Computer graphics processing and selective visual display system – Computer graphics processing – Graph generating
Reexamination Certificate
1998-02-18
2001-03-06
Powell, Mark R. (Department: 2772)
Computer graphics processing and selective visual display system
Computer graphics processing
Graph generating
Reexamination Certificate
active
06198489
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the use of a computer to produce an image that appears to be produced by hand, and more specifically, to the use of a computer to generate a watercolor image or to modify an existing image that is not a watercolor so that it appears to be a watercolor.
BACKGROUND OF THE INVENTION
Watercolor is unlike any other artistic medium. It has a translucent, luminous quality. It can be applied in delicate layers to achieve subtle variations in color, and it can be used to create a wide variety of beautiful textures and patterns.
Since it is possible to characterize the most important effects of watercolor, it is clearly desirable to simulate these characteristics automatically, with a computer. It is apparent that computer generated watercolor can be used as part of an interactive watercolor paint system, or alternatively, as a method for automatic image “watercolorization.”
A watercolor simulator should be largely empirically-based; it may incorporate some physically-based models, but need not be a strict physical simulation. Instead, the simulator should enable the user to re-create, synthetically, the salient features of watercolor in a way that is both predictable and controllable. Absolute physical realism is only a secondary goal.
There exists a long line of important work on simulating artists' traditional media and tools. However, the prior art does not address certain areas. Relative to the prior art on this topic, a more sophisticated paper model, a more complex shallow water simulation, and a more realistic rendering and optical compositing of pigmented layers is needed. Further, a novel approach is required that will provide many additional watercolor effects such as edge darkening, hard edges, backruns, and separation of pigments. These effects are necessary to produce a look that is much closer to real watercolors, and to capture more of the feeling of transparency and luminosity that is characteristic of the medium. An optical compositing method is particularly important for getting the appropriate look for transparency of multiple layers.
In the commercial realm, certain watercolor effects are provided by prior art products such as Fractal Design Painter and by a plug-in for Adobe Photoshop. Yet, these products do not appear to give as realistic watercolor results as would be desired. Others working on this topic have explored the effects of sumie painting, and that work has been applied to calligraphy. A related prior art model of ink diffusion through paper resembles, to some extent, the water simulation techniques useful in producing the characteristic pigment flow of watercolor.
Other research work on modeling thick, shiny paint and the effects of bristle brushes on painting and calligraphy is also relevant to this problem, by providing a plausible simulation of traditional artists' tools. It may be desirable to include a brushstroke model in the simulation process to enable the creation of the bristle patterns in a dry brush effect. The brushstroke model would thus serve as a complement to paper texture effects in the dry brush model.
SUMMARY OF THE INVENTION
In accord with the present invention, a method is defined for producing a watercolor simulationi that represents at least one wash applied to paper, each wash including at least one pigment dispersed in a fluid. The method includes the step of providing a fluid model that includes a plurality of layers for each wash included in the watercolor simulation. As a function of a plurality of predefined parameters and in accord with the fluid model, the manner in which each wash flows over the paper is specified to determine quantities of each pigment deposited at a plurality of discrete spatial locations on the paper. An optical composite model is applied to render the watercolor simulation as a function of optical characteristics of each pigment and as a function of the quantities of each pigment deposited at the plurality of discrete spatial locations on the paper.
The plurality of parameter-s preferably include physical properties of each pigment employed in producing the watercolor simulation, physical properties of the paper used for the watercolor simulation, and/or predetermined values used in defining the fluid model. Furthernore, the quantities of each pigment deposited at the plurality of discrete spatial locations are preferably stored in an array. A data structure that includes the arrays for the pigments used in a wash comprises a glaze.
A Kubelka-Munk model is preferably employed as the optical composite model for rendering the watercolor simulation. The plurality of layers in the fluid model include a shallow fluid layer (or in a preferred embodiment, a shallow water layer) in which the wash flows above a surface of the paper; a pigment deposition layer where a pigment is adsorbed and desorbed by the paper; and a capillary layer within the paper in which the wash is diffused by a capillary action. The step of specifying how each wash flows over the paper includes the step of determining a movement of the wash in the shallow fluid layer, as a function of a velocity of the wash as it moves over the paper. In addition, this feature of the invention preferably includes the step of reducing a divergence of a velocity field for each wash by redistributing the wash, so that the divergence in a specific time interval is less than a predetermined value.
In specifying how each wash flows over the paper, the method provides for reducing the quantity of pigment at the discrete spatial locations as a function of the velocity of the wash away from the discrete spatial locations. A rate of absorption and desorption for the pigment in each wash is determined as a function of the predefined parameters and is used to determine the quantities of each pigment deposited at the discrete spatial locations.
An edge darkening effect is determined as a function of the predefined parameters, by increasing the quantities of each pigment at discrete spatial locations corresponding to edges of areas in the watercolor simulation covered by a wash. Also, the method may include determining a diffusion of a wash through the capillary layer, thereby increasing a wet-area in the watercolor simulation, to simulate a backrun effect.
A user interface is provided that enables a user to selectively adjust parameters in at least a subset of the predefined parameters. The user interface further enables the user to select pigments that will be used in the watercolor simulation and the user can specify a color of a pigment based upon how the pigment appears when applied over a specific background color. In implementing this option, the user specifies the color components of the pigment as a function of its appearance as applied over both a black background and a white background; the color components arc then used to determine a set of absorption coefficients and scattering coefficients for each pigment.
When creating a watercolor simulation interactively, the user selects at least a subset of the predefined parameters that will be applied to each layer, and provides a wet-area mask for at least one layer.
Alternatively, the watercolor simulation may be based on a color image that is not a watercolor. In this case, the method further includes the step providing a color image for conversion to the watercolor simulation. Mattes are extracted for specific portions of the color image. An ordered set of pigments to be applied to the mattes, and color separations for the ordered set of pigments are determined. Automatic brushstroking is applied when rendering the color separations. It is also contemplated that the color image may comprise a three-dimensional geometric model.
A further aspect of the present invention is directed at a system that includes a memory for storing a plurality of machine language instructions and a device for rendering the watercolor simulation. A processor is coupled to the device and to the memory and executes the machine instructions to implement a
Anderson Sean
Curtis Cassidy
Salesin David H.
Seims Joshua E.
Nguyen Thu
Powell Mark R.
University of Washington
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