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Matysh
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Audit finding P2. At a grazing sun the wedge lost the two invariants it was supposed to keep: one end of the GLASS started at opacity 0, and the two sides of one shaft came out 5.41 and 84.19 long — the long one 31 % LONGER than the pre-cut 64, not 30 % shorter. The cause was the axis. The gradient ran along `dir` from the middle of the window span, so the geometry had to be skewed (each end extruded by a different amount) to make both far corners land on the same offset. That buys the iso-alpha far edge with the other two requirements. The light is a bundle of PARALLEL rays: the distance a point has travelled from the glass is depth/cos, an affine function of the point, whose level sets are lines PARALLEL TO THE WALL. So the correct linear gradient runs along the wall's INWARD NORMAL, starts on the window line and is `len·cos(incidence)` long — SunRay.normal / SunRay.depth. A point `source + dir·u` then lands on offset u/len, whichever ray it rode in on. All three invariants hold at once: * the whole pane of glass is at depth 0 → peak alpha end to end; * alpha depends only on how far that point's own ray has run; * rayQuad() is an honest parallelogram again (both ends extruded by the same `len`), and its far edge — parallel to the wall — IS the gradient's last iso-alpha line, so a bright kerb is impossible by construction and the −30 % holds for every side of every wedge. windowLit() gets a real threshold instead of the 1e-9 epsilon: RAY_MIN_COS = 0.05, i.e. the sun must clear the plane of the wall by ~2.9°. Below it glass reflects nearly everything and the shaft would be a sliver thinner than the wall it came through — nothing is drawn, and the gradient axis can never degenerate to a point. Tests: rayQuad now asserts equal, full-length sides and a wall-parallel far edge; new unit tests replay the auditor's repro with his numbers (both sides 44.8, offsets 0 at both ends of the glass, offset = travel / len for arbitrary rays) and the RAY_MIN_COS cut-off. smoke_sun_soft measures the same facts off the DOM gradient end to end and fails by name on the old bundle (9 named failures). docs/SUN.md carries the new contract and the finding.