[us-commits] [ehb54/ultrascan3] 2c58b4: SAXS extrap-to-c0: Primus mode = weighted merge, n...

[emre brookes]

  Branch: refs/heads/ehb54-issue-920
  Home:   https://github.com/ehb54/ultrascan3
  Commit: 2c58b44034274c0cff1b5c5a19dd8db216b47a6a
      https://github.com/ehb54/ultrascan3/commit/2c58b44034274c0cff1b5c5a19dd8db216b47a6a
  Author: ehb54 <brookes at uthscsa.edu>
  Date:   2026-07-11 (Sat, 11 Jul 2026)

  Changed paths:
    M us_somo/develop/src/us_hydrodyn_saxs_iqq_extrap_c0.cpp
    M us_somo/develop/src/us_hydrodyn_saxs_iqq_extrap_c0_conc.cpp
    M us_somo/somo/doc/manual/somo/somo_saxs_extrap_c0.html

  Log Message:
  -----------
  SAXS extrap-to-c0: Primus mode = weighted merge, not per-q extrapolation

Primus mode previously did a per-q linear fit of the scaled intensity vs
concentration and took the c=0 intercept. Once the curves are scaled onto
the reference their residual concentration dependence is negligible, so
that per-point fit mostly tracks noise -- it drove the high-q intensity
negative (25 negative points on the asyn test set), and since the error
column is the copied reference sd, those near-zero/negative points showed
enormous error bars on a log plot. This is what a colleague reported as
"Primus error bars too large vs ATSAS"; the sd column itself was already
byte-identical to the ATSAS reference.

Replace the per-q extrapolation with the inverse-variance weighted MERGE
of the scaled curves (matching ATSAS almerge). On the asyn data this
removes all 25 negative points (ATSAS has 0), eliminates all 37
huge-error-bar points, and matches the ATSAS zc_001.dat curve far better
(median rel. diff low/mid/high-q 0.008/0.032/0.104 vs 0.026/0.133/0.470).

The error column (reference sd), reference scaling, Conc tagging, and the
I*(q) handling are unchanged. The regression pop-up now draws a level
(slope 0) at the merged value. Zimm mode is untouched. Comments, editor
message, dialog tooltip, and manual page updated.

Co-Authored-By: Claude Fable 5 <noreply at anthropic.com>



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