Flow-field particles · deepseek-v4.1-flash

Let me write the index.html.

Design:
- Canvas full viewport, resize handling.
- 2000+ particles stored as typed arrays or array of objects. Use object array simpler but 2000 fine.
- Perlin noise field: angle = noise(x*scale, y*scale, t)*TWO_PI*2.
- Fading trails: no clear each frame, draw translucent rect. But background with alpha creates fading trails naturally. Avoid growing history.
- Colors vary over time based on position/time using HSB.
- Field evolves gradually: t += small amount per frame.
- Recycle at boundaries: when particle leaves, respawn at random edge / random position.
- Pause/Resume and Restart buttons, responsive.

Use p5 global mode. Need retina handling maybe.

Performance: 2000 particles, drawing small lines/points each frame with alpha background. Use stroke with HSB color. At 60fps drawing 2000 lines is fine.

Let me ensure background fade doesn't fully accumulate. Use blendMode maybe. Simpler: each frame fill(0, 12) rect. Trails fade.

Colors: use HSL via colorMode(HSB). Each particle has a hue offset; hue shifts over time. Set stroke each particle — that's expensive but okay. Could batch by hue buckets. 2000 is fine.

Let me write it.

I'll use canvas sizing to windowWidth/windowHeight with pixelDensity(1) for performance.

Particle struct: x,y,px,py,speed, hueOffset, life. Use arrays.

Restart: reinitialize positions randomly, clear background.

Let me write the file.

For coherent nearby motion: use noise field based on position so nearby particles share direction.

Let me create index.html.

There's an error: "Cannot read properties of null (reading 'appendChild')" — p5's createCanvas trying to attach? Possibly p5 runs before DOM ready, or the parent element. Actually `c.parent('sketch')`? The error mentions appendChild. p5 when creating canvas appends to container. Hmm. The default canvas container appendChild works usually. Maybe the issue is p5 in this environment loads differently. Let me check console more. Possibly the script runs before body parsed? No, script at end of body.

Wait — the error could be from p5's `createCanvas` when auto-append to `document.body`. But body exists. Let me inspect more by checking if canvas exists in DOM.

Actually the issue: the runner likely injects/inlines the script differently. But error "reading 'appendChild'" — could be p5 trying `this._userNode.appendChild` where _userNode null and new p5 auto? Hmm.

Let me check the DOM for canvas.

No canvas appears. The error is likely from p5's auto-start. Let me check the p5.min.js version behavior. Actually, common issue: p5 global mode auto-initializes on DOMContentLoaded, but because we assign window.setup inside an IIFE after p5 loads, should be fine.

"Cannot read properties of null (reading 'appendChild')" — In p5, `_createCanvas` calls `document.body.appendChild`? Let me search for appendChild in p5.min.js.

Actually likely culprit: my `createCanvas(...).parent('sketch')`. But the div exists. Hmm.

Wait, maybe the error happens before my code even — from p5's own initialization trying to access `document.body` when body is null. But body is parsed since script is at end.

Let me look at how the runner loads. Perhaps it inlines p5 and my script into a sandboxed iframe using document.write or something, and the timing differs. The error might even be from the runner, not my code.

Let me test by evaluating in console? I don't have a console-eval tool directly. Let me add some debugging. Actually, let me check p5 source around appendChild.

The key line: `var i,a=r||d.P2D,r=o._userNode||document.body;for(i in n||r.appendChild(s.canvas)...`. Here `r` could be null if `document.body` is null. That means p5's setup runs before body exists. This happens if the runner inlines scripts into head, or p5 auto-start triggers on load before body? Actually p5 global mode starts on window load or DOMContentLoaded.

Hmm, but the DOM shows hud/controls, so body exists at inspection. The error occurs at frame 1. Maybe p5 auto-init happened before my `setup` was assigned? No, that would cause different error.

Wait: `_userNode` is set by `.parent()`. Not here.

Actually the error could be from p5's Friendly Error System (FES) / loading screen. The line `d._loadingScreenId` ... `(d._userNode||document.body).appendChild(r)`. If document.body null → error. This happens during p5 construction. p5 auto-constructs on DOMContentLoaded. If the runner inlines the HTML in a way where p5 script runs and immediately constructs before body... 

Hmm. Let me test: is the problem caused by p5 being loaded and auto-starting before my setup? Actually, the standard robust fix is to use instance mode with a container element, and not rely on global auto-init. But instance mode still uses document.body if no node.

Better: wait for DOM ready explicitly, and create p5 instance. Let me switch to instance mode and pass the container element directly. But if document.body is null at construction... we can wait for DOMContentLoaded / window load.

Actually let me first determine whether it's really body being null. Let me add a try/catch and log. Simpler: use instance mode created on window load.

Let me check p5 version.

p5 1.9.4. Let me reproduce locally with node? Hard. Let me just make the code more robust by using instance mode and constructing after DOM is ready, passing the container. The error might be from something else though.

Actually wait, the error "Cannot read properties of null (reading 'appendChild')" — in p5's canvas creation: `r=o._userNode||document.body`. If we pass `_userNode` explicitly it won't use document.body. That avoids the issue if body is null. But creating p5 instance itself requires document.

Perhaps the real issue: the runner executes the HTML's scripts but the p5 auto-init runs at a moment when `document.body` is null because the runner uses an iframe and writes content incrementally. The safest: inline instance mode and instantiate on `window.addEventListener('load', ...)` — by then body definitely exists.

Actually p5 global mode waits for window load already. Hmm.

Let me look at what triggers p5 construction. In p5, `_globalInit` is set on `window.addEventListener('load', ...)` if document.readyState !== 'complete', else setTimeout. Actually let me check.

Let me test a minimal page to isolate. Write a test file and reload. Actually the browser tool always loads index.html. Let me temporarily modify index.html to a minimal p5 test.

Better approach: add error logging with more detail. But I can't see stack. Let me just try instance mode with explicit container and DOMContentLoaded.

Let me rewrite the script portion to instance mode.

Actually, I suspect the FES "friendly error" loading screen may be involved: `_loadingScreenId` appendChild. Or the p5 constructor's `_start` uses document.body.

Let me try a quick experiment: minimal p5 global page.

Basic p5 global mode works fine. So the error is in my code. Let me find what appendChild with null. In my setup, `c.parent('sketch')` — parent() does `document.getElementById(e)` which should exist. Fine.

The error occurs likely during p5 auto-init at window load. But it works in minimal. So what's different? My IIFE assigns window.setup/draw inside an IIFE which executes immediately — fine.

Hmm, `createCanvas(windowWidth, windowHeight)` fine. `colorMode(HSB,...)`.

Wait — the error might happen before setup: p5's global init also processes `window.preload`? No.

Actually — the error is "Cannot read properties of null (reading 'appendChild')" and DOM shows no canvas. In minimal, canvas created. Difference: my CSS has `#sketch { position: fixed; inset:0 }`. Not relevant.

Maybe it's the `backdrop-filter`? No.

Could be `document.getElementById('pauseBtn')` null? No, that's not appendChild.

Let me reconsider: p5 constructor's `_start` for global mode. Actually the error could be from p5's FES. My code likely runs and throws first? Let's add console logging to find where. Actually simpler: test my full page but progressively. Let me restore and wrap in try/catch, and log stages.

Actually, a likely culprit: `pixelDensity(1)` called, fine.

Hmm what about `createCanvas(...).parent('sketch')`. The `parent` method: `p5.Element.prototype.parent = function(p){ ... if(typeof p==='string'){ if(p=document.getElementById(p)){ p.appendChild(this.elt);} } ...}`. If getElementById returns null, condition fails silently. Not error.

Wait, actually the parent function code from grep: `g(1)),e=document.getElementById(e)):e instanceof ... &&(e=e.elt),e.appendChild(this.elt),this` — it calls `e.appendChild` UNCONDITIONALLY! Look: `if("string"==typeof e) ... (e=document.getElementById(e))`, then `e.appendChild(this.elt)`. If getElementById returns null → appendChild of null. That matches! So `document.getElementById('sketch')` returns null.

Why would it be null? Because... the setup runs and 'sketch' div exists. Unless p5 starts before body parsed. But minimal worked with the same structure. Hmm, but in minimal it worked.

Wait, actually in my full file I load p5, then put my script, then `#sketch` div is BEFORE controls and before script. Yes: `<div id="hud">`, `<div id="controls">`, `<script src=p5>`, `<script>`. But where's `<div id="sketch">`?! Let me check my file... I defined `#sketch` CSS but I don't think I added `<div id="sketch">`! Let me check.

Looking at body: hud, controls, scripts. No `<div id="sketch">`! That's the bug. `createCanvas` auto-appends to body fine, but `.parent('sketch')` returns null and appends to null.

Fix: add `<div id="sketch"></div>` before scripts, or remove `.parent('sketch')`. I'll add the div. Actually createCanvas already appends to body; adding div is fine.

Let me restore backup and fix.

Now it works. I can see coherent flow. Let me sample motion frames to check trails and color evolution. Use motion action.

Excellent — coherent nearby motion, fading trails, evolving colors and field. 60fps with 2200 particles. Now test Pause/Resume and Restart.

Let me click Pause (at ~535,755). Then screenshot twice to confirm frozen. Then resume. Then restart and confirm trails clear.

Paused; button says Resume. Colors have shifted to purple/red. Now take screenshot after a moment to confirm frozen. Then click resume.

Let me wait and take a screenshot.

Frozen (identical). Pause works. Now resume and check it moves again.

Resumed (button says Pause). Now test Restart. Click restart and immediately screenshot to see cleared trails.

The Restart button is at around x=644, y=755. Let me click it and immediately screenshot. The click action returns a screenshot after one frame, so it should show mostly cleared.

Restart cleared trails and reset particles, now short dashes with new field (fieldTime randomized). Good. Let me wait a couple seconds to confirm new arrangement builds.

New arrangement, green/red palette this time (different fieldTime start), trails rebuilding. All behaviors verified. No console errors.

One consideration: the HUD text on reload initially says "2000 particles" then updates to 2200. Minor cosmetic. Could fix initial text to 2200. Let me update the static HTML to 2200 to avoid inconsistency. Also the task says at least 2000; PARTICLE_COUNT is 2200. Let me fix HUD initial text.

Also the field evolution speed: fieldTime increments 0.0022/frame → slow evolution, good.

Let me update hud initial.

All good, no console errors. Final file present. Let me verify the file exists and is self-contained (no external URLs).