Airbrush Simulation for Artwork and Computer Modeling Jonathan Konieczny∗ Gary Meyery University of Minnesota Department of Computer Science and Engineering Digital Technology Center Abstract is very easily varied mid-stroke, allowing the production of artis- tic effects that are difficult to achieve with a bristle brush (see the A computerized airbrush system with a full three dimensional air- fade away stroke in Figure 2 for an example of such effects). These brush interface is presented. The position and orientation of an unique properties make the airbrush a commonly used tool in the electronic airbrush tool is tracked in space, and, when the trigger artistic and design worlds. While most photo-retouching is now is pulled, paint is sprayed onto two and three dimensional objects done using programs such as PhotoShop, the airbrush is still em- displayed on a computer monitor. The experimentally derived paint ployed by commercial artists to produce architectural renderings spray model used for the airbrush takes into account factors such [Dombek and Porter 2003], medical illustrations [Misstear 1984], as air to paint ratio, viscosity, and distance of the airbrush from the advertising signs, and automotive graphics. Fine artists use the air- work. Paint mixing between colors applied to the surface is mod- brush to apply ceramic glazes [Misstear 1984], to decorate textiles eled using Kubelka-Munk theory. Computerized stencils, including [Maurello 1955], and to create wall murals. Model makers add semi-permeable stencils, can be manually positioned by the artist detail to a wide variety of objects including train dioramas [Caiati or projected onto the object’s surface. Two and three dimensional 1985], costume masks, stuffed animals [Mitchel 2008], wax figures, examples of traditional airbrush artwork, produced using the sys- and fish lures. tem, are presented. The system can also be used as a modeling tool While bristle brush painting has been fairly well researched in com- to decorate three dimensional objects. puter graphics, airbrush has not. Computerized airbrushing has sev- eral differences with simulation of bristle brushes. First, no haptics CR Categories: H.5.1 [Information Interface and Presentation]: are needed to perform a proper simulation. While the feel of a bris- Multimedia Information Systems—Artificial, augmented, and vir- tle brush against the surface of the painted object is an important tual realities feedback device in bristle painting, an airbrush artist works almost entirely via visual feedback. This significantly lessens the equip- Keywords: Airbrush, Painting, Virtual Reality, Augmented Real- ment requirement for full airbrush simulation versus brush simula- ity, Art tion. Second, most of the potential advantages a real airbrush has over a bristle brush still apply: brush stroke size and rate of appli- cation are easily varied, and a smooth final appearance is achieved 1 Introduction - brush bristles and resulting brush patterns on the canvas need not be simulated. Airbrushing also stands as a prime example of a true Allowing artists to ply their craft on a computer has always been a three dimensional real world interface. Studying its success as an major goal within the field of computer graphics. However, while interface could provide interesting insights into virtual interface de- there are programs that permit artists to create virtual artwork and sign. decorate virtual models, far fewer software packages allow artists to do so using skills they learned with real-world tools such as a paint In this paper, an airbrush simulation tool is introduced that closely brush or sculpture chisel. For example, most drawing and paint- replicates the process of real airbrushing. This is done not only to ing applications available today provide a brush that generates an give a virtual version of the real world tool, as [Baxter et al. 2004] airbrush spray pattern, but few of these programs work with three does for paint brushing, but also to give a new way to decal virtual dimensional objects and none of them provide a true airbrush inter- models and supplement tools such as Photoshop for texture map face. In this paper we present an airbrush tool with correct spray creation. In order to create as realistic a simulation as possible, paint dynamics and a complete three dimensional interface, and we many aspects of airbrushing are explored. First, an existing spray demonstrate how it can be used to produce both traditional airbrush particle simulation is optimized for airbrush simulation. Second, a artwork and texture detail for three dimensional models. Kubelka-Munk paint simulation is provided for accurate rendering of both wet and dry paint layers as they are sprayed onto the canvas The airbrush has several advantages over a traditional paint brush. in real time. Third, a realistic airbrush interface is created which First, it leaves no brush stroke marks, permitting the creation of a closely mimics what a real airbrush looks and feels like. Fourth, much smoother and more realistic image. Second, the rate of ap- interface/artistic tools which airbrush artists use are added into the plication can be easily varied, allowing for very smooth paint gra- system, such as true three dimensional stencils and easy paint color dients. Finally, the width and density of a single airbrush stroke selection. Finally, display of the canvas or model, as it is painted, is provided on a computer monitor. The result is a WYSIWYG sys- ∗e-mail: [email protected] tem which allows existing airbrush artists to create virtual artwork ye-mail: [email protected] as well as non-photorealistic model texture materials. This system has been evaluated by professional airbrush artists, and some of the resulting artwork created from those trials is presented. 2 Relevant Work Traditional brush painting and paint mixing have been heavily re- searched in computer graphics, and both are relevant to the work presented in this paper. Below is a summary of some of this re- search. 2.1 Brush and Spray Particle Simulation Several researchers have simulated the physics of a brush being ap- plied to the surface of a canvas and have used those results to pro- duce painting applications. While the physics of an airbrush are quite different from that of a bristle brush, the end goal is similar: to create a program that realistically simulates the use of that artistic tool by properly modeling the physics of the device. [Chu and Tai 2002] and [Saito and Nakajima 1999] both gave a deformable brush model for simulating oriental style ink painting. [Baxter et al. 2001 and 2004] gives both a deformable brush model as well as a haptic interface device for simulating oil painting. Figure 2: A range of various brush strokes and beginner exercises. [Rudolf et al. 2003] presents both a simulation of wax crayons as Top-Left: Red lines of differing paint thickness are drawn over blue well as the physics of the canvas surface to provide a simulation of lines, created by varying how far the trigger is pulled back. This crayon drawing. also shows the Kubelka-Munk paint mixing. Top-Right: Dots of [Konieczny et al. 2008] provides a simulation of spray paint for au- differing size, but similar thickness, obtained by pulling back from tomotive paint application and training. Our system uses a similar the canvas, but varying the trigger. Top-Right: A fade away stroke, particle based solution for modeling the physics of an airbrush, but obtained by pulling away from the canvas while moving the brush. enhances it for the finer detail and control required for airbrushing Bottom: A shading exercise creating a smooth transition from dark instead of industrial spray painting. to light paint thickness. 2.2 Paint Simulation layer along with all previous layers is condensed into a new single In addition to simulating the physics of the particular paint brush, layer in order to speed the simulation. The solution presented in the paint being used must also be modeled in order to provide a this paper also provides a real time GPU based implementation of convincing result. Some of these models are intended to be used K-M paint mixing. [Rudolf et al. 2003] also uses K-M mixing to along with the brush physics models, while others just take existing simulate wax crayons, but does not do so in real time. images and alter them to appear as if they have been painted using Finally, [Hertzmann 1998 and 2002] uses an algorithmic approach some desired brush pattern. to simulate the appearance of various brush strokes. However, this [Curtis et al. 1997] used shallow water diffusion equations to solve approach only alters the appearance of existing images. watercolor paint mixing. Curtis also presented an implementation for Kubelka-Munk (K-M) paint mixing. The result was a convinc- 2.3 Professional Paint Suites ing simulation of watercolors, but the results could not be displayed to the user interactively. There are a number of professional painting programs that have an airbrush tool, such as Adobe Photoshop. However, these tools The work most similar to our own with respect to its simulation appear to simply mimic a random spray pattern on a two dimen- of paint mixing and layering is [Baxter et al. 2004]. This work sional surface, they do not actually simulate spray particles in three uses a GPU implementation of K-M mixing. As one paint is added dimensions, and they do not give the user a true airbrush interface. onto another, they are mixed and the result of that mix is presented Programs such as BodyPaint 3D or Z-Brush do allow a user to paint back to the user interactively. Once a particular layer has dried, that directly on a three dimensional surface, but they employ a standard two dimensional input device such as a mouse or a tablet PC and they do not provide three dimensional stenciling capabilities.