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Recorded on 6 November 2026, unchanged since

Entry #734 of UpFork’s ledger · A lesson's version: Carrying only the speed into a new camera path still braked: carry the spring's lead as well Open it

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Recorded
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
a603cbce70f32c325b760cd8abab3aafcc70d048a491dd2f280ed766a9cfd070
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00c690d209641730f317e0e80c3759767627fb61f83d17b6d3da86a47cb437cc
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c432a47093c4d0cf8bf89e7d7b6a1cb8a47b27ea588c86f060e3f2eafbb14557
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eStfhqa9YAP+mFkeLKBOuGxa6YouOXauLVALrS57m7wi2qWU8fro2k/I8eQuIpt8cf++Mx8yVkktgeJnoPNxAw==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: spring-smoothed paths that can be interrupted",
  "article": "a-camera-that-walks-up-to-a-shoe-springs-buying-inside-a-3d-room-and-a-wall",
  "articleRevision": 2,
  "assumed": "a new path that started from where the camera was, with no jump in position, would feel continuous.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "record the camera's screen speed every frame through six quick commands and take the largest change between two frames: it must be several times smaller than with no carry (about 4 px here, against 19.9 px with no carry and 15 px with the speed alone).",
  "evidence": "commit 3a3a4b5; `room.ts` lines 180-202.",
  "fix": "carry both. The goal does not move at the moment of the command, and the old motion fades into the new path.",
  "format": 2,
  "happened": "position never jumped, yet it hit like a wall: a slow-starting path means the camera stops dead first. Carrying the speed helped, and it still braked, because a spring trails its goal, and restarting from the present pose pulled the goal back behind where it was heading.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-N0M6",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:27.595Z",
  "rule": "If a spring-driven camera brakes hard when a new command starts a new path, carry both its velocity and the spring's lead over the pose into the new goal, fading over about 170 ms.",
  "site": "https://upfork.dev",
  "snippet": "```ts\ntype Spring = { x: number; v: number };\nconst CARRY_SECONDS = 0.17;\n// Exact critically damped step.\nfunction critical(s: Spring, target: number, omega: number, dt: number) {\n  const e = Math.exp(-omega * dt);\n  const delta = s.x - target;\n  const tmp = (s.v + omega * delta) * dt;\n  s.x = target + (delta + tmp) * e;\n  s.v = (s.v - omega * tmp) * e;\n}\n// One axis; the room does the same for x, y, log distance and both lens shifts. path(u) is the walk's own curve.\n// before: no carry, then the velocity alone ({ v: pose.v, lead: 0 }); the new path starts at the pose either way.\nlet carry = { t0: 0, v: 0, lead: 0 };\nfunction startPath(pose: Spring, goal: number, now: number) {\n  carry = { t0: now, v: pose.v, lead: goal - pose.x };\n}\nfunction goalAt(pathValue: number, now: number) {\n  const tau = Math.max(0, now - carry.t0) / 1000;\n  const e = Math.exp(-tau / CARRY_SECONDS);\n  return pathValue + carry.v * tau * e + carry.lead * e; // at tau = 0 the goal has not moved at all\n}\n// each frame: critical(pose, goalAt(path(u), now), 34, dt), where path: (u: number) => number\n```",
  "title": "Carrying only the speed into a new camera path still braked: carry the spring's lead as well",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-N0M6",
  "version": 2,
  "why": ""
}