1985), Tominaga and Wandell (1989Wandell ( , 1990), and others). Inhomogeneous materials are composed of more than one material with different refractive indices, usually there exist a vehicle dielectric material and embedded pigment particles. ...
Statistics-based colour constancy algorithms work well as long as there are many colours in a scene, they fail however when the encountering scenes comprise few surfaces. In contrast, physics-based algorithms, based on an understanding of physical processes such as highlights and interreflections, are theoretically able to solve for colour constancy even when there are as few as two surfaces in a scene. Unfortunately, physics-based theories rarely work outside the lab. In this paper we show that a combination of physical and statistical knowledge leads to a surprisingly simple and powerful colour constancy algorithm, one that also works well for images of natural scenes.
From a physical standpoint we observe that given the dichromatic model of image formation the colour signals coming from a single uniformly-coloured surface are mapped to a line in chromaticity space. One component of the line is defined by the colour of the illuminant (i.e. specular highlights) and the other is due to its matte, or Lambertian, reflectance. We then make the statistical observation that the chromaticities of common light sources all follow closely the Planckian locus of black-body radiators. It follows that by intersecting the dichromatic line with the Planckian locus we can estimate the chromaticity of the illumination. We can solve for colour constancy even when there is a single surface in the scene. When there are many surfaces in a scene the individual estimates from each surface are averaged together to improve accuracy.
In a set of experiments on real images we show our approach delivers very good colour constancy. Moreover, performance is significantly better than previous dichromatic algorithms.
... From a measured set of vectors r r r r and the already calculated r r onebounce , r r nobounce should be derived. For this purpose, the quarter-circle analysis suggested by Tominaga and Wandell [6] is employed. Following this method, all the measured r r r r on surface A are projected onto the v v 1 2 ...
In images, interreflections can change the color and the intensity that is reflected from the surfaces. An approach is presented that minimizes the interreflections in a color image and takes shadows into account. Using the ### color space the color image is segmented into homogeneous regions. All segmented regions are examined for the occurrence of shadow and interreflections. The one-bounce model is used to describe the interreflections in the scene. An analysis of the singular value decomposition is carried out for all pairs of regions that are expected to be affected by interreflections. Results of the minimization are presented for real color images.
... The main problem with the combination of region segmentation and colour object recognition is the need of colour constancy. Although some approaches already exist to achieve colour constancy (e.g.: ( 48]), ( 21]), ( 20]), ( 72]), etc.), the methods are computationally expensive and/or they need assumptions that are rather restrictive. The discussion of colour constancy is out of scope of this review but a robust and e cient algorithm is needed for that purpose. ...
Image segmentation, i.e., identification of homogeneous regions in the image, has been the subject of considerable research activity over the last three decades. Many algorithms have been elaborated for gray scale images. However, the problem of segmentation for colour images, which convey much more information about objects in scenes, has received much less attention of scientific community. While several surveys of monochrome image segmentation techniques were published, similar comprehensive surveys for colour images, to our knowledge, did not emerge.
... They assumed that the specular component of reflectance is constant with wavelength and show that the hue of a surface is constant with respect to changing geometry. Tominaga and Wandell [133] also considered scenes which have a spectral reflection component and varying geometry. ...
This thesis addresses the problem of estimating the surface reflection model of objects observed in a terrestrial scene, illuminated by natural illumination; that is, a scene which is illuminated by sun and sky light alone. This is a departure from the traditional analysis of laboratory scenes, which are illuminated by idealised light sources with positions and radiance distributions that are precisely controlled. Natural illumination presents a complex hemispherical light source which changes in both spatial and spectral distribution with time, terrestrial location, and atmospheric conditions. An image-based approach to the measurement of surface reflection is presented. The use of a sequence of images, taken over a period of time, allows the varying reflection from the scene due to the changing natural illumination to be measured. It is shown that the temporal change in image pixel values is suitable for the parameters of a reflection model to be estimated. These parameters are estim...
... : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 25 5.3 Angle between (0) and (1) , and angle between (2) and the plane described by (0) and (1) (shaded area) in three-dimensional RGB space. 29 6.1 Relative radiant power distributions of the D 65 standard illuminant and a 3200K Planckian (blackbody) radiator. : : : : : : : : : : : : : 31 6.2 Relative response curves for CIE 1931 standard observer lters. ...
... While interre ection can provide additional information for approaching the problem of colour constancy 6,8,13,12], simultaneous solutions invariably lead to poorer estimates of shading and interre ection elds than could be attained by approaching interre ection as a problem in its own right. With the moderate success and continuing research being performed on colour constancy 14,16,20,29,30], it is not unreasonable to assume the relative spectral power distribution of the illuminant is known. However, we need not know the actual intensity of this source. ...
Interreflection (or mutual illumination) occurs when two or more object surfaces are illuminated both by a light source and the light reflected from other surfaces. As the distance or angle between two interreflecting surfaces decreases, the intensity of interreflected light increases, with a corresponding shift in colour known as colour bleeding. For computer vision algorithms that assume spatially invariant surface reflectances, this plays a confounding role. As an example, in the presence of interreflection, "shape-from-shading" methods will incorrectly reconstruct surfaces such that the orientation of their surface normals will appear to be closer to the direction of the illuminant than they actually are.
... However, the number of filters used to acquire a visible spectrum is too large. According to some references, five to twelve principal components estimated by principal component analysis [5] (PCA) are enough for general color recording. We may manufacture specific color filters which have the same characteristics as the estimated principal components. ...
Conventional image acquisition is usually based on the theory of tri-chromacy. Three channels are not enough to record color information. One approach to solve this problem is using multi-spectral image acquisition. We want to establish this system based on some researches [1] [2] in foreign. First step is the measurement of spectral reflectance by spectroradiometer. Second, the measured spectrum will be analyzed by principal component analysis [3] [4] (PCA) to estimate the number of dimensions that needed to representation. Third, the spectral reflectance will be captured using digital camera with 7 filters. Forth, the coefficients of spectral reflectance in new vector space and the digital counts will be analyzed by linear regression to get the transformation matrix between these two spaces. Finally, multi-spectral image acquisition is performed to capture any pictures. Using the digital counts and the transformation matrix, the coefficients respect to the principal components are going to be calculated. According to the coefficients and estimated principal components, the spectral reflectance of objects can be calculated through a linear combination of the principal components directly.
There are two broad classes of colour constancy algorithms: statistical and physics-based. The former attempt to correlate the statistics of the colours in an image with statistical knowledge about light and surfaces. If there is good colour diversity in a scene then the statistical approach often works well. The latter, physics-based algorithms are founded on an understanding of how physical processes such as specularities and interreflection manifest themselves in images. The theory behind physics-based algorithms is both elegant and powerful. Indeed, colour constancy becomes possible even in scenes with as few as two surfaces. Unfortunately the theory rarely translates into practice; most physics-based algorithms do not work outside the lab.
