Jungfraujoch/docs/RUGNUX_OVERVIEW.md

What Rugnux does

The map of a run, in the order it happens — one paragraph per stage, each linking into the data-analysis reference where the algorithm lives. The walk-through is a rotation run with the defaults; stills differences are at the end.

Open the dataset. The geometry, wavelength and goniometer come from the file (What Rugnux reads). A goniometer axis that turns makes it a rotation run; none, or one that stands still from frame to frame, makes it serial stills — nothing is asked of the user.

Pre-scan. A projection of frames (60 by default) spread over the first part of the sweep, the part the geometry pre-pass integrates, finds the beam-stop shadow and masks it (§1.5), masks pixels its frames show to be defective (§1.6), measures the beam centre from the isotropy of the scattered background and compares it with the file's (§1.4), and reads how wide this crystal's spots are, which sets the integration radius (§9.5), and how much longer they are along their radius than across it, which is the beam's bandwidth where the file does not state one (§9.6), and how much wider still they grow away from the beam, which sets the integration footprint (§9.1). In a GPU build the pre-scan runs on the card.

Spots. Every image is decoded — on the GPU straight from the compressed chunk (§0) — and one fused pass computes the azimuthal profile and finds the spots against each image's own per-resolution-ring noise (§2–§3). The ice-ring score is read off the same profile.

Indexing. The spots of a sample of frames are rotated back to a common crystal frame and the FFT search looks for periodicity over thousands of directions; candidate cells are Niggli-reduced, classified by Bravais lattice, refined both constrained and triclinic, and decided on how many validation frames each actually indexes (§4–§7). A de-novo run also tries a second hypothesis with the shortest accepted axis lowered from 10 to 5 Å, for small-molecule cells. The first pass is indexed a second time at the beam centre the pre-scan measured; the file's centre is kept unless it indexes nothing, gives an axis harmonic of the measured centre's lattice, or loses to it when both first passes are merged (§1.4). A failed pass triggers the discrete rescues — the rotation-axis sign, the first-pass depth ladder, the long-axis rescue — before anything is given up on.

First integration pass. At the geometry in the file, every frame of the first half of the sweep, at least 120° of it (--prepass-fraction) is predicted (§8) and profile-fit integrated (§9); partials are combined into fulls, scaled and merged (§10).

Geometry post-refinement. From those reflections the detector distance, beam centre and the cell scale / rotation axis are refined over all those frames at once, each step committed only if it improves a held-out residual (§7.5).

Second pass. The sweep is re-indexed de novo and re-integrated at the refined geometry; this pass is the canonical output, and a guard compares the two passes and keeps the better one (reported as PASS= / PASS_DECISION= in the report).

Space group. On the P1 merge of the final pass, the point group is scored operator by operator on resolution-normalised intensities and the screw axes, glide planes and centring are read from the systematic absences, with the centre of symmetry from the intensity distribution where the absences leave it open (§13.1); twinning and translational pseudo-symmetry are checked beside it (§13.2). CANNOT_DETERMINE and an enantiomorphic pair are real answers here, not evasions.

Scale and merge. In the determined group: per-frame scales, the cross-validated correction surfaces (decay, absorption, modulation), the error model and ISa, outlier rejection, the CC1/2-based resolution cut, the anisotropy description, French–Wilson amplitudes and the R-free flags (§10, §13.3–§13.5).

Model: validation and molecular replacement. Only with a model (--model or --mr, PDB or mmCIF). With --model the model is placed as a rigid body and checked against a null; where that does not place it, or the model is a prediction, it is placed by molecular replacement instead, and --mr goes to molecular replacement straight away. The search places as many copies as the cell holds, in every space group the merge cannot tell apart and in the twin hypothesis' subgroup; the placed model then gives R-work/R-free and σA-weighted maps. The model never changes how the data are processed (Model and reference, §14). It runs on every GPU of the machine, and on a Mac its model density and rotation function on the Apple GPU (GPU data analysis).

Write. The merged .mtz / .cif / .hkl, the unmerged MTZ and the results report land next to each other (Output files); with a model, the maps and the placed model are written too, and where molecular replacement chooses another space group or the twin hypothesis' subgroup, that merge is written beside the answer as <prefix>_mr.mtz/.cif with the model placed there, <prefix>_mr_model.pdb.

Stills instead. Serial data skip the two-pass machinery: each image is indexed independently (with the known-cell ffbidx indexer where a cell is given), partiality comes from a per-crystal orientation-tilt post-refinement rather than a rocking curve, and a merohedral indexing ambiguity has to be broken per image, at integration time, against a reference or a model (Advanced ▸ the indexing ambiguity).