Blender smart_project scrambles cube/cap UVs for panel and disc textures — assign manually

bpy.ops.uv.smart_project works well for organic meshes and irregular shapes, but produces wrong-looking results in two specific cases relevant to retro / low-poly asset authoring:

1. Cube props with a "panel" texture (washer, dryer, arcade cabinet, monitor — anything where the texture has a clear top/bottom and front/back orientation): smart_project picks per-face rotations from its own heuristics, so the bottom vent grille ends up on the top face, the readout ends up sideways, etc. The texture is "applied" correctly in the loose sense — every face has UVs in [0,1] — but no face reads right.

2. N-gon cap faces built as a triangle fan (hex prism coin, circular button, lens, disc): smart_project treats each triangle of the fan as a separate UV island and spreads them across UV space. A single painted logo on the cap renders as N disjoint wedges instead of one coherent stamp.

smart_project's heuristic is "minimise stretch, pack islands". For a cube, "minimum stretch" allows arbitrary 90° rotations per face — no concept of "this texture has a canonical up". For a triangle fan, each triangle's normals are identical but the operator still treats triangulated faces as separate islands when packing. For cube panel props: build the cube with bmesh in known face order, then iterate faces and write UVs by hand. Pattern:

# Assume standard ±X / ±Y / ±Z faces. For each face's loops, assign a UV
# corner so the texture's +V direction aligns with world +Z on side faces,
# and the front/back (±Y) faces use mirrored UV corners so the painted panel
# reads correctly from both sides.
front_uv = [(0,0), (1,0), (1,1), (0,1)]
back_uv  = [(1,0), (0,0), (0,1), (1,1)]   # mirrored so texture isn't flipped

For radially-symmetric cap faces: after smart_project (or instead of it for the cap), walk faces whose normal is close to the cap axis and rewrite UVs from the vertex's planar position on a unit disc:

# For each cap vertex (radius R, normal along ±Z):
u = 0.5 + (v.co.x / R) * 0.45
v_uv = 0.5 + (v.co.y / R) * 0.45
# Mirror U on the back cap if the stamp should read the same on both sides:
if normal.z < 0: u = 1.0 - u

The 0.45 (vs 0.5) leaves padding so the design isn't clipped at the rim. Side-face quad strips are fine to leave to smart_project.

Verify with a viewport screenshot from multiple angles before shipping — single-island UVs only look right if every face of the prop reads the design correctly. Applied to washer/dryer (panel texture) and to a hex-prism token (logo on cap). Both read correctly from all viewing angles after manual UV pass. User confirmed visually. 3D Asset Pipeline