[2.1.8] Spidertron Legs Unstable Jittering Over Dense Entities

[fawniux]

Apologies if this is a duplicate of bug report 134383.
I am describing a slightly different issue but I expect the underlying cause is similar.

What Happened?
After updating to the latest experimental 2.1 patch, spidertrons' legs are sometimes unable to find a stable position when moving over densely-occupied terrain. This causes prolonged 'jittering' of the legs which can fail to settle even after all movement keys are released. The screen shakes and the spidertron's position can either fluctuate in place, or be passively transported given specific entity configurations. (See videos below as demonstration.)

This seems more severe with higher movement speed. A legendary spidertron with 14 legendary exoskeletons experiences the issue very frequently in compact bases. A default-speed legendary spidertron seems to also cause the issue, but it is less noticeable since the movement fluctuations are slower and less disruptive. I didn't test any intermediate speeds or other quality levels.

Platform Details
Steam Factorio distribution
version: 2.1.8
OS/hardware: linux x64, ryzen CPU, AMD GPU
build: 86744 expansion, linux64
mods: base, all Space Age mods, no 3rd party mods

What Was Expected?
That the spidertron settles to a static position within 1 or 2 seconds after releasing movement keys, as it did in 2.0.77 and before.

What Was Actually Observed?
After moving to an area with dense entities, the spidertron continuously tries to re-plant its legs if there are few open spaces. For example, spaghetti pipes connected to buildings or adjacent entities with thin collision gaps, or large entities the spider must stretch over (e.g. rocket silos, big demolisher remains).

Sometimes it finds a stable position after several seconds, sometimes it seems to never settle, although I didn't test for more than 60 seconds in any case.

Occasionally it can even be perpetually transported by the constantly changing pressure of the legs, as is the case for the purple spider below. After initially squeezing between the rocket silos, no keys are held down but the spider ambiently moves up the entire line. The rest of the video shows some static fluctuations; some entity layouts in my other saves caused more intense fluctuations than in the video but it was generally unpredictable.

This issue doesn't always happen; often the spider will settle smoothly as in 2.0. But running around a large base with compact production blocks makes it frequent enough to be distracting. Visually the screen shaking is unpleasant, and mechanically the cursor moves around which makes building frustrating.






Replication Steps

I've attached an editor world save with a default- and max-speed spidertron, and a couple of entity layouts. Walking over the entities in some random pattern causes the issue quite quickly and consistently.

spidertron_bug_legs_jittering.zip

reproduction example save file

(4.62 MiB) Downloaded 5 times

There is nothing special about these specific entity layouts. Any surface with sufficiently sparse empty tiles should be able to replicate the problem when moving over it.

[ramyalexis]

Looks like "same" bug viewtopic.php?t=134383

[fawniux]

Yes, I'd assume the same mechanism is causing both bugs due to some change in the spidertron's pathing algorithm. If I had to guess it's perhaps because of the new patrol features?

My experience suggests it doesn't just cause slight timing differences but also meaningful unintended behaviour. I was most surprised that they can get pushed a notable distance from their intended position with the right setup.

[Rseding91]

Thanks for the report. With Boskid's help we were able to identify the issue and it's now fixed for the next release.