1. Introduction
　　"Sumi-Nagashi" provides painters (system users) a method for creating digital art by using tactile feelings of digital colors and a fluid canvas (Fig. 1). Conventional digital tools appeal only to the senses of sight and sound. We employed a force feedback system, which we call Proactive Desk [1], as a digital canvas to incorporate the tactile sensation in the digital creation process.
　　We believe that the experience of using this system will prompt new inspiration and give digital painters the opportunity to experience again how important touch is to creativity in art [2].

2. Concept of Sumi-Nagashi
　　Artists create an attractive piece of art by deeply understanding and interacting with the materials and tools through physical contact [3]. The importance of corporeality in the field of art is well known by all artists. However, most of the artistic forms expressed through current computer technologies involve only the senses of sight and sound. Consequently, it has been difficult to incorporate into artworks the artist's sense of corporeality, although this is critical to the creative process.
　　Today's "Digital Artists" have had to perceive tactile feelings artificially, using non-tactile tools and digital materials with only visual attributes, along with their imaginative powers, to create the illusion of corporeality. This may lead one to assume that there are limitations to the new media art in comparison with the classical arts. But we think that this new art form is only in its infancy.
　　The "Sumi-Nagashi" system is designed to bring corporeality into the field of digital art by refining the relationship between digital colors as virtual materials and the sense of touch. We created this system to expand upon the kind of corporeality that can only be achieved through digital technologies. This implies that our ultimate goal is to present the same level of experience that painters feel. They precisely grasp the viscosity of paints and the texture of the canvas through a delicate sense of touch, gradually forming the desired expression.

3. Implementation using Proactive Desk

Figure 1 : Installation of "Sumi-Nagashi."

　　The user is able to draw a picture naturally on a physical desk. The desktop is actually a type of digital canvas. A digital picture is drawn through interaction with the user, and it is projected onto the desktop by an overhead projector. The user holds a paintbrush-shaped device and moves it directly across the desktop as though it were a real brush. Its trace is colored by digital paints, and the system updates the image on the desktop. In addition to a layer for applying colors, this canvas has another layer defined by a virtual flow (Fig. 2). This layer affects the painted images by making them change constantly, from moment to moment.
　　The force feedback system, Proactive Desk [1] developed by ATR, is employed as the digital canvas. It calculates the force resulting from the action of drawing and provides haptic feedback, through the paintbrush-shaped device, on the physical force along the desktop surface. The force that the user feels reflects virtual tactile attributes of digital colors and the floating canvas, as explained in the next section.

Figure 2 : Structure of Sumi-Nagashi's canvas.

4. Drawing Functions and Force Generation
　　Various drawing functions are implemented in our system. "Paint" and "Smudge" are functions for applying a selected color and for mixing and spreading colors already on the canvas. They work in the same way as in typical digital painting software. Additionally, there is another function for gradually removing away colors placed on the canvas by an effect perceived as "swinging water." This new expression developed and enhanced by digital media will inspire the user as he or she creates artworks.
　　When the user moves the paintbrush-shaped device, he or she is able to perceive a change in the tactile sensation through the paintbrush-shaped device as well as seeing the change. Here, we modeled the physical haptic feeling from digital colors and canvas as a sensation deriving from three kinds of virtual force: inertia force, color friction, and flow resistance (Fig. 3).

Figure 3: Basic concept of virtual sensations.

　　The user is able to feel tactile impressions of colors while drawing. We implemented this feeling by mimicking "inertia force." The general idea of the "weight" of a color derives from the psychological impression of color. We designed it as the most basic attribute of color. Some people get a "heavy" feeling from dark colors and a "light" feeling from light ones. In other words, they depend on the luminosity of the color. Next, "color friction" gives the user a feeling for the textures on the canvas. It depends on contrast and changes in colors. For example, the user will feel more friction from a striped area and perceive an edge at the boundary of colors. Finally, "flow resistance" is defined as the resisting force against the virtual water stream. This determines how much the user's paintbrush-shaped device is pushed along the flow direction.
　　The user will feel a large force in the following cases: a dark color is selected; rapid movement of the brush occurs; the brush crosses a high-contrast area and the edge of colors; and the brush runs quickly with streaming flow of color.

5. Conclusion
　　The Sumi-Nagashi system has been demonstrated at several exhibitions such as SIGGRAPH 2003, Japan Media Arts Festival 2003, and L'ORÉAL Prize Workshop 2004, and many people have actually used the system in this installation. In the future, we will verify the usefulness of our approach by considering comments from digital artists who use this tool to create artworks.

References

[1]	H. Noma, S. Yoshida, Y. Yanagida and N. Tetsutani: The Proactive Desk: A New Haptic Display System for a Digital Desk Using a 2-DOF Linear Induction Motor, Presence, Vol. 13 No. 2, pp. 146-153, 2004.
[2]	S. Yoshida, J. Kurumisawa, H. Noma, N. Tetsutani and K. Hosaka: Sumi-Nagashi: Creation of New Style Media Art with Haptic Digital Colors, ACM Multimedia 2004, pp. 636-643, 2004.
[3]	S. Zeki: Inner Vision: An Exploration of Art and the Brain, Oxford University Press, 2000.