Fluid simulation · qwen3.8-27b

Need to build a stable-fluids (Jos Stam) simulation in a single index.html. Use a grid-based grid (e.g. about 96×64), velocity field + dye field, semi-Lagrangian advection, a few iterations of Gaussian splat or simple projection. For performance, Canvas 2D using ImageData, or use WebGL. Canvas 2D with a coarse grid upscaled via drawImage is simple and fast enough: N≈128×90 → about 11500 cells, Gauss-Seidel 20 iterations for pressure — 11500*20 ≈ 230K ops/frame, no problem in JS.

Plan:
- Grid W=128, H=80 (or adaptive to aspect ratio).
- Fields: u, v, u0, v0, dyeR, dyeG, dyeB (+ temporary arrays).
- Steps: add forces (from mouse drag), velocity advection, projection, advection... standard stable fluids: vel_step: add force, diffuse (skip, use light damping), project, advect, project. Dye: advect, fade.
- Rendering: ImageData on a small offscreen canvas at grid size, putImageData, then drawImage upscaled to the full screen with imageSmoothingEnabled.
- Controls: color input, pause/resume button, clear button.
- Initial seed: several dye blobs + vortices.
- Damping so it dissipates.

Use a simple projection with ~14 iterations of Gauss-Seidel. Actually, to be safe on performance, 16 iterations with 128×80.

Let's write the code.

Grid: N = 96 wide? Use N=120, M=80. Cells = 9600. Pressure iterations 18 → about 170K. No problem.

Semi-Lagrangian advection using bilinear interpolation.

Forces: on mousemove during drag, add dye and velocity to cells near the pointer using a splat radius.

Fade: dye *= 0.995 per frame; velocity *= 0.999.

Rendering: map dye to ImageData with clamping, add a bit of ambient background color.

Also add curl (vortex) forcing to maintain vortices? Optional; a good idea for keeping "persistent vortex flow." Add a small curl force.

Controls are a top overlay, with pointer events on the canvas.

Let's write it.

Code structure:

```js
const NW=120, NH=80;
size = (NW+2)*(NH+2) with borders.
idx(i,j) = i + (NW+2)*j
```

Functions: addSource, advect, project, set_bnd.

Standard Stam code. Then render.

Pointer events: pointerdown → dragging, track last position; pointermove → splat along the line from the last position to the current position; velocity = (dx,dy) scaled.

Splat: for cells within radius r (about 3 cells), add dye color * w and velocity.

Initial seed: 3 dye blobs with circular velocity.

Pause toggle: skip the physics, keep rendering.

Clear: zero out all fields.

FPS guard: fixed 1 step per frame, dt clamped.

Write index.html, then do browser test: check screenshot, drag, inspect via console. The browser tool provides click/drag. Use the browser tool: screenshot, drag.

Alright, let's write it.

Test in the browser: screenshot, drag, verify the flow after release, drag in the opposite direction, clear, drag again.

The canvas is almost pitch dark. The seed's dye should be visible. The seed value is about 0.9 * wgt — at the center wgt=1, rr=0.9 → 1-exp(-1.26)≈0.72 → the color should be visible. But in the screenshot, nothing is displayed other than the dark background. Maybe the dye just hasn't been rendered yet? Or maybe the seed's dye is too dark to be seen. 8+247*0.72 = 186. It should be a bright cyan blob. Hmm, maybe something failed — a console warning? Let me check with inspect for errors and wait a bit. Maybe the canvas is being drawn correctly, but the image is very small... Let me inspect the DOM / check for JS errors.

no console errors. It's possible that the seed blob is spreading out and fading (1.5 seconds × 0.995 fade ≈ 0.995^90 ≈ 0.64, so it should still be visible). Or maybe the dye is too dark? A 120x80 blob should be visible. Wait — there might be a bug: in render, k starts from 0 and loops over i=1..NW, j=1..NH — this is correct, and fills the NW*NH pixels of the off-screen canvas.

Hmm, could the dye be getting wiped out by advection? No, the dye is preserved. Let me try using browser tools to evaluate JS in the page? The browser tool doesn't have an eval feature... only reload/inspect/click/drag/key/type/scroll/screenshot/motion.

Maybe the screenshot is being taken before rAF draws the first frame? "Frame 1" — the screenshot might be of the first frame before the animation runs. But the seed dye should appear after a few frames. Let me wait a bit and take another screenshot — use the motion action to capture a frame over time.

Still pitch black even after 2 seconds. Something's wrong with rendering. Possible causes: canvas is black, but the drawImage of off — could the dye field be zero? Let me check: seed writes r/g/b. render reads them. Hmm — but wait: in `render`, the background is drawn as 8+247*(1-exp(-rr*1.4)). If rr≈0 then the pixel would be (8,10,16) — dark, which matches the background. So the dye must be 0. Why?

Check the `step` call: the first rAF frame has dt>0 → step executes. Dye advection uses u,v — that's fine. But the seed is called before the first frame... it should be visible at frame 1.

Wait — there's a bug: is the canvas being cleared? No, drawImage covers everything. Hmm, if img has been putImageData'd onto off and drawImage'd onto canvas, but the canvas size is dpr-scaled... should still be fine.

Possible bugs: `toCell` is unrelated. Let me debug: can I not evaluate JS directly... actually, could I add a temporary debug log that writes to the DOM and check it? Or use the browser's "type" action? No, there's no eval tool.

Let me carefully re-examine the code for bugs.

`const img = octx.createImageData(NW, NH);` — 120x80. In render, k advances 4 per cell, NW*NH*4 = img.data.length. Good.

seed(): loops j,i, adds dye. r[id] += ... with max 0.9. Should render as cyan.

Hmm, wait — `data[k] = 8 + 247 * ...` — Uint8ClampedArray, no issue.

Oh! I see: inside `step`, damping `u[i]*=vDamp`... no problem.

Could rAF not be running? The motion tool shows frame 2 at +712ms — the tool captured the frame, but the page's JS might be running. If a JS exception occurred at startup, the canvas would remain black (body background is #05070c — that dark color). In the DOM the HUD is rendered (the HTML). A JS exception before the first rAF could leave the canvas black. The console only shows warnings. But an exception should be logged as an error.

Wait — the console only shows one warning. If there were an exception it would show up. Maybe... errors are being captured. Hmm.

Let me think about `resize()`: canvas.width = innerWidth*dpr, innerWidth in an iframe with a 1200 viewport is 1200. Fine.

Actually — the issue might be: `ctx.imageSmoothingQuality` is fine.

Let me test the rendering path: drawImage(off, ...) — off is 120x80 with img data. Should be visible.

Or maybe dye exists but is too dim? A 0.9*exp(-d2/90) blob on a 120x80 grid upscaled to 1200x800 — the blob would be about 40 pixels wide on screen, cyan around (186,245,239). Should be clearly visible.

Could the sim have run and the dye vanished instantly? The fade is 0.995/frame — nope.

Wait, maybe there's a big bug: in advect, dtx = dt * min(NW,NH) = 0.016*80 = 1.28. x = i - 1.28*u. Initial u is around ±0.02*14 = 0.28 → x shifts slightly. No issue.

project: h = 1/80. OK.

Curl (vortex) forcing: amp = 0.55*dt*4 = 0.035*c. Small.

Hmm, everything looks correct. Let me add a debug overlay: temporarily put the dye max in the DOM to check state. Actually there's an easier way: add `window.__dbg = {get maxR(){...}}` — but I can't eval it. I could add a small text element that updates with the dye total each frame, reload, and read it via inspect.

That dbg snippet using Math.max(...[...u]) would spread 12288 elements — no problem (under the limit). Actually, more simply, I should have just taken the max value. Oh well, it's temporary anyway.

Is the debug element not showing up in the DOM dump? The DOM shows "Dye Pause Clear drag to stir the fluid", but there's no dbg text. The debug element should be empty in the initial state (the first update is at 500ms). Also, there's no green text in the screenshot at +0ms. Let me wait — I'll use motion to capture at 500ms intervals and check the dbg text.

Interesting! Debug shows maxDye≈0.2, sumDye≈23–31 — meaning the dye field has values (seeded at 0.9, currently faded and diffused). Velocity is ~0 (vortex has dissipated). But the canvas is still black! That means rendering isn't displaying the dye. With maxDye 0.26 → pixel = 8+247*(1-exp(-0.26*1.4)) = 8+247*0.304 = 83. That's a visible dark grey-green on a few pixels... Actually, the dye is now spread across many cells with low values, so each cell is around ~0.2 → 83/255 ≈ 30% grey, which should be visible. And yet the screen is black!

So there's a problem with the rendering pipeline: either putImageData on octx isn't happening, or drawImage is failing. Wait — maybe the dye is being sampled by my dbg sampling (i += 7) — but the fact that the sum is small means the dye is thin. Still, max 0.26 → at least 1 cell is ≈83. But the whole image is black.

Hmm! Maybe the problem is this: drawImage(off...) — the off canvas is fine. Could `ctx` be referencing the wrong canvas? No, that's not it.

Wait, actually — is rAF running? The screenshot shows a completely black canvas. If rAF isn't running, no frames would be drawn at all... but the debug updates come from setInterval, which is running. Maybe rAF is being throttled in this headless environment! The "motion" tool shows frames over 2 seconds — rAF might be running at low frequency, or being completely throttled. But the first screenshot was also black.

Let me test: change render to also update the debug text (e.g., count of rendered frames). If the frame counter doesn't advance, then rAF is throttled. Alternatively, run the main loop with setInterval instead of rAF for robustness.

Actually, the simplest and most robust fix is: keep rAF but also draw... let me add a frame counter to the debug line.

If rAF is throttled, a better design: use a time-based interval fallback. Actually, before that, let's first check the frame count.

At the +0ms mark, the dbg div is not shown in the DOM — it's probably empty until 500ms. Wait 1 second via motion and read it.

frames=437 (running at about 60fps), dye max 0.2, but the canvas is pitch black. So rendering is being executed, but drawImage of `off` isn't producing visible output. Why?

Hypothesis: putImageData on `octx` is working, but when I check: the image data is computed per (j,i). Wait — is the dye only contained in the cells being sampled? No, sumDye is over 1/7 of the cells.

Hmm — the max pixel value is 0.2*1.4 → 8+247*(1-exp(-0.28)) ≈ 8+58 = 66. That's a subtle dark gray — but the background is (8,10,16). A region of value 66 spread over many cells (sum 23 distributed over cells) — in a 1200x800 image, the blob is small and faint, and it should show up as a faint gray blob in the screenshot. But I don't see anything.

Maybe the canvas is being covered by something? #view has width 100vw height 100vh. The screenshot is 1200x800. The canvas is black — maybe drawImage is throwing an exception? There's no error in the console. Or the canvas element isn't visible at all, and what I'm seeing is the body's background, and the canvas is... hidden by z-index? The canvas has no z-index and comes before the HUD in the DOM; the HUD is on top. The canvas should be visible.

Wait, maybe the canvas has a size but drawImage is failing silently? Or the `ctx` variable... `const ctx = canvas.getContext("2d")`.

Oh, wait. I got it! Inside `resize()` I'm setting ctx.imageSmoothingEnabled — no problem there. Inside render() it's also fine.

Let me think about `octx.putImageData(img, 0, 0)` — img = octx.createImageData(NW, NH), 120x80. off.width=NW, off.height=NH. No problem.

Could the off-canvas be hidden? No, it's not appended to the DOM.

Let me test directly: draw a red fillRect on the main canvas and see if it shows up. I'll modify render temporarily to do a red fillRect before the drawImage.

Wait, fillRect(0,0,50,50) will be covered by drawImage if drawImage works. The order is wrong — put fillRect after drawImage so it can be confirmed. Swap the order: drawImage first, then fillRect. Let me fix it.

Hmm, the red rectangle will get overwritten by drawImage unless I put fillRect after it. So the order is correct: drawImage, then fillRect — if fillRect executes and the canvas gets displayed, then red should be visible in the corner.

the red rectangle is showing (in the upper-left corner) — the main canvas is being drawn. But the `drawImage(off, ...)` of the off-canvas renders nothing (it's a black region, not even the background (8,10,16)... in fact the rest of the canvas looks uniformly dark — could that be exactly (8,10,16)? The body background is #05070c = (5,7,12), and the canvas background is (8,10,16). The canvas region is clearly slightly brighter than the top strip? Hard to tell. The HUD background is semi-transparent.

So the fillRect works, but drawImage(off) contributes nothing — meaning either `off` is blank or black. So either the putImageData to octx isn't producing the result, or... the img data is actually all background color. With maxDye=1.7, some pixels should be bright (8+247*(1-exp(-2.4)) ≈ 240). At frame 6, the seed blob should be bright cyan. So the img data should contain bright pixels — unless the render loop's img writing is wrong.

Wait, is the dye in a cell that gets rendered? Rendering iterates j 1..NH, i 1..NW — all interior. The seed places blobs at cell coordinates cx = 1+(0.30-0.5)*120 = 1-24 = -23!! Bug! toCell formula: I wrote cx = 1 + (s.x - 0.5) * NW, but for s.x=0.3 that gives 1-24 = -23. Off the grid! Should be 1 + s.x * NW. Same for y: 1 + 0.4*80 = 33. I used (s.x-0.5) — wrong. So the blobs landed outside the field (i0=-26, so the loop skips most, and only some cells near the boundary received dye — sumDye 71 concentrated near the boundary, and it gets wiped out by setBnd).

But even so, some boundary cells did get dye (e.g., seed 1's x range -26..-20 → i is clamped to ≥1: i0=max(1,floor(-23-20))=1, i1=min(120, ceil(-23+20))=-3 → i1 < i0, no cells. Seed 3's x: 1+0=1? cx = 1+(0.5-0.5)*120=1 → dye at the left edge; then wiped by setBnd (dye's setBnd(d,0) copies from inside... the edge itself persists). maxDye 1.724 is in the i=1 column — a 1px-wide vertical strip at the left edge, which when scaled up is about 10px wide and thin... it might be at the left edge, hidden behind the HUD? It's a column across the full height at x≈0. Looking at the screenshot... the left edge is dark. Hmm, hard to see at 10px width and max brightness 240. Actually a 12px-wide bright cyan strip at the left edge should be visible. Let me stop over-analyzing; fix the bug.

Also, the maxU debug earlier was ~0 — velocity decays; no problem.

Fix the seed coordinate calculation, remove the red rectangle, remove the debug. Also maybe boost the seed's brightness a bit. Let me apply the fix.

Wait — but if the seed is outside the grid, where did the sumDye=71 come from? Seed 3's cx=1 → column 1, cy = 1+(0.3-0.5)*80 = 1-16 = -15 → outside. Seed 2: cx=1+0.18*120=22.6, cy=1+0.05*80=5 → near the top edge, cells i 1..? dy is at most -1..? d2<400: at row j=5, the cells in the range j=1..5, |dx|<20 receive dye, but the j<1 boundary cells get cleared. So a small clump near the top edge — consistent with sumDye~70. OK, the bug is confirmed.

Reload and check.

still dark — but this is the +0ms screenshot, and at that point the seed blob should be in the center. It's suspicious that it's dark even at frame 1. Could it be that the screenshot is being taken before the first rAF callback? The red test was visible at +0ms of the previous reload (frames=6 already at 0ms, and the red corner was also visible). So at reload time the screenshot happens after a few frames. And yet this time the dye isn't visible??

Hmm, wait — in the previous test with the red rectangle, was there even any dye visible in the first place? No — only the red corner was visible. In other words, at that point even a bright cyan blob (maxDye 1.7) was not displayed by drawImage. This suggests that drawImage(off) itself is rendering nothing!

But now the seed is in the center... still dark. So the problem is in drawImage of the off-canvas. Why would fillRect work but drawImage(off) not? Maybe `off` is a 120x80 canvas — a small canvas. drawImage should work. Could it be... that `ctx` has a scale transform on it? No, it doesn't.

Wait a sec — maybe the problem is that the image data really is a dark background color: the render loop is writing to `img.data`... `let k = 0` — correct. But wait: is the dye really contained in `r[]`? In the previous debug, maxDye came from r+g+b — that was 1.724. So the data has values, and putImageData...

Hmm, actually — could it be that putImageData is failing because `img` was created with octx before... no, that's not it.

Let's think more carefully about drawImage: `ctx.drawImage(off, 0, 0, canvas.width, canvas.height)`. In a headless environment, canvas.width = 1200*dpr. If dpr=2 then → 2400x1600, and the screenshot is 1200x800 — the content is at the same position and just scaled. No problem.

Maybe the screenshot is only compositing part of it... no, the red rectangle was showing up.

Actually wait — was it really showing up? In the upper left of the screenshot there's a red square at (0,0)-(50,50) — yes, and the HUD overlaps it. So fillRect is working.

That means drawImage(off) is drawing black. Which means the off-canvas is black → the putImageData data is background color. But the dye values exist... unless render() is running before step/seed? No, seed() runs at startup.

Wait a sec — did the edit break the placement of render? Let me re-read the current file and check whether render is intact, and whether the seed fix is reflected. Maybe something got messed up by a bad edit.

The code seems fine. So why isn't drawImage displaying the off-canvas? Wait... maybe there really is no problem, and the screenshot at +0ms is taken before rAF, and in the previous test frames=6 was displayed at 0ms because of the debug interval... but the reason red was visible in the previous screenshot was because render had run 6 times. In other words, rAF is running, render is running, and the dye exists (it's in the center this time), yet the canvas is black. Which means the drawImage output must be black → the off-canvas is black → the img data is being overwritten??

Ah, wait. I've got it! Inside `render()`: `const data = img.data;` ... but `img` was created at startup with `octx.createImageData(NW, NH)`. No problem there...

Hmm, let me look at render once more — maybe there's an offset in the k index: after `let k = 0`, in the loop i=1..NW → data[0..] corresponds to (i,j)=(1,1)... that's correct.

Hmm, maybe... `octx.putImageData(img, 0, 0)` — no problem.

Is `off` even being displayed in the first place? Let me put in a direct test: inside render, also draw `octx`'s getImageData... can't easily do that. Alternative: skip the off-canvas entirely — create a NW*NH-sized ImageData on the main canvas? No, the canvas is 1200x800, but putImageData on a large region can't be done partially... Actually putImageData can place at an offset, but you can't scale.

Alternative test: after putImageData, run `const px = octx.getImageData(60, 40, 1, 1).data` and write it into dbg. This would let me isolate whether the off-canvas has data in it.

Wait, actually — maybe the real issue is: the screenshot tool is capturing the page but the canvas content is... no, red was displayed.

Let me rethink: maybe the dye is actually zero, and the "maxDye" I measured earlier was from a broken seed near the boundary. With the seed fixed, the blob is in the center — the screenshot at +0ms shows the center as black. But maybe rAF hadn't run even once before the screenshot? In the previous test, frames=6 was visible at the moment of the +0ms screenshot, but that's because the debug interval had already fired, and rAF was running at 60fps → 100ms = 6 frames. So at the time of the +0ms screenshot, render should have run 6 times and the central blob should be displayed. But it's black!

So the blob is not being rendered. It's something about the render itself. Let me test putImageData/getImageData with a debug readout. No wait, let me look at render() exactly as written in the file — there might be a subtle bug I've overlooked.

render looks correct. But wait—about the dye: the seed blob is cyan (0.15, 0.85, 1.0)*0.9. The max value of gg is 0.765 → pixel g = 10+245*(1-exp(-1.07)) ≈ 10+155=165. It should be clearly visible.

Is there a possibility that the canvas is displayed with CSS width 100vw/100vh but the actual pixel size is 1200*dpr, and the screenshot is a capture at CSS scale? Red fillRect(0,0,50,50) (in device pixels) appears at CSS 0-50... in the screenshot it's 50px? The red square in the screenshot looks about 50px → dpr=1.

So drawImage(off, 0,0,1200,800) should scale a 120x80 image to 1200x800. That's 10x. It should be visible.

Hmm, wait—did the screenshot tool capture before the first rAF in the "reload" case? In the red test, at +0ms, frames=6 was displayed in dbg. So at the time of the screenshot, rAF is working. But there's no dye.

Maybe... `img`'s data is being written but putImageData isn't being reflected in `off` because the off canvas's size was... off.width=NW was set before createImageData. It should be fine.

Let me do a decisive experiment: bypass putImageData's tone mapping and render a solid fill directly to octx? That is, temporarily replace render with octx.fillStyle="lime"; octx.fillRect(0,0,NW,NH). If lime appears, the putImageData path is broken; if it doesn't appear, drawImage(off) is the problem.

Actually there's an even quicker method: keep both tests—fill octx red and putImageData a lime img... too complicated. First, solid fill test.

wait a sec, this is going to fill the entire screen with lime — if the lime is visible, then drawImage is working, and the problem is in the img data path. Gonna reload and take a screenshot.

DrawImage(off) works — a lime-colored canvas is displayed. Therefore putImageData(img) is the problem. Why does putImageData generate black/dark? `img` is created with `octx.createImageData(NW, NH)`. Then render writes to `img.data`. And then `octx.putImageData(img, 0, 0)`.

Ah, wait. Now I get it!!! Inside render, the background pixel (8,10,16) — a very dark navy. In the screenshot, the "black" canvas might actually have been (8,10,16) all along! The dye blob is small (radius about 10 cells = 100px), but the max value of the dye is 0.77 → luminance about 165... it should be visible. Unless the dye was erased before the first render!

The first render call happens on the first rAF. Before that, step has not been executed (step is called before render within the same frame — step(dt) with dt>0). In frame 1, step runs: velocity is small; advect; project. Dye is mostly preserved.

Hmm, but wait: was the previous screenshot (before the lime test) actually the background of img (8,10,16), or was it the default transparent canvas? The background was a slightly bluish dark color... consistent with (8,10,16). So drawImage WAS drawing the image data — the background was visible! The problem is that the dye wasn't in the data.

But dbg showed maxDye 1.724 (the old broken seed at the frame 6 point). Hmm, that dye is near the boundary at the left edge; the previous render loop is... wait a sec, that dye is at column i=1 → pixel column k=0 → the left edge. In the red rectangle screenshot, the left edge region was partially covered by HUD. The dye column should be at x=0..10, and the rest of the top-left was covered by the red rectangle... the region below the red rectangle (below y=50) at x<10: looking at the screenshot, the leftmost edge is dark. The dye is at max 1.724 → pixel about 240 cyan. A 10px-wide cyan band — I don't see it.

Hmm, wait, maybe the dye was only at row j=1? The broken seed #3: cx=1, cy=1+(0.3-0.5)*80 = -15. The loop is j 1..NH: at j=1, dy=-16, d2 = dx²+256 <400 → dx<12. So the dye is at row j=1, columns 1..12 → a thin horizontal band at the very top, y≈0..10px. The top edge is at the top of the HUD region... The first row of the screenshot, from x 0..120 — the left part is hidden behind the HUD, the rest at x>230 is dark. Hmm, the band should have been visible at y≈5px between the HUD and the right edge. It wasn't.

OK, another thought — maybe putImageData IS working, and the dye WAS being drawn, but the dye had dissipated... no, in the broken seed case, maxDye persisted for several seconds (18-31 at the 2 second point).

Let's do a direct check: revert the lime hack, and add a temporary debug that reads octx.getImageData at the pixel of the blob center and outputs maxDye, and where (argmax) that is. This will settle it.

The center of the off canvas (60,40) roughly corresponds to cell (61,41) → id = 61 + 122*41 = 5603. Seed 1's blob is at cx=37, cy=33 (0.3*120+1, 0.4*80+1) — not at the center. Seed 3 is at (61, 25). Hmm, the pixel (60,40) is the center of the grid; seed 2 is at (82.6, 45) — no wait, 1+0.68*120=82.6, 1+0.55*80=45. So the center pixel (60,40) is between the blobs, and may end up dark. A better approach: output a few pixels and argmax. The imgpx indices I output: data[244],data[245],data[246] — this is pixel (61,0)? Whatever, it's an approximation. Actually, let's just look at argmax and the center pixel; if the argmax cell is at, say, (37,33), then the pixel would be (36,32). For now let's just run it and check the console.

Key data: `DBG offpx 0 0 0 maxDye 1.727 at cell 81 44 imgpx 8 10 16`.

- The off canvas pixel at (60,40) reads back as (0,0,0) — pure black, not the (8,10,16) background!
- The argmax of the dye field is at cell (81,44) — seed 2, which is currently in the correct range.
- The `img` data pixel I printed is the background value (that pixel index isn't the blob).

So putImageData is writing zeros to the off canvas?! The img data has the correct values (imgpx shows the background 8,10,16 at that index... wait, imgpx = data[244..246] = 8,10,16 — that's the background color so the img data is being written. But reading back from the off canvas gives 0,0,0.

Interesting: (60,40) on the off canvas = (0,0,0) but img[60 + 120*40] should be ≥ background 8. putImageData put zeros. Why?

Hmm, is putImageData silently failing on this canvas? Maybe because `off`'s width/height (120,80) — but the lime fillRect worked on the same canvas. putImageData on a canvas that's not in the document should also work, but...

Wait a minute. Could this environment's canvas implementation (skia-canvas or some polyfill?) have putImageData broken? The lime fillRect worked. Let me test: write a single bright pixel with putImageData and read back the whole thing... no, actually let me test like this: after putImageData, run getImageData at the blob's pixel (80,43) and check. If putImageData is broken, I'll need another rendering path.

Alternative rendering that avoids putImageData: create ImageData and... putImageData is the only way to fill in pixels. Alternative: use ctx directly on the main canvas via createImageData with the main canvas's dimensions, then putImageData the full frame (1200x800 = 960k pixels — a per-frame loop of 960k*4 is heavy, ~ms, maybe fine, but it means scaling the dye grid manually — that's actually more work).

A better alternative: build a Path2D per cell? No.

Alternative: manually scale the dye grid into a Uint8ClampedArray at screen resolution, and do one putImageData to the main canvas at full size. That's 1200*800 = 960,000 pixels, each needing bilinear sampling — about 4–8ms in JS. Acceptable.

But first confirm the putImageData hypothesis. Let me log: after putImageData, read getImageData(80,43,1,1) and img.data at the same offset before put. Also test putImageData with a fresh ImageData filled with 123s.

Actually — wait. Could `img` be getting corrupted between the write and the put? The write loop assigns data[k], k is 0..95999. That covers it all.

Hmm, one more wild idea: in this headless environment, canvas width/height is in device pixels and maybe putImageData clips at 32767... no, 120x80 is tiny.

Let me just run the experiment.

(Internal note) Wait, `putImageData(t)` — t.data length is 120*80*4, and setting 123 for everything → RGBA 123,123,123,123, semi-transparent. Read (5,5) after putImageData. But note: this debug runs at render._n===10, and it destroys the off-canvas with 123 — no problem, it's temporary.

Interesting: testPx 122 122 122 123 — putImageData works (the fresh ImageData with willReadFrequently… well, it works). But the `img` I wrote gets read back as zeros?!

Wait… `img` was created at the top with `const img = octx.createImageData(NW, NH);`. I'm writing to `img.data`. But `blobPx 0 0 0` — I read the cell (80,43) on off after putImageData(img). And testPx proves that putImageData works. So does that mean `img.data` at that offset is zero at the point of putImageData?

Oh, wait. I think I've got it! The `imgpx` I read in the previous run showed 8,10,16 for the background — so the data was written. But now blobPx is zero…

Hmm, hmm. Let me check: in the previous run, I read `data[240+4]` = pixel 61 (x=61, y=0) = 8,10,16. Good, so img.data contains the written values (background). The background of the whole image should be (8,10,16) everywhere. Yet offpx at (60,40) reads (0,0,0)!!! So putImageData(img) wrote zeros to the canvas, but img.data has the background values?? That's contradictory… unless the data is being corrupted between the write and the put — within the same function, though.

Could it be that… `img.data` isn't being written where I think it is. Ah, wait!!! Found it!!

In `render()`:
```
for (let j = 1; j <= NH; j++) {
  let k = 0;
  for (let i = 1; i <= NW; i++) {
```
This writes img.data[0..95999] — all of it. That's fine.

Hmm, but the readback at (60,40) is 0,0,0, and the background should be 8,10,16. 0,0,0 isn't the background I wrote either. So the canvas contents are completely zeroed out. And yet putImageData(t) works.

The difference between `img` and `t`: `img` was created once at startup… no wait. The `imgpx` from the previous run — I read data[244..246] and got 8,10,16, so the img data was being written.

Is `img.data` a Uint8ClampedArray and `octx`… no, that's not it.

Hmm, wait, actually there's a subtle point in this run: `c` (offpx) was read after the putImageData(img) within this frame, and blobPx was also read after the put — both zero. Then t was put → 123.

So putImageData(img) produced an all-zero canvas. But img.data (from the previous run) had the background values… unless the previous run's read happened in a different order: in the previous run I read `data[...]` — `data` is a reference to `img.data`, read within the same render call, after writing, before putImageData. Values present: 8,10,16. ✓

So: img.data is filled → putImageData(img) → canvas all zeros. But putImageData(t) (a fresh ImageData) → works.

Difference: `img` was created with `octx.createImageData` before… hmm, same context.

Wait, is the issue the `willReadFrequently` warning? No.

Let me think… in this browser, could `createImageData` return a buffer that gets reused/zeroed when another canvas operation happens? For instance, is `img` created before `off`'s width/height are set? Order in the code:
```
const off = document.createElement("canvas");
off.width = NW; off.height = NH;
const octx = off.getContext("2d");
const img = octx.createImageData(NW, NH);
```
No problem.

Hmm, hold on — what if render() ran multiple times per frame and…? No.

Actually, let me double-check: could the readback at (60,40) be wrong because getImageData on a canvas that willReadFrequently is false returns zero? But testPx worked on the same canvas right after. So the readback is working.

The sequence within frame 10:
1. Write img.data (background + blob)
2. putImageData(img)
3. getImageData(60,40) → 0,0,0
4. getImageData(80,43) → 0,0,0
5. putImageData(t) → 123 ✓

If img.data is correct, 3 should be ≥8. Unless the write loop isn't writing where I think it is. data length = 120*80*4 = 38400. k goes up to 38400. OK.

Wait. Ah, wait!!! I got it!!!

`const data = img.data;` — but there's another variable… inside `render()` I'm using `let k = 0` — no shadowing. Hmm.

Let me look again… `data[k] = 8 + 247 * (1 - Math.exp(-rr * 1.4));` — no problem.

Hmm, actually, wait: could `img` be a different object than I think? The declaration: `const img = octx.createImageData(NW, NH);`. Yeah, that's right.

Let me add a readback right after the write and before the put: `data[0..3]` and, say, the blob pixel within img.data, then compare with the canvas read after the put. This will pin down where the data is being lost.

There's a possibility that putImageData(img) is broken due to the old canvas context. Rewrite render() to create a new ImageData every frame, and also remove the debugging code.

I need to check the actual current text of the render function.

Wait—hmm, before blindly switching to a fresh ImageData: the testPx from earlier proved that a fresh ImageData works with putImageData. The persistent img data read back as zero. Weird, but a fresh per-frame ImageData is the safest bet. 120*80*4 = 38400 bytes per frame—the GC cost is no problem.

Let me edit precisely using the actual lines.

There's an extra closing brace `}` on the line after the debug block (a leftover from the previous edit). I need to rewrite the entire render function block cleanly.

Also, remove the now-unused `let rr/gg/bb` — already done. Check whether there are any other references to the global `img`. Line 153 has been deleted. Grep for "img" references.

Hmm — "new Function" compiled the code. However, the top-level "use strict" and such are also fine. Next, I'll reload and take a screenshot.

Still dark at +0ms. But wait — at +0ms of "reload" the screenshot might be taken before rAF has even run once (in previous runs, dbg frames=6 was visible at 0ms, but that's because the interval fired after ~100ms... maybe the reload screenshot is actually captured a little after the reload). Previously (when there was a seed), maxDye=1.724 at the time of the screenshot. If render now works, the blob should be visible. Unless the seed's dye vanished again?

No wait — maybe the screenshot is at a moment before the first rAF (0 frames). The previous dbg run showed frames=6 at 0ms — that was because the screenshot was taken after ~100ms. Hmm.

For now, I'll wait with a motion frame.

After 2.3 seconds it's still pitch black. Meaning either the dye has vanished, or the rendering is still failing. Wait — is the dye still being retained? In the previous (broken) state, the dye persisted for a long time. In the new seed, the blob is in the center with vortex velocity (u≈0.28). After about 130 frames, could the dye have been advected out? No — advection is conservative within the domain, and only the boundary wipe removes a tiny amount. The blob should still be there.

So the rendering is still not displaying anything. But a fresh ImageData works in the test! Differences between my test's ImageData and the one inside render: the test filled it with the constant value 123 and put it. render: writing pixel values... could `1 - Math.exp(-0*1.4)` = 0 → data = 8,10,16 — no problem.

Wait. Actually — is `putImageData(img, 0, 0)` being called on the same off canvas? Yes.

Hmm, let me reconsider: could it be that the dye field r/g/b is actually zero, and the earlier dbg maxDye=1.727 "at cell 81 44" was from the broken seed? No — the seed fix was applied before that debug run (the reload after the seed fix was displaying... wait, the timeline:

1. Seed fix (cx = 1 + s.x*NW) → reload → black.
2. Lime test → lime displays (drawImage is OK).
3. Add offpx debug (this version already had the fixed seed) → offpx 0 0 0, maxDye 1.727 at cell 81 44 — cell (81,44) ≈ seed 2 position (82.6, 45). So the dye exists in the field. blobPx reads 0,0,0 → putImageData(img) is losing the data.
4. Test t (a new all-123 ImageData) → testPx 123 ✓.
5. Replaced with a fresh ImageData inside render → still black?!

In step 5, img is fresh. It should behave the same as t. Unless the data written in render is all zero! `data[k] = 8 + 247*(1-exp(-r*1.4))` — for r≈0 → exactly 8. Not zero. But what the screenshot shows is... 8,10,16 ≈ #080a10, which is very close to the body background #05070c. At this screenshot's compression, (8,10,16) and (5,7,12) are indistinguishable!

And the blob's pixel: max dye 1.7 → 8+247*(1-exp(-2.4)) = 8+247*0.909 = 233. Should be a bright cyan, about 100px wide. Not visible.

So either the dye disappeared between frame 10 and the screenshot (about 1–2 seconds), or the put is broken. Dye at 0.995/frame decays → 0.59 after 100 frames — still bright.

Is the sim killing the dye? Let's check advect with the new seed: u velocity 0.28, dtx = 1.3*0.28=0.36 — no problem.

OK, I can't keep guessing. Do one definitive diagnostic: render a checkerboard pattern directly to img.data (independent of the field), putImageData, and take a screenshot. If the checkerboard appears → the field/rendering write is broken. If it doesn't appear → something else (e.g., off canvas size mismatch, drawImage is broken in a different way).