Fluid simulation · qwen3.6-plus
Create a real-time fluid simulation on a full-screen canvas where dragging the mouse pushes swirling colored dye through the fluid.
transcript · 10 messages
Create a real-time fluid simulation on a full-screen canvas where dragging the mouse pushes swirling colored dye through the fluid.
thinking
The user wants a fluid simulation - a classic Navier-Stokes based fluid simulation with mouse interaction. This is a well-known technique popularized by Jos Stam's "Stable Fluids" paper. I need to create a single self-contained HTML file with: 1. Full-screen canvas 2. Navier-Stokes fluid simulation (velocity field + dye/density field) 3. Mouse interaction to add dye and force 4. Multiple dye colors that swirl and mix 5. All CSS and JS inline I'll implement a GPU-free version using canvas 2D with a grid-based approach: - Velocity field (u, v components) - Dye fields (r, g, b channels) - Advection (semi-Lagrangian) - Diffusion (Gauss-Seidel relaxation) - Projection (make velocity divergence-free) - Mouse adds dye color and force Let me build this comprehensively with good visual quality.
/tmp/ws/index.html (14326 bytes)Successfully wrote 14326 bytes to /tmp/ws/index.html
thinking
The code has a bug - the `renderClean` function uses `offCanvas` which is defined after the function. Also, there's a rendering issue: I'm putting image data at 0,0 but then drawing the main canvas to itself (scaled up) which creates feedback. Let me fix this properly. Let me restructure the rendering to use the offscreen canvas approach correctly.
/tmp/ws/index.html (11653 bytes)Successfully wrote 11653 bytes to /tmp/ws/index.html
thinking
I need to fix the file. The second write succeeded, but looking at my code I see the file starts with `<!DOCTYPE html>` correctly. Let me check the file content.
Let me check the file looks correct:
{"path":"/tmp/ws/index.html","limit":5}<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> [360 more lines in file. Use offset=6 to continue.]
thinking
The file is written successfully. Let me verify it's complete by checking the end of the file.
{"path":"/tmp/ws/index.html","offset":355}dyeStep(dG, dG0); dyeStep(dB, dB0); render(); requestAnimationFrame(loop); } requestAnimationFrame(loop); })(); </script> </body> </html>
thinking
There's an issue in the render function - I'm using `canvas.width` and `canvas.height` but I should be using `displayWidth` and `displayHeight` which don't exist anymore after my refactor. Let me fix that. Actually wait, I removed `displayWidth` and `displayHeight` variables but I'm using `canvas.width` and `canvas.height` which should work since they're set in resize(). Let me check... Actually there's another issue - in the `ambient` function I'm using `for (let j = 1; j <= N; j += 2)` and `for (let i = 1; i <= N; i += 2)` but then I'm indexing `u[IX(i,j)]` and `v[IX(i,j)]`. The step of 2 is fine for performance but I need to make sure the curl noise looks okay. Wait, actually there's a problem - the `render()` function references `canvas.width` and `canvas.height` but in `drawImage` I'm drawing from `off` to the same canvas. Let me check: `ctx.drawImage(off, 0, 0, canvas.width, canvas.height)` - this should work, it draws the offscreen canvas scaled to the display canvas. But wait, there's a potential issue where `ctx` is the same canvas context and we're also using it for putImageData... no, putImageData is on `offCtx`. So this should be fine. Actually wait, there's a subtle bug. In the `render()` function I use `ctx.drawImage(off, 0, 0, canvas.width, canvas.height)`. But `ctx` is the 2D context of `canvas` (the main visible canvas). `off` is a separate canvas. So this should correctly draw the offscreen buffer onto the main canvas. Yes, this is correct. But wait - `canvas.width` and `canvas.height` are set in resize to `window.innerWidth` and `window.innerHeight`, so they should be correct. Let me verify by looking at the resize function... yes: `canvas.width = window.innerWidth; canvas.height = window.innerHeight;` The file looks correct. Let me verify there are no obvious issues by looking at the complete file one more time to check the structure. Actually, I should double-check that the `off` canvas is used correctly. Let me look at the flow: 1. `offCtx.createImageData(N, N)` creates ImageData of size NxN 2. We fill `px` (which is `imgD.data`, same as `offImgData.data`... wait, let me re-check) Looking at the code: ``` const imgD = offCtx.createImageData(N, N); const px = imgD.data; ``` Then in render: ``` offCtx.putImageData(imgD, 0, 0); ctx.drawImage(off, 0, 0, canvas.width, canvas.height); ``` Wait, there's an issue. `imgD` is created by `offCtx.createImageData()` which creates a NEW ImageData each time. But `px` is a reference to `imgD.data`. Then we modify `px` in the render loop and call `offCtx.putImageData(imgD, 0, 0)`. This should work. Actually, `imgD` is created once, `px` references its data array. In each render call, we modify `px` and then call `putImageData(imgD, ...)`. This is correct. But there's one issue: `imgD` is created by `offCtx.createImageData(N, N)`. But this allocates N×N pixels = 256×256 = 65536 pixels × 4 bytes = 262144 bytes. And we loop from j=1 to N and i=1 to N, which is N×N = 65536 iterations. The `px` array has N*N*4 = 262144 elements. So pi goes from 0 to (N*N*4 - 4) = 262140. That's correct. Wait no, the off canvas is N×N = 256×256. But the imageData from createImageData(N,N) has width N and height N, so N*N*4 bytes. Our loop fills N*N pixels. Yes, this matches. One concern: the off canvas is N×N but we only fill (N+2-2)×(N+2-2) = N×N pixels in the imageData. Wait, let me look more carefully: - off.width = N = 256 - off.height = N = 256 - imgD = offCtx.createImageData(N, N) → creates 256×256 image - We loop j from 1 to N (256 iterations) and i from 1 to N (256 iterations) → 65536 pixels - px has 65536*4 = 262144 elements But wait, pi increments by 4 each iteration, starting from 0. So pi goes: 0, 4, 8, ..., 4*(256*256-1) = 4*65535 = 262140. That's valid. But we're only filling 256*256 = 65536 pixels in a 256*256 image. That's the entire image. Good. Everything looks correct. The file should work fine. Actually, let me reconsider one thing. The `off` canvas dimensions are N×N = 256×256. The `imgD` is also N×N. We fill all pixels in the loop. Then we draw the off canvas onto the main canvas at full screen size. This should work correctly. I think the file is correct and complete. Let me verify it exists and give a summary.
`/tmp/ws/index.html` is ready — 365 lines, zero external dependencies. **What it does:** - **Navier-Stokes fluid sim** on a 256×256 grid (Jos Stam's stable fluids method): diffuse → advect → project steps each frame - **RGB dye channels** — three independent density fields for red, green, and blue - **Click & drag** paints swirling colored dye; color hue shifts as you move - **Right-click drag** pushes the fluid without adding color (for mixing) - **Touch support** for mobile - **Ambient curl noise** + periodic random vortices keep the fluid alive even when idle - **Semi-Lagrangian advection** with bilinear interpolation for smooth, stable flow - **Gauss-Seidel pressure projection** keeps the velocity field divergence-free (incompressible) - Renders the sim grid to an offscreen canvas and **scales it up** with high-quality smoothing to fill the full screen
TAKE IT TO CODE
A single self-contained HTML file — copy it and open in any browser, or reuse the prompt with your own pi-agent run.
RUN
EVALUATION
Fluid dye sim reacts to drag and wheel with bright swirling color, but click/keys barely visible and dye fades to near black over time.
Vision grade · anthropic/claude-sonnet-5
MEASUREMENTS
Raw readings from the capture, not a verdict — the grade above is the only judgement. Percentages are how much of the screen changed; the measure under-reports sparse motion on a dark background.