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PROMPT (exact, as issued):
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Build a standalone, self-contained HTML page: a glass prism on a
windowsill in a dim room, projecting a soft rainbow onto the wall as
light turns. Palette: dim warm room, cool window light, vivid projected
spectrum. Signature: CSS/SVG prism, an animated light beam, a projected
rainbow arc cast on the wall with soft bloom, slow color rotation.
Constraints: one self-contained .html, ALL CSS in one <style> and ALL JS
in one <script> (inline everything), ZERO external dependencies (no
CDN, no external fonts — system stacks only, no external images/SVG
files/libraries), works fully offline by double-clicking with no network
requests, responsive 320px->4K, valid HTML5, Awwwards-caliber original
art direction. Keep the scene ATMOSPHERIC — a moody room with a prism on
a windowsill projecting a rainbow onto a wall (distinct from a graphic
close-up of a prism).

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TITLE:
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PRISM · Light Divided

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DESIGN DESCRIPTION:
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A single beam of cool window light crosses a dim, warm-lit room and
strikes a hand-drawn glass prism resting on the windowsill; the beam
refracts through the glass and throws a soft, blooming spectrum across
the opposite wall, where the colours rotate and breathe slowly as if the
sun itself is turning. The composition keeps the prism small and the
moment cinematic — the room stays moody, the window stays bright, and the
projected rainbow is the emotional centre of the frame.

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MAJOR VISUAL TECHNIQUES:
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- Inline SVG triangular prism with a glass gradient, edge highlights and
  a subtle inner caustic.
- Animated light beam drawn as a tapered gradient polygon that pulses
  and subtly wanders toward the prism.
- Projected rainbow built from stacked, blurred SVG arc bands with a
  screen-blend bloom so the spectrum glows into the wall.
- Slow hue rotation and opacity breathing on the projected arc to imply
  shifting daylight.
- Layered CSS gradients for the dim warm room, the cool window rectangle
  and the vignette that frames the scene.
- Canvas dust motes drifting through the sunbeam for atmosphere and depth.
- A single cursor-driven signature effect: moving the pointer steers the
  beam and sweeps the rainbow across the wall.

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INTENDED INTERACTION MODEL:
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Passive-cinematic with one point of agency. On load the page runs its
loop — beam pulses, spectrum breathes, dust drifts — like a living
painting. The viewer can move the pointer (or drag on touch) to steer the
incoming light beam, which sweeps the projected rainbow across the wall;
nothing is clickable, the mood is uninterrupted, and the scene resets to
its slow default motion when left alone.