Spec prompting, the state list

Core idea. Describe the film as a table of states with numbers (w×h, radius, fill, content and time) rather than adjectives. "Grows into a card" can mean anything; "880 × 600, radius 48, at 1.5 s" can't, and you can check it.

Morph rules

• One spring track per property.
• Content enters after a morph starts and leaves before the next one starts.
• Fills are interpolated in OKLab (lib/color.js), not RGB, which goes muddy at the midpoint.

Loops, correctly. For duration D at F fps, render frames 0 … N−1. The state at t = D equals the state at t = 0, but t = D is never rendered, because a duplicated frame causes a hitch. Velocity must match at the seam too.

Testable specs. check.py renders each state's time, measures the container, and fails if it's off by more than 2 px. Florios morph: 8/8 states OK, and the loop value and velocity differences were exactly 0.

A check only proves what it measures. Every check passed, but text inside the shapes was clipped at its baseline. The geometry check looked at boxes, not text. Fix: masks cover the font's full descent, plus a new check that re-renders each state with masks off and requires identical pixels.

Fact-check of the article

• ❌ "@verbove's film used an XML spec." His own post says he showed Opus one viral post and wrote a one-line prompt.
• ❌ "The last frame must equal the first": see the loop rule above.
• ⚠️ The will-change gotcha is real for DOM layers but irrelevant to canvas rendering.
• ✅ XML tags help structure mixed prompts (Anthropic's docs). Markdown works too.