Gwyddion Path Level Compatibility

Status and scope

gwyddion_path_level records the registered Gwyddion pathlevel Path Level tool within the frozen, finite, non-empty, full-field campaign. The reference is the installed tools.so module, SHA-256 4711c360dd42e3e16257bf0e86d8bd41852b43d1d34540bf097736a603146237, Build ID 600b16d9857946609b567704b406abcc74aea698, whose debug source matches the frozen pathlevel.c SHA-256 4c0411c73f7ca883d4d03f35b38ef81a02ea3c7688620754992cb58e8825326f.

The evidence consists of 18 field/selection families, four thicknesses, 72 logical cases, 144 fresh external executions, and an independent Python oracle with 72/72 bitwise-exact arrays. This is not a universal-equivalence claim.

Selection identity and coordinates

Path Level consumes an ordered collection of straight GwySelectionLine objects, not GwySelectionPath. GwySelectionPath belongs to unrelated path/spline tools. Every line is (x0, y0, x1, y1) in physical data-field coordinates. The tool maps horizontal coordinates as x*xres/xreal and vertical coordinates as y*yres/yreal; field origin offsets do not participate.

Each endpoint is floored. If the first Y is greater than the second Y, both endpoints are swapped. X endpoints and Y bounds are clamped to the field. The active transition domain is y0 < row <= y1; consequently horizontal lines are excluded.

Numerical contract

For each ordered line object, the tool creates a start and an end change point. They are ordered by row, then starts before ends, then object ID. This makes line IDs and user-supplied object order scientifically relevant. Duplicates and overlap retain multiplicity.

For an active line, the column at each row transition uses the source integer formula with C signed integer division truncated toward zero. A thickness window is inclusive and asymmetric: (thickness - 1)//2 samples on the lower column side and thickness//2 on the upper side, clamped to valid columns. Its range is 1..128; the future public default is 1.

Row differences are accumulated as explicit scalar current - previous samples in line-object and increasing-column order, then divided once by the sample count. The per-row differences are cumulatively summed left to right. That correction is subtracted from every column of its row. There is no interpolation, mask, ROI, path, spline, or profile operation in this contract.

Publication semantics

Gwyddion mutates and publishes the selected data field in place, with undo and tool logging. The future SPMKit kernel returns a new corrected array and does not mutate its input. This intentionally follows SPMKit immutable-return convention; it does not claim GUI, undo, logging, or publication parity.

Evidence and non-claims

The frozen fixture preserves external input/output records, independently regenerated inputs/cases, oracle comparison records, physical ranges, ordered lines, normalized endpoints, and bitwise uint64 hashes. It covers signed zero, line-order sensitivity, overlap, clamping, fractional coordinates, and all four frozen thicknesses.

No claim is made for NaN or infinity, masks or ROI, spline/polyline paths, profile extraction, volume line-leveling, align_rows equivalence, performance parity, other Gwyddion builds or versions, or universal equivalence.