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Unity Fracture

https://user-images.githubusercontent.com/36990593/118142411-a53f6500-b40a-11eb-9942-4a6b0c1c1a42.mp4

FractureThis.cs script takes all meshes in its gameobject and merges them. This global mesh is send to nvblast for fracturing into chunks. The inside part of the chunk has new UVs generated for material to be applied. Original gameobject is hidden and each chunk mesh is converted to gameobject with rigibody. Neighboring chunks are tethered with fixedjoints.

Requirements

How does this work?

  1. Cut the mesh into smaller meshes chunks
  2. Add rigidbody component to each chunk
  3. Connect chunks with fixed joints that break with force

1) Cut the mesh into smaller meshes chunks

I stumbled upon this forum thread https://forum.unity.com/threads/nvidia-blast.472623 where someone figured out how to use Nvidia blast library in the Unity. Feed the library with mesh (must have vertices, triangles, uvs and closed without missing any faces) to this library and receive mesh chunks.

2) Add rigidbody component to each chunk

Convert each mesh chunk into a gameobject with rigidbody. Without anything holding the chunks together they crumble to the ground. Connect the chunks with fixed joints, so they stay in place. Take each chunk and its neighbors (chunks that are in close proximity or in touch) and connect them with fixed joints.

Issue #1 - Structure is wobbly

This is not ideal, the joints are not 100% fixed in place due to how PhysX resolves collisions. There is a big deal of springiness within the joints. The structure ripples when force is applied and acts like a it is made out of jello.

Solution #1 - Freeze rigidbodies

https://answers.unity.com/questions/230995/fixed-joint-not-really-fixed.html advised to freeze the rigidbody. Freezing the rigidbodies makes them stay in place (they drift apart after some time #1) and the rigidbodies still register impact and the joints can be broken.

Issue #2 - Rigidbodies float in the air

Rigidbodies will stay in place even though there is no support under them. They will only resume movement when all joints are broken.

Solution #2 - Introduce anchors

Let's create a graph of connected chunks. Traverse graph each frame and unfreeze chunks disconnected from anchors (kinematic body)

Chunk Graph Anchored chunks are red