Show HN: Inkwash, a watercolor sketching app and explanation

Inkwash is a 'Show HN' project presenting an innovative browser-based watercolor sketching application, distinguished by its deep fidelity to physical painting processes and an exceptionally well-crafted interactive explanation. The author, John O' Whitaker, designed Inkwash to recreate the experience of nature journaling with a pen and waterbrush, transforming a personal artistic practice into a sophisticated digital tool.

The app's core relies on a stack of floating-point textures and WebGL2 fragment shaders to simulate real-world elements like pigment density, wetness, and water velocity. This approach allows for nuanced effects that traditional digital art tools often miss:

• AI-Assisted Creation: The project began as an exploration with Anthropic's Claude Fable 5, which generated the core codebase and much of the explanatory prose, highlighting AI's potential in software development.
• Fluid Dynamics: It employs Jos Stam's "Stable Fluids" algorithm, using semi-Lagrangian advection, pressure projection, and vorticity confinement to create realistic water movement, swirls, and eddies.
• Paper Physics: A "wetness" field governs pigment mobility and velocity, mimicking how ink behaves on damp or dry paper. Water slowly evaporates, and a "drying" mechanic freezes the fluid simulation, capturing the essential nature of watercolor.
• Chromatography: A key feature is the simulation of color separation (chromatography) by allowing different color channels to diffuse at varying rates, producing the characteristic "halos" seen in real watercolor.
• Mark-Making: Strokes are rendered using gaussian splats, with ink densities adding and wetness blending using a MAX operation. Pressure and speed influence brush effects, and a subtle cursor lag enhances the feel of a real brush.
• Layering and Fixing: A distinct "fixed" layer allows dried pigment to bond, enabling traditional watercolor layering. White gouache is simulated destructively, bleaching underlying colors when applied and fixed.
• Realistic Rendering: The display shader uses the Beer–Lambert law for accurate color blending, preventing muddy overlaps. It also generates realistic paper grain, granulation, edge darkening, and wet sheen.
• Device Agnostic Input: The app supports various input methods, including Apple Pencil with finger as a water brush on iPad, stylus barrel buttons, Force Touch on trackpads, or speed-based pressure faking.
• Technical Elegance: The application itself is a single HTML file with no dependencies, running efficiently even on mobile devices, and the interactive explanatory page ingeniously runs multiple WebGL instances within a single context.

The Inkwash project stands out not just as a functional art tool, but as a compelling demonstration of how complex physics can be rendered interactively in a browser, and how AI can significantly accelerate the creation of such intricate software.