Reported as "the wheels are in the wrong place, it is floating", and it was
three separate faults in one wheel.
The one that showed: the part declares `origin` at the axle so `setRoll` turns
the wheels in place, and a part with an origin has its geometry written
RELATIVE to that point — the anchor supplies the rest. The geometry was ALSO
translated by the axle height and position, so both were applied twice. The
wheels sat 304 mm off the ground at twice the axle height, and the cart stood
on its two shaft tips.
The one hiding behind it: the felloe segments were turned into place and then
moved out to the rim. That looks equivalent to moving out and then turning and
it is not — `rotateZ(a)` sends local +X to (cos a, sin a) and local +Y to
(-sin a, cos a), so a piece placed at (cos a, sin a) has its ARC pointing
radially. Every rim segment was a stubby block sticking 89 mm past its own
felloe, straight through the iron tyre meant to bind it. `cart-wheel` has had
the correct order all along, with a comment saying why; this model did not
follow it.
And the hub: a lathe is built around Y, the wheel is drawn in the XY plane, so
the hub stood upright inside the wheel like a little drum instead of running
along the axle. Invisible behind the spokes, which is how it lasted.
Then a fourth, found while checking the fix. The ground meets a pole's SURFACE,
not its axis: the shafts ran their axis to y = 0, so the square-cut end of an
inclined pole buried its low corner 13.7 mm, and the cart rested on two buried
tips with its wheels still fractionally clear. The axis now stops one tip
radius short and wheels and shafts share one floor.
`restsOn` is the check that would have caught all of it. The support check asks
whether every piece is connected down to the ground, and a cart standing on its
shaft tips answers yes: bed hangs off shafts, wheels hang off bed, nothing
floats. It reported 0 failing cases throughout. Connectivity cannot tell a
wheel from a decoration, and the renderer cannot either — the contact shadow is
cast from the model's own lowest point, so the shadow looked right the whole
time.
What the geometry cannot know, the author does. Naming the parts that carry the
weight turns "it rests on its wheels" into arithmetic, at 1 mm, which is tight
enough to have caught the 13.7 mm as well as the 304 mm. Declared on the cart,
the trestle table, the bench and the market stall.
Two models that each fill a hole the catalogue could name.
The kit had a torch and a lantern and no HEARTH, which is the wrong way
round: a cooking fire is what everyone in a medieval scene is facing and
the only light in most of them. The cauldron hangs from a tripod over a
ring of hearth stones, with embers that `setLit` puts out. Its belly is
wider than its mouth, it has three short feet because it stands in the
ashes as often as it hangs over them, and it hangs by a bail rather than by
its rim.
And there was a `cart-wheel` with nothing to put it on -- while every
container in the catalogue, crates and sacks and barrels and baskets, is a
thing you would MOVE. The handcart's axle sits aft of the bed's centre so a
load leans on the puller rather than away from him, and its shafts leave
the bed at an angle and rest on the ground, which is what lets one person
lift them to waist height without the load shifting.
The cauldron found a fault in the mill on its way in. Both build their
splayed legs the same way -- lean about X, swing about Y, translate out
along the radius -- and the lean was POSITIVE, which carries the top the
same way the translate does. The two add, so the head ends up at the full
outboard radius and the foot at the axis: the tripod stood on its apex and
the mill's quarter-bars stood on their heads, splayed over the cross-trees
and gathered at the post, which is precisely backwards for a brace whose
job is to carry a load out to the ground. Nothing had caught it in the
mill, because a bar joins post to cross-tree either way round and the
bounding box is identical; in the cauldron it separated the tripod from the
ground and the support check finally had something to say.
Lowering the pot to where it could actually cook then broke its chain, and
for a reason this session has seen before: the link radius was derived by
dividing the drop by the link COUNT while the links are laid out over
count-1 gaps, so each link came out a little smaller than half its own gap
and the chain came apart as soon as the drop grew. The tavern sign's chain
had the same slip.
31 models, 25 221 triangles, all checks pass, 0 floating cases, GLB
round-trip clean, Conformant, build clean.
