Comparing Absolute Distance using GlobalTransform Not Working as Expected

[TomPridham]

I'm trying to implement a rope using Verlet transforms and enforcing constraints via Jakobsen's method. I was working off this post: https://www.owlree.blog/posts/simulating-a-rope.html. Things were working mostly correctly when I was using normal Transform. The main issue there was that the first(parent) node and the second(child) node weren't offset correctly because the Transform for the parent is relative to the origin and the child is relative to the parent. I thought that using GlobalTransform to compare the distances would be the correct thing to do, but now the translation for the cubes I'm rendering very quickly approach the max for a f32, overflow and disappear. I could solve this by have a separate parent element that is basically just a container for the rope elements, but that doesn't seem like it should be required. Is that just the way?

You can see what I am talking about here: https://github.com/TomPridham/bevy-cable/pull/3/files. The pull request has the bad behavior using GlobalTransform and main has the normal behavior using Transform. You can run cargo run --example basic to see it in action. The problem of the translation being offset because of the parent isn't present in the example because the parent is centered around the origin

[DJMcNab]

I haven't done any debugging of the code, but when your cables in the child move the parent, they will also change their own GlobalTransform by the same amount, effectively cancelling out the move on the parent.

This seems like a possible mechanism for things to zoom off.

[TomPridham]

I don't think that's it. I just tried spawning them all without a parent and saw the same behavior

[TomPridham]

I think it's related to the last paragraph here: https://bevy-cheatbook.github.io/features/transforms.html. The system is fine for a single iteration, but gradually gets more and more out of sync because the Transforms are getting changed twice for every GlobalTransform update. i think i will just have a container element that isn't technically part of the cable and use normal Transforms to change everything