Blood flow simulation

Blood Flow Simulation Overview

This section describes how the blood flow simulation was developed.

Mechanism of the Blood Flow Simulation

The core mechanism behind the blood flow simulation is that an actor can move smoothly along a predefined path, known as a spline in Unreal Engine. These splines are carefully hand-drawn to trace the most significant veins and arteries within the human body.

After creating these splines, we built an actor containing skeletal meshes of arteries and veins. Skeletal meshes allow for animation, which is essential for creating dynamic simulations. To organize the splines, we randomly selected the artery skeletal mesh to serve as the parent for all spline components, resulting in the following structure:

CirculatorySystem
    ├─ Veins skeletal mesh 
    ├─ Arteries skeletal mesh
    │   ├─ Spline
    │   ├─ Spline
    │   ├─ Spline 
    │   └─ Spline

This actor inherits from the CPP_BloodFlowSimulation class, allowing it to interact with the simulation manager and respond to simulation events.

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To represent individual blood cells, we created a CPP_Cell class. This class includes a static mesh to represent the blood cell, which is why we named it CPP_Cell.

Next, we developed a Blueprint that inherits from the CPP_Cell class. For easier maintenance, we implemented the blood cell’s movement along splines within this Blueprint. For more information on this implementation, refer to this video.

Spawning Cells

In the simulation, the Blueprint version of the cell (BP_Cell) is spawned into the world. If we spawned CPP_Cell instead of BP_Cell, we would need to implement spline movement logic entirely in C++, which would be difficult without the help of Unreal’s timeline feature.

To specify the Blueprint version of the blood cell, we added a new member field in CPP_Cell that holds the type of Blueprint to spawn. This Blueprint is assigned in the editor, as the property is set to EditAnywhere.

In addition the cells are spawn once during event begin play. If we were to spawn them every time when simulation starts and despawn them when it stops, it would be much more unefficient.

Passing Splines

The cell itself is unaware of which spline to follow. To specify the spline in CPP_BloodFlowSimulation, we iterate over all the children of the arterial skeletal mesh, as this is where the splines are located. We then retrieve a pointer to the spline componet and pass it the cell via the void SetSpline(TObjectPtr<USplineComponent> spline); function

End of depricated section

Possible Future Improvement: Currently, splines are created manually using Unreal Engine’s spline creation tool. We trace the paths of arteries and veins, looping them through the heart region to simulate blood flow. While this setup creates a convincing effect, an improvement would be to dynamically generate splines directly from the artery and vein meshes. However, implementing this is challenging, as these meshes are constructed from arrays of vertex positions with no inherent hierarchy. This lack of structure makes it difficult to automatically generate splines that follow the natural flow within these complex anatomical shapes. We have explored voxelizing the meshes into a sparse voxel octree (SVO) structure. This approach could offer a more structured representation of the arteries and veins, potentially simplifying the process of spline generation by enabling traversal of a hierarchical, voxel-based approximation of the mesh.