fixed xynova texture generation problem

docs/xynova-texture-investigation.md

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+# Xynova camera-projected patch investigation
+
+## Intended pattern
+
+The requested squares are defined in the image seen by `cam_sensor`, not in the
+flat compute PNG or in physical surface distances. The current 1000×1000 camera
+and 80×80 grid define nominal 12.5×12.5 camera-pixel cells. Silhouette and occlusion
+boundaries may clip those cells.
+
+Changing PNG width/height or forcing UV shapes to squares is not a correction of
+the camera pattern. An independent raycast using Blender loop triangles, a BVH,
+and triangle barycentric UV interpolation measured the following centre-region
+steps (camera samples (0.5,0.5), (0.5125,0.5), (0.5,0.5125)):
+
+| UV layout | Horizontal step in texture pixels | Vertical step | Ratio |
+|---|---:|---:|---:|
+| compute | 59.3465 | 50.7074 | 1.17037 |
+| produce | 106.1594 | 90.2955 | 1.17569 |
+
+These are local mapped edge lengths, not axis-aligned bounding boxes. They show
+that camera-square elements can legitimately look rectangular in the flat UV
+artwork. Curved surfaces also make the distortion position-dependent.
+
+## Confirmed interpolation defect
+
+The investigated blend contains 4,818 triangles and 16 quads with no modifiers.
+A valid quad does not need to be removed or manually triangulated to be usable.
+However, the original `get_uv_from_face()` always selected a polygon's first
+three vertices, even for a ray hitting the other tessellated half of a quad. It
+also formed barycentric weights from unsigned triangle areas, which are not a
+valid containment-aware extrapolation outside that first triangle.
+
+At an actual tessellated triangle centre on compute polygon 4826:
+
+- Correct UV: approximately `(0.0480605, 0.2511341)`.
+- Original generator UV: approximately `(-0.0245351, 0.3876915)`.
+
+The error is hundreds of output pixels and can leave the tile. The original
+periodic-wrapping heuristic can then relocate the bad polygon into another part
+of the texture. Another tested compute polygon (2876) produced a UV error of
+approximately 0.1471, mostly in V.
+
+## Separate seam-construction weakness
+
+The original generator connected independently raycast camera-cell corners into
+one UV polygon (or their convex hull near the silhouette). That loses the face
+and UV-chart identity of each sample. A cell spanning a UV seam cannot generally
+be represented by one polygon connecting all its samples. A UV-span threshold
+or a final raster mask can hide some symptoms, but neither repairs the mapping
+and a raster-only mask leaves incorrect SVG/PDF geometry.
+
+The approved correction uses triangle-local camera-cell intersections, keeps
+separate UV fragments across seams, and applies perspective-correct interpolation.
+Verification must check the vector fragments against the actual UV triangles and
+check their reprojection against the camera grid, not merely inspect the PNG.
+
+## Source preservation baseline
+
+Source: `sensors/finger-xynova/r01/source/geometry.blend`
+
+SHA-256 recorded before implementation:
+
+`574e1e4501db224e025a0b44327afa53726d71a25c36aa8c36d8c45a6f0317b2`
+
+The blend was already modified relative to Git HEAD when this investigation
+started. That user modification must be preserved. No camera configuration,
+mesh editing, UV re-unwrapping, staging, or commits are part of this repair.