[ii-impy]

Closes #161.

A new shader player Setup, rainbow_poop. About every 5 s, a rainbow poop appears at the top of the sky over each visitor. It drifts down slowly enough to watch and splats on their silhouette. The splats stick to the person, drip, and fade after 20 s. If the visitor turns round after it appears, it misses and lands on the floor at their feet.

Spec: docs/superpowers/specs/2026-11-08-rainbow-poop-design.md. The first build had a flock of birds dive-bombing; the birds were removed on review (see the spec's "Revisions" section).

How it works

• Buffer B (a small texel grid) holds, per person:

  • a countdown to their next poop
  • an anchor: their silhouette's centre of mass and size, measured from this frame's Body inside the tracker's box. Raised arms, strides and box jitter don't move splats, and they don't lag.
  • their poop while it's falling (one at a time)
  • a ring of 8 splats, stored in the person's own frame so they ride along with them

  It also holds a ring of 8 floor puddles.

• The fall: the poop starts just above the top of the screen. The drop point is over the person's centre of mass, ahead of them by their velocity × the fall time, and always on screen. It falls at a steady FALL_SPEED (0.2 screen heights/s) with a gentle sway, drifting toward its person at up to HOMING_MAX (0.18 screen heights/s). Standing or fidgeting gets you hit; walking or turning away dodges.

• Image draws paper and clouds, floor puddles, people (warm-white fill with an ink outline, as in hunter_bird), splats clipped to the silhouette, then falling poops.

• Sizes: splats are 0.15 × the person's size (about their height) in radius, and puddles 0.1 screen heights.

• Ages and countdowns advance in 1/32 s steps with golden-ratio rounding, which keeps them exact in 16-bit buffers.

• Shadertoy: Buffer D is left free for the blob painter.

• Every tunable is at the top of common.glsl.

Testing

• test_rainbow_poop.py: 22 headless GL tests using the real BodyBlobs tracker on a fake clock. They cover:
  • the empty room
  • first hit and cadence
  • the fall: starts at the top, takes over a second, one at a time, drawn while falling
  • a fidgeting, person-shaped silhouette (narrow head, legs, an arm in and out) is always hit
  • splats stay put on the body while arms go up and out and the person walks
  • two people both getting hit
  • splats following a walker, fading, keeping time, and fading on loss
  • a reused slot starting clean
  • dodging and the floor puddle
  • clipping to the silhouette
  • bounded state through huge time steps
• They pass on this machine's Intel GPU and under llvmpipe.
• Full Python suite: 464 passed. test_hunter_bird.py::test_person_has_white_fill_and_ink_outline fails on this machine on a clean main too, but passes in CI.
• Frame rate on the show machine (the Intel Ultra 9 mini PC): shader_app.py --mock-camera --windowed --setup rainbow_poop holds the 30 fps cap (30.2 fps; render 640×360).

Not verified

• How the cadence, fall speed and dodging feel with the real camera and real visitors.
• Pasting into Shadertoy.

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