Calculate global stiffness matrix

I understand you're working on calculating the global mass and stiffness matrices for a 3D object represented by a grid of cuboids. This is a common problem in finite element analysis and structural mechanics. Let me provide an overview of how you might approach this:

1. Mass Matrix:
The mass matrix represents the distribution of mass throughout the structure. For your discretized 3D object:

- For each cuboid marked as "inside":
- Calculate its mass based on its volume and material density
- Add this mass to the diagonal elements of the global mass matrix corresponding to the degrees of freedom of that cuboid's nodes

The result is typically a diagonal or lumped mass matrix, which is computationally efficient.

2. Stiffness Matrix:
The stiffness matrix represents how the structure resists deformation. This is more complex:

1. For each "inside" cuboid:
- Determine its material properties (Young's modulus, Poisson's ratio)
- Calculate its local stiffness matrix (this involves the cuboid's geometry and material properties)
- Transform the local stiffness matrix to the global coordinate system
- Assemble this into the global stiffness matrix

2. The assembly process involves adding the contributions of each element's stiffness to the appropriate locations in the global matrix, based on the node numbering scheme.

To actually implement this, you'd need to:

1. Define a node numbering scheme for your grid
2. Implement functions to calculate local mass and stiffness matrices for a single cuboid
3. Create a data structure to represent the global matrices (likely sparse matrices due to their size)
4. Implement the assembly process to build the global matrices

This is a complex topic, and the details can vary based on the specific finite element formulation you're using. Would you like me to elaborate on any specific part of this process? Or perhaps provide a simple Python code structure to get you started?