Left open earlier as do-not-fit. Closed by adding observables rather than tuning
a parameter.
The draw s vertex buffer carries positions and colours at the same instant, so
all four quantities must agree on one t. Solving independently: quad A x gives
357.88, quad B x 357.58, quad A alpha 355.75, quad B alpha 354.09. The alphas are
about 50x less precise per unit of time because alpha is a byte changing by only
0.27 to 0.33 levels per keyframe unit, so a single level of quantisation is worth
1.5 to 1.9 units, which at 4 px per unit is 6 to 8 px of sweep. The 11.5 px was
that.
At the position-derived t = 357.7 every observable lands: centres within 0.70 and
0.48 px, both alphas inside one level, and the parent alpha is 0 throughout. And
there is no pivot correction to find -- the leaf pivot is (200,90) against a
399x180 sprite, so the pivot is the sprite centre and rotation displaces it by
nothing.
The methodological point is the one this exchange started with, inverted. Earlier
a rule looked confirmed because it was checked against alpha, the insensitive
field. Here the same insensitivity manufactured an apparent 11.5 px error. The
insensitive quantity does not merely fail to falsify, it invents residuals. Solve
on the fastest-moving field and check the slow one, never the reverse.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QsEPXWVaEpyfudtR6re1Pd
f0fc4afd10