CONFIGURAL AND PERCEPTUAL FACTORS INFLUENCING THE PERCEPTION OF COLOR TRANSPARENCY THÈSE NO 3181 (2005) PRÉSENTÉE À LA FACULTÉ INFORMATIQUE ET COMMUNICATIONS Institut de systèmes de communication SECTION DES SYSTÈMES DE COMMUNICATION ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Peggy GERARDIN licence de sciences cognitives, Université Lumière, Lyon 2, France et de nationalité française acceptée sur proposition du jury: Prof. S. Süsstrunk, directrice de thèse Prof. M. D'Zmura, rapporteur Prof. M. Herzog, rapporteur Dr K. Knoblauch, rapporteur Lausanne, EPFL 2005 Peggy GERARDIN THÈSE NO 3181 (2005) Acknowledgments Many people supported me during the completion of this thesis with criticism, helpful assistance and refer- ences. This work would have never been possible without them. First of all, I would like to express my gratitude to my supervisor Prof. Sabine Süsstrunk who gave me the opportunity to join her group, the best research environment I ever had. I would like to thank her for her guidance, encouragement and excellent advice throughout this study. I owe a special thank to Dr. Kenneth Knoblauch (INSERM U371, Lyon, France), whose patience, assistance and motivation were greatly appreciated. I stopped counting the numerous emails we sent to each other, the abundant lunches we shared and the million discussions we had on color vision. I would like to give a special thank to Prof. Michael D’Zmura, who welcomed me in his laboratory at the University of California, Irvine (USA). He proposed me a very interesting subject and helped me during all my stay. I thank him for the nice barbecues we had together and private concerts at home. I also thank Prof. Don Hoffman (UCI, USA) for the nice classes I attended to, and for the interesting discussions we had facing some good sushi. I thank all members of my thesis committee, including Prof. Michael Herzog, for accepting to read the thesis and to comment my defense. In particular, I would like to thank my colleague Roberto for being a very quiet officemate, not disturbing me with phone calls and not singing all day long, especially when I had to concentrate at work. I also thank Philippe for continuing working with me, even after his diploma. Many thanks to Vladan for the numerous coffee breaks, and to Jocelyne, Laurence, Patrick, Luciano, Andrea, Williams, Martin and all LCAV members, and for their interesting questions after each of my talks, such as ‘Do Inuits see colors like Europeans’? I really enjoyed working in this group. All my gratitude to my friends who shared these four years in Lausanne with me, in particular to David, Véronique, Elena, Gianluca, Gloria, Daniela and Stefan. I enjoyed travelling and partying with them, especially when I was discouraged and wanted to give up and become a florist. Also many thanks to Erin, for her gentleness and for the numerous rides we had in California. I also thank all my friends in France for their precious support. Last but certainly not least, I would like to thank my parents and my brother for their support and their numerous encouragements for each step that a thesis involves. I really enjoyed our relaxing and refreshing barbecues in our country house. Their visit in California was also one of my best memories. Finally, my warmest thanks go to Nicolas, for his understanding and his fantastic support. We endured numerous geographical distances during all my studies. During my thesis, a total of 72,240 km has been done to see each other. This does not include the time when I was in California. I dedicate this thesis to him, with love. iii iv Acknowledgments Abstract The mechanisms by which the brain represents colors are largely unknown. In addition, the large number of color phenomena in the natural world has made understanding color rather difficult. Color transparency perception, which is studied in this thesis, is precisely one of these interesting phenomena: when a surface is seen both in plain view and through a transparent overlay, the visual system still identifies it as a single surface. Processes of the visual system have widely inspired researchers in many domains such as neuro- sciences, psychology, as well as computer vision. The progress of digital imaging technologies requires research engineers to deal with issues that demand knowledge of human visual processing. To humans, an image is not a random collection of pixels, but a meaningful arrangement of regions and objects. One thus can be inspired by the human visual system to investigate color representation and its applicability to digital image processing. Finding a model of perception is still a challenging matter for researchers among multidisciplinary fields. This thesis discusses the problem of defining an accurate model of transparency perception. Despite the large number of studies on this topic, the underlying mechanisms are still not well understood. Investigating perceptual transparency is challenging due to its interactions with different visual phenomena, but the most intensively studied conditions for perceptual transparency are those involving achromatic luminance and chromatic constraints. Although these models differ in many aspects, a broad distinction can be drawn be- tween models of additive and subtractive transparency. The General Convergence Model (GCM) combines both additive and subtractive color mixtures in showing that systematic chromatic changes in a linear color space, such as translation and convergence (or a combination of both), lead to perceptual transparency. However, while this model seems to be a necessary condition, it is not a sufficient one for transparency perception. A first motivation of this thesis was to evaluate and define situations more general than the GCM. Several chromatic changes consistent or not with the GCM were generated. Additional parameters, such as configural complexity, luminance level, magnitude of the chromatic change and shift direction were tested. The main results showed that observers’ responses are influenced by each of the above cited parameters. Convergences appear significantly more transparent when motion is added for bipartite configurations, or when they are generated in a checkerboard configuration. Translations are influenced by both configuration and motion. Shears are described as opaque, except when short vector lengths are combined with motion: the overlay tends to be transparent. Divergences are strongly affected by motion and vector lengths, and rotations by a combination of checkerboard configuration with luminance level and vector length. These results question the generality of the GCM. We also investigated the effects of shadows on the perception of a transparent filter. An attempt to extend these models to handle transparency perception in complex scenes involving surfaces varying in shape and depth, change in conditions of illumination and shadow, is described. A lightness-matching task was performed to evaluate how much constancy is shown by the subject among six experimental conditions, in which shadow position, shadow blur, shadow and filter blending values were varied. The results showed that lightness constancy is very high even if surfaces were seen under both filter and shadow. A systematic deviation from perfect constancy in a manner consistent with a perceived additive shift was also observed. Because the GCM includes additive mixture and is related to color and lightness constancy, these results are promising and may be explained ultimately by this model. v vi Abstract Version Abrégée Les mécanismes par lesquels le cerveau se représente la couleur sont encore méconnus. De plus, le grand nombre de phénomènes colorés traités rend la tâche encore plus difficile. La perception de la transparence colorée, qui est étudiée dans cette thèse, est justement l’un de ces phénomènes : lorsqu’une surface est vue à la fois sous un filtre et en dehors, le système visuel l’identifie comme une seule et même surface. Les processus de traitement du système visuel ont largement inspiré les chercheurs dans des domaines aussi variés que les neurosciences, la psychologie et les sciences de l’informatique. Les progrès grandissants des technologies de l’imagerie numérique incitent les chercheurs à acquérir des connaissances sur le traitement visuel humain. Pour lui, une image n’est pas une collection de pixels, mais un arrangement de régions et d’objets qui a un sens. Il serait donc utile d’en être inspiré pour explorer la représentation de la couleur et ses applications dans le traitement de l’image numérique. Cette thèse traite du problème de définition d’un modèle précis de la perception de la transparence. Malgré le grand nombre d’études sur ce sujet, les mécanismes sous-jacents à ce phénomène sont encore peu compris. Explorer la perception de la transparence est un défi , dû à ses interactions avec de nombreux phénomènes visuels. Cependant, les modèles les plus étudiés sont ceux relatifs à la luminance achroma- tique et aux contraintes chromatiques. Même si ces modèles diffèrent par de nombreux aspects, ils peuvent être divisés en deux: les modèles de transparence additifs et soustractifs. Le Modèle Général de Con- vergence (GCM) combine ces deux mixtures de couleurs et montrent que des changements chromatiques systématiques dans un espace de couleurs linéaire, tels qu’une tranlation ou une convergence (ou bien la combinaison des deux), permettent la perception de la transparence colorée. Seulement, ce modèle est une condition nécessaire mais non suffisante à cet effet. Nous avons évalué et défini dans cette thèse des situations plus générales que celles qu’englobe le GCM. Plusieurs changements chromatiques cohérents et non cohérents avec ce modèle ont été générés. Des paramètres supplémentaires, tels que la complexité de la configuration, le niveau de luminance, la magni- tude des changements et leurs directions ont été testés. Nos résultats montrent que les réponses des sujets sont influencées par chacun de ces paramètres. Les convergences apparaissent plus transparentes lorsque le mouvement du filtre est ajouté pour les configurations de type bipartite, ou bien lorsqu’elles sont générées pour des configurations de type damier.