The alternating per-frame scaling used to run a fixed three rounds of a Cauchy-reweighted fit
against a reference that included half a sweep's rocking-curve tails. Three rounds left a short
sweep merged in P1 far from its answer (a 90 deg tetragonal sweep refused its 422 with the P1
scales anti-correlated with the converged ones), and more rounds did not help: the fit had no
fixed point. Two things made it walk. The objective is invariant under G -> cG with the reference
-> reference/c, so every round moved every scale by a constant factor; and the robust loss, iterated
against a reference refitted each round, drops the strong reflections of a frame whose scale is off
by a third (ten-sigma residuals) and lets the weak ones carry it further off - measured on a 360 deg
sweep the scales shrank 10-30% per round for thirty rounds and the H ratio of a genuine 222 read 21x.
Now the loop pins its gauge every iteration (G divided by the precision-weighted typical frame
scale G_ref = sum G^3 / sum G^2, one definition shared with the CC1/2 weight), fits the plain
weighted least-squares slope with the weights the reference uses and only on the observations the
reference is built from (the partiality floor), and stops when the rms |log(G_new/G_old)| over the
frames falls below 1e-3. With the same weights on both sides the alternating fit is exact
coordinate descent on one objective and cannot climb; measured, the 360 deg sweep settles in 19
rounds and the 90 deg one in 30-50, each pass's partials and fulls loops alike.
--scaling-iterations is now the cap (default 100). A loop that reaches it is logged, the report
prints SCALING_ITERATIONS and raises SCALING_NOT_CONVERGED, and the correction surfaces run to the
same tolerance under their own cap. On the GPU the loop runs one iteration per call so the pin and
the step test read the same numbers as on the host; the fulls' reset is split out of ScaleFulls.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nW6FNRP1bBJJ8pfHiByAT
leonarski_fandClaude Fable 5.1
61afff891b