A Computer-Assisted Colorization Algorithm based on Topological Difference

National audienceRegion-based approaches to cel painting typically use shape similarity and topology relations between regions of consecutive animation frames. This paper presents a new colorization algorithm based on topological differences defined over a hierarchical graph of adjacent regions, which allows an almost full automatic colorization process. Also this paper discusses other attributes that improve the solution of the image association problem

Le Rendu Expressif Assisté par l'Ordinateur

Date de rédaction: du 01 mai au 30 août 2010 - Date de soumission: 01 septembre 2010Digital edition, creation, and design bring the computer's extraordinary flexibility to the act of creation. The near absence of penalty for experimentation can be, however, both a blessing and a curse. Although the unprecedent flexibility of the computer encourages exploration of artistic ideas, it also can be a factor of great distraction to the artist. A great challenge for the development of graphics softwares today is, thus, to take advantage of computer's unprecedented flexibility, while offering to the user the adapted level of control that she or he is looking for. The difficulty when designing such adapted system is highly influenced by the expectations users place on the job performed by computers. As the computer places itself in the role of an assistant in the creation process, we expect it to be 'smart enough' to deserve this place. Usually, we do not accept well when the result of such assistance is not satisfactory. Nevertheless, the level of expectation, and consequently, satisfaction, is not the same for all users. Novice users tend to be less exigent and are often happy when the computer help them to create interesting results without the need of a strong user intervention. Professional users, on the other hand, usually have different needs. They are looking forward to softwares that are good enough in automatizing tasks, but also that provide intuitive controls to manipulate their results. For these users, the bigger is the impact of the work generated by the computer in the final result, the more these tasks are subjected to have control parameters. In this thesis, we demonstrate that the computer can be placed as an incomparable assistant into the visual creation process once well-adapted interaction systems are provided. Adaptation is thus an important aspect because, in different levels, users want to be able to have the control over the result in order to give their touch and express their creativity. The thesis presents two different scenarios demonstrating that controllability plays a major role in expressiveness: a real-time technique to cluster a dynamic 3D scene in order to achieve an automatic, yet controllable output that can then be used as input to any rendering style; and a number of adaptations to a drawing vector graphics primitive called Diffusion Curves that constrain and control the creation of complex color gradients. The thesis proposes, on one hand, mathematical and computational well-adapted representations to provide the user with different levels of control over algorithms of expressive rendering in order to manipulate two- and tridimensional scenes; and, on the other hand, to reproject this technical support into user-friendly software solutions.L'édition, la création, et le design digital mènent la flexibilité des ordinateurs vers l'acte de création. La quasi-absence de conséquences par l'expérimentation peut être à la fois une bonne et une mauvaise chose. Bien que la flexibilité des ordinateurs encourage l'exploration d'idées artistiques, elle peut également la freiner, en proposant trop d'options mal-adaptées. Un grand défi pour le développement des logiciels graphiques aujourd'hui est de tirer parti de la flexibilité sans précédent des ordinateurs, tout en offrant à l'utilisateur le niveau de contrôle adapté à ses besoins. La difficulté quand on développe un tel système est fortement influencée par l'attente que l'utilisateur place dans le travail effectué par l'ordinateur. L'ordinateur étant considéré comme un assistant dans le processus de création, on attend de lui qu'il soit 'assez intelligent' pour mériter ce rang. Habituellement, nous n'acceptons pas bien quand le résultat d'une telle assistance n'est pas satisfaisante. Cependant, le niveau d'attente et, par conséquence, de satisfaction, varie en fonction de l'utilisateur. Les utilisateurs novices ont tendance à être moins exigeants et sont satisfaits quand ils peuvent créer des résultats intéressants avec peu d'intervention humaine. Les utilisateurs professionnels, d'un autre côté, ont des besoins différents. Ils recherchent des logiciels assez performants pour automatiser leurs taches, mais qui leur laissent assez de contrôle sur le résultat obtenu. Pour cette catégorie d'utilisateurs, plus l'impact du logiciel sur le résultat est important, plus ils veulent avoir le contrôle sur la façon dont est obtenu ce résultat. Dans cette thèse, nous démontrons que l'ordinateur peut être un assistant précieux dans le processus de création visuel du moment que des systèmes d'interaction adaptés existent. L'adaptation est donc un aspect important car, à différents niveaux, les utilisateurs veulent pouvoir contrôler le résultat afin d'exprimer leur style et leur créativité. La thèse présente deux scénarios différents qui démontrent que la contrôlabilité joue un rôle majeur dans l'expressivité : une technique temps-réel pour grouper des scènes 3d dynamiques pour obtenir automatiquement un résultat sur lequel on peut facilement appliquer un style de rendu arbitraire; et plusieurs dérivées d'une primitive de dessin vectoriel appelée Diffusion Curves qui contraignent et contrôlent la création de dégradés de couleurs complexes. La thèse propose, d'un coté, des représentations mathématiques et algorithmiques adaptées avec différents niveaux de contrôle sur les algorithmes de rendu expressif qui manipulent des scènes bi et tridimensionnelles; et, d'un autre coté, de traduire ces outils en interface intuitive pour l'utilisateur

An image-based shading pipeline for 2d animation

Shading for cel animation based on images is a recent research topic in computer-assisted animation. This paper proposes an image-based shading pipeline to give a 3D appearance to a 2D character by inspecting the hand-drawn image directly. The proposed method estimates normal vectors on the character’s outline and interpolates them over the remaining image. The method does not limit the animator’s creative process and requires minimal user intervention. The resulting shading pipeline can be easily applied to photorealistic and non-photorealistic 2D cel animation. In the proposed method, the animator can easily simulate environment reflections on the surface of 2D reflecting objects. As far as the authors are concerned, the proposed technique is the only one in the literature that is genuinely an image-based method for 2D animation

A Computer-Assisted Colorization Algorithm based on Topological Difference

National audienceRegion-based approaches to cel painting typically use shape similarity and topology relations between regions of consecutive animation frames. This paper presents a new colorization algorithm based on topological differences defined over a hierarchical graph of adjacent regions, which allows an almost full automatic colorization process. Also this paper discusses other attributes that improve the solution of the image association problem

A Computer-Assisted Colorization Algorithm based on Topological Difference

Region-based approaches to cel painting typically use shape similarity and topology relations between regions of consecutive animation frames. This paper presents a new colorization algorithm based on topological differences defined over a hierarchical graph of adjacent regions, which allows an almost full automatic colorization process. Also this paper discusses other attributes that improve the solution of the image association problem. 1

Diffusion Constraints for Vector Graphics

Session: Research papersInternational audienceThe formulation of Diffusion Curves [Orzan et al. 2008] allows for the flexible creation of vector graphics images from a set of curves and colors: a diffusion process fills out the parts of the image that are away from curves. However, this model has limitations in certain situations and does not always seem to agree with how an artist wants to use the software. First, the diffusion itself cannot be controlled, only the colors. Further, the fact that color needs to be defined everywhere along the curve can lead to tedious and nonintuitive interactions. In this paper, we present a number of adaptations to diffusion curves that constrain how color is spread across the image. Specifically, we argue for the utility of controlling the speed and direction of the color diffusion, and the ability to have barriers that can be defined without the need to specify a particular color along these curves. We also describe how this can be implemented by solving a linear system, and demonstrate the effectiveness of our solution on a number of examples

Controllable Diffusion Curves

Thème : Acquisition, Representation and Transformations for Image SynthesisThis report introduces new drawing tools for controlling color diffusion in an image. It builds on the Diffusion-Curve algorithm, which computes images by diffusing colors from user-defined constraint curves. However, control of the diffusion process was limited in significant ways. In this report, we adapt the representation of Diffusion Curves by allowing artists to specify color strength, diffusion directions, and non-diffusing barriers. We also describe the algorithmic changes necessary for their efficient implementation. We demonstrate how these extensions give artists more control, and streamline the editing process in common situations.Dans cet rapport nous présentons plusieurs outils de contrôle de la diffusion de couleurs dans une image. Ces outils sont mis en œuvre sous forme d'extensions de l'approche "courbes de diffusions" qui consiste à calculer une image à partir de contraintes placées le long de courbes. Nous proposons ici d'étendre la représentation par courbes de diffusion pour permettre à un artiste de dé finir localement la puissance de la diffusion, sa direction ainsi que des courbes barrières qui peuvent stopper la diffusion. Nous décrivons de plus les modifications algorithmiques nécessaires pour une implémentation efficace

3D Dynamic Grouping for Guided Stylization

International audienceIn art, grouping plays a major role to convey relationships of objects and the organization of scenes. It is separated from style, which only determines how groups are rendered to achieve a visual abstraction of the depicted scene. We present an approach to interactively derive grouping information in a dynamic 3D scene. Our solution is simple and general. The resulting grouping information can be used as an input to any "rendering style". We provide an efficient solution based on an extended mean-shift algorithm customized by user-defined criteria. The resulting system is temporally coherent and real-time. The computational cost is largely determined by the scene's structure rather than by its geometric complexity