Path constraint and .json code

Chris_solo

Hi Spine's Team!
I work on slot machine games, which are made in Unity, and I have a question about a specific case.
I have firecrackers with physics applied to them as parents to bones on a shape (constrained path). For the animation, I made the shape move at the bottom point, which causes the firecrackers to react with the physics.
Everything works fine and is integrated into Unity, but I would like to know if it is possible to move the bottom point of the shape using a numerical value in Unity code instead of the predefined animations (which I created) using Spine export? Can we see the shape in the .json code
or only the bones?
Thank you for your feedback.

Spinebot

I can help you understand how path constraints work in the JSON export and what's possible with Unity code!

Path Data in JSON

Yes, paths are exported to the JSON file! The path vertices and their deformations are included in the skeleton data. You can find:

The path definition in the skeleton data under the paths section
Deform timeline keys in animations under deform timelines if you've animated the path vertices

Moving the Path Programmatically

There are a few approaches you could take to manipulate the path at runtime in Unity:

1. Using Animation Mixing
Instead of creating entirely new code, you could create multiple animations with different path deformations and blend between them or select them based on numerical values at runtime. This keeps your workflow in Spine while giving you runtime control.

2. Direct Vertex Manipulation
You can access and modify path vertices at runtime through the Spine API. The path attachment has vertex data that you can modify programmatically. However, this requires understanding how the path vertices are structured and calculated.

3. Bone-Based Control
A potentially simpler approach: instead of directly manipulating path vertices in code, you could:

Set up bones that control the path using weights
Use SkeletonUtilityBone components in Unity set to Mode Override to control these bones from your C# code
This way, moving the bones in Unity code will deform the path, which will affect the physics-constrained bones

Recommended Approach

For your use case (moving the bottom point based on numerical values), I'd suggest the bone-based control method. It's much cleaner than trying to manipulate vertices directly, and it maintains the visual workflow benefits of Spine while giving you the programmatic control you need in Unity.

You would essentially:

Weight the bottom part of your path to control bones
Access those bones via SkeletonUtilityBone in Unity
Move them based on your numerical values
The path deforms automatically, physics reacts accordingly

Would you like more specific details about implementing any of these approaches?

Harald

@Chris_solo Please note that instead of JSON you should rather export your skeletons in binary .skel.bytes format in your production setup for smaller file size and faster loading.

I would highly suggest creating a setup in Spine where you can modify a Bone position to achieve your goal rather than manipulating path points. I'm not sure I understand your setup completely though.

If it helps, you could alternatively also create a Transform Constraint and modify its Mix value from 0.0 to 1.0 (0% to 100%) programmatically in Unity to blend between two positions, if you prefer that to moving a Bone around.

Chris_solo

Thanks for the feedback.
It's this point of the shape that I would like to be able to move numerically in the code in Unity every time something hits the whole thing. Is that possible? I'm not sure if that's clear (FYI, there's no optimization issue related to your first paragraph, We have enough hardware to support it, no need to over-optimize.).
I can only show the bones, as the project is under NDA.!
(Can't copy videos to the forum? Every time I tried, it was rejected, even with 28 Mo!)
video link below
https://www.dropbox.com/scl/fi/omsim6jkwg4kzrryttyuw/export.mp4?rlkey=q6gvow6oum193tppz2n3ozi0t&st=kqhje8s2&dl=0

Misaki

Chris_solo Thank you for the additional explanation. That video is very helpful in understanding what you want to do.
And after watching that video, I think Harald's advice seems appropriate. Path attachments can be bound to bones and have their weights set. So, I think it would be better to bind that path to some bone, set the bottom vertex to have 100% weight on a specific bone, and make the path controllable that way.
What I'm talking about is a setup like this:

By the way, it is possible to attach videos directly to this forum (like the video above). However, I believe the maximum file size limit for attachments is 20MB. So you should be able to attach it by shortening the video slightly or splitting it into multiple files.

Chris_solo

Misaki
Fourth question: to create weight points on the shape, did you create mesh points? I can't figure out how you did that...!

Chris_solo

Thanks Misaki, that helped me see a solution when I looked at the hierarchy of your video....I didn't know you could put a bones (Path controller) at the end of the Path slot.

Chris_solo

@Misaki
No, it's not built like yours! I created a shape/path, and next to it I created a chain of bones that I put in Path constrained on the shape, and automatically the bones were placed on the shape. You can't see them in the video because they're small. After that, I parented the external bones (firecrackers) to certain bones of the shape and put physics on them... .
I can't figure out how to do what you did!!?....
How do you attach the second bone at the bottom to the bottom shape?

Chris_solo

@Misaki
Hmm... I understand, I had to search for it! I looked closely at your example, and I had completely forgotten to bind the bones from the shape.

Misaki

Chris_solo Ah, I'm sorry I seem to have confused you! I mentioned weight settings in my previous reply, so I assumed that would make it clear.
Yes, you can bind a path attachment to a bone and set weights for each vertex and handle. The same technique is used in our official example skeleton, Stretchyman, and it's explained on this page, which might be helpful:
Spine Example Projects | Stretchyman