The one I fixed by taking the ground away

The last post was about a height map being asked a question it could not answer, cars driving along the tops of roofs, and nine places getting a second map built to answer the right question. There are ten places. The tenth is the one I had fixed three weeks earlier by taking its collider away.

That is the mistake the same post is about, one step further on. Understand a cause, correct the instance, write a careful note, move along. What the note did not say is that removing a collider does not leave a car on the ground. It leaves it on the flat backstop plane that sits under every capture as a last resort, and that plane holds one number for an entire town.

That number had been set by looking at the ground in exactly one spot: where the car appears. Five centimetres out there, which is why it looked right, and wrong everywhere else. Within forty-five metres of the spawn, 94% of the scanned ground stands above it, by a metre and a bit on average. The road climbs the hill. The car did not.

Ludlow is also the place people actually drive, and it had been doing this since the ninth of August.

Worse than that, the previous post named it the worst-driving capture in the set, on the strength of a measurement: 14% of a simulated lap taking more than half a g vertically. The measurement was honest and it was measuring the plane. On the baked ground the same lap reads 4%, the sustained roughness drops from 1.01 g to 0.31, and the share of the run that is over the capture at all goes from none of it to all of it. So a public criticism of a photograph of a town turned out to be a bug in my own floor.

The rest of the week went on a second thing that had been hiding behind the first: even with a correct ground under every place, eight of the ten cars were starting underneath the road.

A car is built the instant a place opens. The ground it stands on is a file, and files arrive when they arrive. So for the first fraction of a second the only thing under 1245 kg is that backstop plane, and now that the plane is only a backstop it is free to sit a long way down. At the Aix spawn the road is 6.8 metres above it. In Lisbon, 3.4. In Ludlow, three centimetres, which is the one reason this was never seen in the place everyone drives.

The car falls. The ground then loads above it, and the physics engine will not push a body up through a surface that arrived over it, quite reasonably. That is the report about the car spawning below the map and pressing R putting it right: R returns the car to its starting pose, which by then has a road underneath it.

The instinct is to spawn higher. That was measured and it does not work. Falling time goes as the square root of the height, so twenty metres of drop buys about two seconds of margin, which is still not a guarantee, and it costs a landing at seventeen metres per second on a car that is supposed to be parked.

It is a race, so the answer is a wait rather than a correction. Gravity is switched off on the chassis while its ground is still in the air, and the car is set back down on its exact starting pose the moment the ground lands. A body with no gravity and no contacts does not move, so this costs nothing per frame and the driving code does not know it exists. A ground that never answers counts as a landing after fifteen seconds, because a car parked in mid-air over a broken download is worse than a car on the plane, which at least drives.

The check on all this deliberately avoids the physics grid, since a grid quietly disagreeing with the picture is the entire class of bug. It reads the capture itself, takes the points in the column under each wheel, and asks how far the wheel is from the road you can see. All ten places now sit within three centimetres of it. Before this, eight of them were between 0.67 and 6.54 metres under it, every one of them resting exactly on the plane.

The small one I like most: the suspension thump a car makes when it settles used to fire at the moment the car was created, which is the moment it begins falling. It is wired to the landing now, so it is the sound of the thing it is a sound of.