Yiwei Tu
Masters of Science Capstone Project, June 2023

[Proposal    Report    Presentation]

Watercolor, as a well-known style of artistic painting, is appealing due to the translucent patterns formed by the spreading of its coloring water-based solution. These translucent patterns are produced by two basic brushing techniques, wet-on-wet and wet-on-dry, and due the stochastic nature of liquid mixture motion, each watercolor painting is distinctive. For newcomers, challenges in creating watercolor paintings include soiled canvas and wrong brush strokes that cannot be corrected due to the limitations of the paper and inconvenient tools.

To offer more freedom to watercolor creation, simulation of watercolor painting has been extensively studied including physically-based methods. The physically-based approach presents watercolor patterns by emulating the physical dynamics of paint and water flow and rendering an image based on the simulated results. The Lattice-Boltzmann method (LBM) is favored by researchers in watercolor fluid dynamic simulation due to its computation efficiency and stability in dealing with complex boundaries, incorporation of microscopic interactions, and potentials for parallel implementation.

The project follows Chu and Tai’s approach of modeling hydrodynamics with LBM and using the Kubelka-Munk (KM) reflectance model rendering method proposed by Curtis et al. Compared to other methods, this approach strikes a balance between accuracy of the simulation model and execution time. The LBE models fluid flow as a continuous propagation and collision process on a discrete lattice where the multiple lattices can be processed in parallel. By trading-off realism, LBE is capable of presenting relatively realistic watercolor results, while the parallel processing significantly reduces the simulation time.
The implementation of the physically-based simulation requires a platform that supports parallel processing at the per-pixel level, user’s interactive drawing activities, and rendering of the simulation results. Unity3D, as a cross-platform game engine, is chosen for the implementation of the system because of its support for user-defined HLSH shaders that can process pixel operations in parallel, a well-designed editor that can accommodate complex parameter adjustments, and pre-built pipelines that can render simulation results.

The implemented simulation system consists of four components: fluid injection, fluid flow simulation, pigment movement, and pigment composition. The first component receives the water applied to the digital canvas from the brush, and updates the edges between the wet and dry canvas. The fluid flow simulation component then simulates the diffusion of the fluid with a semi-rebounding scheme of LBE. The pigment movement component calculates the movement of the paint. As the final step, the rendering component renders the resulting image based on the KM model with the transmittance and reflectance of the pigment layers as input parameters.

The provided watercolor simulation system allows for the manipulation of brush, paper on canvas, and simulation settings. The system can make images based on the two basic brush approaches with popular watercolor patterns such as edge darkening, purposeful backrun, and pigment granulation by adjusting the brush settings. The mixture of multiple pigments might result in new and distinct colors when the KM model is used. Paper and simulation parameters can be adjusted to allow painting on canvases that do not exist in the physical world.

A novice painter can use the features of our system to make simple watercolor paintings. The system proved that the KM model can render watercolor visuals with color blending on the canvas, and that LBM can reduce the computational load of fluid simulations while maintaining simulation realism. The results of this project lay a solid platform for additional, in-depth research into watercolor simulation.