A Reddit Builder Turned a SIGGRAPH Paper Into a Watercolor Simulator With Claude

A watercolor simulator that models actual pigment optics — not a filter, an implementation of a 1997 computer graphics paper — picked up 2.9K upvotes and 130 comments on r/ClaudeAI within about 17 hours of posting in mid-August 2026. The post, titled "I built a watercolor Simulator based on real physics (V2)" by Reddit user u/kouklimou, links to a live, free tool at sudoaquarelle.com. It's the kind of story explainx.ai keeps coming back to in its Claude Opus 5 game-prompt and 3D cell explorer coverage: solo builders using an AI coding agent to ship things that used to require a graphics PhD and a rendering team.

The builder's own framing, from the post: "Last week-end I built a watercolor simulator based on real physics and shared it online. A lot of people asked to play with it so i improved it and it's now live." A video demo is embedded on the original Reddit thread on r/ClaudeAI — search "watercolor simulator" within the subreddit to find it, since Reddit's video CDN uses expiring signed links that don't survive being copied elsewhere. The builder also posted the same project on X, describing it as built on "Curtis et al.'s watercolor model (SIGGRAPH 1997) with a fluid layer, drifting pigment and wet paper fibers".

TL;DR

What's Actually in the Simulator

Visiting the live site confirms the Reddit description holds up. The interface exposes real watercolor controls, not a generic drawing-app skin: pigment library and reset, a "Water Lift" brush property, paper light settings including a Backlight toggle, three water-behavior modes ("Forever wet," "Breathing," "Rain"), and a lab section with Fiber paper, Salt, Alcohol, and X-ray visualization. Standard tools — undo, dry, clear, save, film, import — sit alongside those physics-specific controls. The site's own footer attributes the model directly to "physics after Curtis et al. 1997."

That paper is the real citation, not a marketing flourish. Curtis et al.'s "Computer-Generated Watercolor" is a genuine SIGGRAPH publication that models watercolor as fluid flowing through the porous fiber structure of paper, carrying suspended pigment that settles, granulates, and dries according to concentration and capillary pressure. u/nickdaniels92 dropped the direct link in the Reddit thread as the source, and noted a detail that frames how much cheaper this has gotten: the original 1997 implementation took 7 hours to process 11 glazes of a 640×480 image on a 133MHz SGI MIPS R4600. Modern multicore CPUs are roughly 10,000x faster — what took most of a workday in 1997 now runs sub-second in a browser tab.

Why Kubelka-Munk Matters More Than It Sounds

Curtis et al.'s paper handles fluid movement. It doesn't fully solve pigment color mixing — how two translucent, layered pigments combine optically is a separate, older problem. That's where Kubelka-Munk theory comes in: a 1931 optical model, still used today in paint, printing, and textile industries, that predicts the reflectance of a layered translucent medium from each layer's absorption and scattering coefficients. Sudo Aquarelle's 52-pigment library is calibrated against Kubelka-Munk research specifically so that mixing two colors on the virtual page produces the same kind of result a real pigment mix would — not just an RGB blend, which is exactly the shortcut most "watercolor" filters take and the reason they look flat.

One commenter, u/RyanTorant, who said they were building something similar, asked directly where the builder sourced the Kubelka-Munk coefficients — a fair, specific technical question that the visible thread doesn't show answered, and worth noting as an open gap rather than papering over it.

The Honest Pushback: "That's Not How Watercolor Behaves"

Two commenters laid out exactly why this simulation is hard, and one pushed back on whether it's succeeded. u/EvaUnit343 asked how "real physics" was actually simplified, since 2D flow through porous media, drying dynamics, and contact-line advancement (the physics of how a wet edge creeps across dry paper) are all non-trivial and computationally expensive to simulate faithfully. u/stddealer added the optics side of the same complexity: spectral reflectance distribution per pigment and optical density, stacked on top of the fluid-dynamics factors EvaUnit343 raised.

Then u/Tight_Banana_9692 said flatly: "Yeah, that is not how watercolor behaves at all." That's worth including precisely because most coverage of viral builds skips the dissent. A simplified, real-time, browser-runnable model of a fluid-dynamics-plus-optics system is necessarily an approximation — the same trade-off every real-time graphics simulation makes, from game water to this. Whether that approximation reads as "close enough to be useful" or "not real watercolor" is a legitimate disagreement, and both reactions showed up in the same thread.

What the Community Actually Wants Next

The clearest signal in 130 comments isn't the praise — it's the fact that the same feature request landed independently from two people: wet-on-wet painting, where you lay down water first and then drop pigment that creeps and blooms into the wet area on its own, the signature watercolor effect. Both dmigowski and u/Mr_Sim_ raised it separately, which is a stronger signal than either raising it once.

Other requests, in rough order of how concrete they were:

• iPad app with Apple Pencil support — the most-requested platform gap, unsurprising for a painting tool currently browser-only.
• A confirmation dialog on the reset button — two separate commenters, BenevolentCheese and Bluehelix, reported losing unsaved work to an undoable reset. Two independent data-loss reports on the same control is the kind of bug report that should jump the backlog queue over feature requests.
• Image export — u/shafiq235 asked for a straightforward save-to-file option, distinct from whatever the site's existing "Save" button currently does.

Praise, Comparisons, and the Honest Middle

The praise in the thread was specific rather than generic: "leaves all the other watercolor simulators to dust," "Procreate should pay you for this engine," "first genuinely useful and unique vibe coded app," and multiple people independently describing it as "relaxing to use." Commenters compared it against Rebelle (dedicated experimental watercolor software) and Krita (the long-standing open-source painting app) — both existing, purpose-built tools, which makes the comparison meaningful rather than a strawman. u/SnooShortcuts3821 mentioned building a similar WebGPU-based watercolor simulator inside their own app, "protoly.io" — a reminder that this exact intersection (real fluid/optics models, real-time in a browser) is an active small niche right now, not a one-off stunt.

What nobody answered: u/granoladeer asked what stack was used to deploy the site. That question sits unanswered in the visible thread — worth flagging rather than guessing at a stack that wasn't confirmed anywhere in the source material. If you're curious, the site runs entirely client-side with no login wall, so your browser's dev tools will tell you more than speculation would.

What to Steal for Your Own Build

This is the actual reason a Reddit watercolor demo belongs on an education-and-building blog: the process generalizes past painting apps.

1. Find a real domain paper before you prompt. Curtis et al.'s SIGGRAPH paper and Kubelka-Munk theory both predate the AI-coding era by decades — the builder didn't invent new physics, they found the right existing model and had Claude Code translate it into a working, real-time implementation. That's a repeatable move: pick a domain you care about (optics, fluid dynamics, acoustics, materials science), find the canonical paper, and use an agent to turn equations and pseudocode into a browser app.
2. Old "too slow" research is often free performance today. The 1997 implementation needed 7 hours per image; the 10,000x hardware speedup since then is why a full physics model now runs interactively. A lot of "impossible in real time" CS research from the 1990s–2000s is quietly runnable in a browser tab now — that gap is where a lot of these viral builds live.
3. Ship a "Code Mode" or equivalent transparency feature. Printing the simulator's real function calls next to the canvas as you paint turns a black-box tool into something a curious user (or a skeptical Redditor) can actually audit. It's a small addition with an outsized trust payoff — the same instinct behind showing a headless test harness's real measurements rather than a "looks right to me" claim, as covered in explainx.ai's breakdown of the Opus 5 procedural desert explorer.
4. Expect specific pushback, and treat it as free QA. "That's not how watercolor behaves" is a more useful comment than ten "amazing!!" replies — it tells the builder exactly where the model's simplifications are visible to someone who knows the domain. Two independent bug reports (the reset button) beat one detailed feature request every time for prioritization.
5. Post the working link, not just a clip. The thread's traction tracks directly to the fact that sudoaquarelle.com is playable, not just watchable — the same pattern that drove engagement on the Opus 5 desert explorer and the img2threejs demo.

If you want a structured path into this kind of build — agent-driven coding with Claude Code, from prompting through debugging real output rather than guessing — explainx.ai runs a live Claude for Work track and an MCP Bootcamp for connecting Claude to real tools and data, alongside the agent skills guide for packaging repeatable workflows like "translate a paper into code" into something reusable.

Related on explainx.ai

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• How a software engineer built a viral AI 3D cell explorer
• img2threejs: procedural photo to Three.js
• Top 10 Claude Opus 5 game prompts
• What are agent skills? Complete guide
• Top Claude live workshops in 2026
• Agentic fatigue: the vibe coding productivity paradox

Primary sources: Reddit — u/kouklimou, "I built a watercolor Simulator based on real physics (V2)," r/ClaudeAI (search the subreddit directly; Reddit's video CDN uses expiring links, so no direct permalink is embedded here) · Sudo Aquarelle, live site · Curtis et al., "Computer-Generated Watercolor," SIGGRAPH 1997 · Lionel Mora on X.

Upvote counts, comment counts, and quoted usernames reflect the r/ClaudeAI thread as captured roughly 17 hours after posting in mid-August 2026 — Reddit vote totals continue to change after publication. The deployment stack was not confirmed in the visible thread as of this writing; verify against the live site if you need current details.