WebGL Water Demo: Check the Old State Before Swapping Buffers

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<html lang="en-US"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1"><title>WebGL Water Demo: Check the Old State Before Swapping Buffers</title><style>:root{--bg:#eaf3f7;--paper:#fff;--ink:#16354a;--accent:#12526f;--tint:#dcedf5;--line:#b5ccd7;--muted:#436171;--button:#fff}*{box-sizing:border-box}body{margin:0;background:var(--bg);color:var(--ink);font:17px/1.55 system-ui,sans-serif}main{max-width:1000px;margin:auto;padding:22px clamp(18px,4vw,42px) 38px}header{border-bottom:2px solid var(--accent);padding-bottom:14px;margin-bottom:18px}h1{font-size:clamp(26px,4vw,36px);line-height:1.15;margin:4px 0 10px}h2{font-size:22px;line-height:1.3;margin:0 0 14px}h3{font-size:19px}p{margin:0 0 14px}.eyebrow{font-size:16px;font-weight:750;color:var(--accent)}.lede{max-width:800px;overflow-wrap:anywhere}.panel{background:var(--paper);border:1px solid var(--line);border-radius:9px;padding:20px;margin:18px 0}.workbench{border-top:4px solid var(--accent)}.grid{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:16px}.grid>*{min-width:0}label{display:block;font-size:16px;font-weight:650;margin:12px 0 6px}input,textarea,select{font:inherit;color:var(--ink);background:var(--paper);border:1px solid #7b8893;border-radius:5px;padding:10px;width:100%;min-width:0}textarea{resize:vertical;line-height:1.45}input[type=checkbox],input[type=radio]{width:21px;height:21px;vertical-align:middle;margin-right:9px}button{font:650 16px/1.4 system-ui,sans-serif;padding:11px 15px;min-height:46px;background:var(--accent);border:2px solid var(--accent);color:var(--button);border-radius:5px;cursor:pointer}button:disabled{opacity:.55;cursor:default}.buttons{display:flex;gap:9px;flex-wrap:wrap;margin:14px 0}.buttons>*{flex:1 1 auto}button:focus-visible,input:focus-visible,textarea:focus-visible,select:focus-visible,summary:focus-visible{outline:3px solid #c05b00;outline-offset:3px}.result{font:600 17px/1.6 ui-monospace,monospace;white-space:pre-wrap;overflow-wrap:anywhere;background:var(--tint);padding:16px;border-left:4px solid var(--accent);min-height:65px;margin:16px 0}.sample{padding:14px 16px;border-left:3px solid var(--accent);background:var(--tint);margin:12px 0}.muted{color:var(--muted)}pre,code{font:16px/1.6 ui-monospace,monospace;white-space:pre-wrap;overflow-wrap:anywhere}pre{margin:12px 0}table{border-collapse:collapse;width:100%;table-layout:fixed;font-size:16px}th,td{text-align:left;vertical-align:top;padding:10px 8px;border-bottom:1px solid var(--line);overflow-wrap:anywhere}th{background:var(--tint)}svg{max-width:100%;height:auto;display:block}details{margin:20px 0}summary{font-weight:700;cursor:pointer}footer{margin-top:28px;border-top:1px solid var(--line);padding-top:18px;font-size:16px}a{color:var(--accent)}.reading{font-family:Georgia,serif}.codeview{border-radius:0;box-shadow:7px 7px 0 var(--tint)}.token{display:inline-block;padding:8px 12px;margin:4px;border:1px solid var(--line);border-radius:4px;background:var(--paper)}.swatch{min-height:30px}.split{display:grid;grid-template-columns:1.1fr 1fr;gap:22px}ol{padding-left:25px}li{margin:9px 0}@media(max-width:600px){main{padding:16px}.grid,.split{grid-template-columns:1fr}.panel{padding:16px;margin:14px 0}h1{font-size:27px}header{padding-bottom:10px}.buttons{gap:8px}.result{padding:13px}th,td{padding:8px 5px}}</style></head><body><main><header><div class="eyebrow">Original practice fixture · local workbench</div><h1>Read old state, write new state</h1><p class="lede">Start a 5 × 5 grid with height 1 at the center and zero velocity everywhere. Compare forward and reverse traversal: both must read the same old state before producing the next one.</p></header><pre id="result" class="result" role="status" aria-live="polite" aria-atomic="true">Initial center: h = 1, v = 0; all other pairs: h = 0, v = 0.</pre><div class="buttons"><button type="button" id="forward">First step · forward</button><button type="button" id="reverse">First step · reverse</button><button type="button" id="reset">Reset both states</button></div><section class="panel"><h2>Interior h/v pairs</h2><p>Keep the 16 border cells at height 0 and velocity 0. For each interior cell:</p><pre>vNew = vOld + 0.25 × (left + right + up + down − 4 × hOld)
hNew = hOld + vNew</pre><pre id="grid">Center: h = 1, v = 0; other interior cells: h = 0, v = 0.</pre><p>All 16 border pairs remain h = 0, v = 0.</p></section><footer><p>This page executes a CPU stencil, not WebGL, GPU readback, or physical-fluid validation.</p><p>Use a prepared HTML explanation in <a href="https://nos-workspace.com/">NOS</a> when reviewing this CPU heightfield reference calculator. Eligible static instructions can be revised in visual editing mode; dynamic outputs follow their controls.</p></footer></main><script>
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let oldH=Array(25).fill(0),oldV=Array(25).fill(0),newH=Array(25).fill(0),newV=Array(25).fill(0);function calc(reverse){const h=Array(25).fill(0),v=Array(25).fill(0),nh=Array(25).fill(0),nv=Array(25).fill(0);h[12]=1;const ids=[6,7,8,11,12,13,16,17,18];if(reverse)ids.reverse();for(const i of ids){nv[i]=v[i]+.25*(h[i-1]+h[i+1]+h[i-5]+h[i+5]-4*h[i]);nh[i]=h[i]+nv[i];}return {h:nh,v:nv};}function step(reverse){const a=calc(reverse),b=calc(!reverse);newH=a.h;newV=a.v;$('#grid').textContent=[1,2,3].map(row=>'row '+row+': '+[1,2,3].map(col=>{const i=row*5+col;return fmt(a.h[i],2)+'/'+fmt(a.v[i],2);}).join(' · ')).join(String.fromCharCode(10));out('First step: center h = 0.00, v = -1.00; cardinal neighbors h = 0.25, v = 0.25; diagonals h = 0, v = 0. Full-state traversal equality: '+String(JSON.stringify(a)===JSON.stringify(b))+'. All 16 borders: h = 0, v = 0.');}on('forward',()=>step(false));on('reverse',()=>step(true));on('reset',()=>{for(const a of [oldH,oldV,newH,newV])a.fill(0);oldH[12]=1;newH[12]=1;$('#grid').textContent='Center: h = 1, v = 0; all other pairs: h = 0, v = 0.';out('Reset both states: center h = 1, v = 0.');});
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