Inspect the morph before Play

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<section class="geometry"><div class="drawing"><svg id="geometry" viewBox="0 0 100 100" role="img" aria-labelledby="geometryTitle"><title id="geometryTitle">Endpoint A, endpoint B, and the current interpolated four-point shape</title><defs><pattern id="grid" width="10" height="10" patternUnits="userSpaceOnUse"><path d="M10 0H0V10" fill="none" stroke="#dbe1e8" stroke-width=".25"/></pattern></defs><rect width="100" height="100" fill="url(#grid)"/><path id="shapeA" fill="none" stroke="#657587" stroke-width=".8" stroke-dasharray="2 1"/><path id="shapeB" fill="none" stroke="#9d6a34" stroke-width=".8" stroke-dasharray=".6 1.2"/><path id="shapeCurrent" fill="#64829833" stroke="#284e70" stroke-width="1.1"/><g id="pointLabels"></g></svg><div class="legend"><span>A · dashed</span><span>B · dotted</span><strong>Current · solid</strong></div></div><div class="panel"><div class="strip"><span class="eyebrow">Current fraction</span><strong id="fraction" class="metric">0</strong></div><label for="t">Interpolation t · 0 to 1</label><input id="t" type="range" min="0" max="1" step="0.001" value="0"><div class="buttons"><button id="zero">A · 0</button><button id="half">Midpoint · 0.5</button><button id="one">B · 1</button></div><div class="buttons"><button class="primary" id="play">Play 2-second pass</button><button id="pause" disabled>Pause</button><button id="reset">Reset fixture</button></div><p id="feedback" class="muted" role="status">Paused at A.</p><h3 class="below">Current d</h3><pre id="readout" class="record">M20 20 L80 20 L80 80 L20 80 Z</pre></div></section>
<section class="panel below"><h2>Endpoint contract</h2><div class="row"><div><label for="a">A · four points in fixed order</label><textarea id="a" spellcheck="false">M20 20 L80 20 L80 80 L20 80 Z</textarea></div><div><label for="b">B · corresponding point order</label><textarea id="b" spellcheck="false">M30 10 L90 30 L70 90 L10 70 Z</textarea></div></div><div class="buttons"><button id="apply" class="primary">Validate endpoints</button></div><p id="error" class="error" role="status"></p><div id="coordinates"></div><p class="muted">Each coordinate is a + (b − a) × t from the original endpoints. The current path retains unrounded numbers; the table uses up to six decimals. This bounded helper accepts uppercase M L L L Z only; it does not align points or normalize curves. The 0–100 view fits the fixture; other coordinates can lie outside it.</p><details><summary>Playback and compatibility</summary><p>No autoplay. A complete pass advances t by 0.5 per active second, stopping at B. Manual selection, invalid endpoints, hidden-page suspension and reduced-motion changes stop playback. Reduced motion keeps manual selection available.</p><p>Matching structure does not guarantee topology or prevent every self-intersection. Incompatible paths are outside this helper; they are not universally invalid SVG.</p><a href="https://www.w3.org/TR/SVG2/paths.html#PathData">W3C SVG path structure and animation rules</a></details></section><footer class="muted below">Local working copy · Changes are not saved after this page closes.</footer></main><script data-app>
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const originalA='M20 20 L80 20 L80 80 L20 80 Z',originalB='M30 10 L90 30 L70 90 L10 70 Z';let A=null,B=null,t=0,playing=false,frame=null,epoch=0,startTime=null,startT=0;const motion=window.matchMedia('(prefers-reduced-motion: reduce)');
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