MathJax example

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  1. Pre-Sim
  2. Post-Sim
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Pre-Simulation

First I've built out the base shape of the rope starting with a simple line and running it through a few point wrangles to get a variable depth, length and number of vertical ropes connecting the parallel ropes.
Next, we cut the geometry at any point, pass the geometry through a vellum hair constraint, and pin its endpoints. Add extra colliders on both sides based on the shape of your terrain. In this example I'm using simple cubes. And finally run everything through a vellum solver.

Post-Simulation

Once the dynamics of the spline are set, we can import our custom plank geometry and copy it onto our simulated points. However, since the plank geometry is not a part of simulation we need to calculate the desired rotation for each plank at every frame. We can do this by orienting each model based on the precomputed point normal vector. Additionally we can add another rotation layer to give each plank a random rotation offset.
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// Define the global upward direction
vector upDirection = set(0, 1, 0);

// Calculate angle between the point normal and global up direction
float angleBetween = acos(dot(upDirection, @N));

// If the normal points downwards in the Z direction, negate the angle
if (@N.z < 0) {
    angleBetween *= -1;
}

// Define the rotation axis as the X-axis
vector rotationAxis = set(1, 0, 0);


// Create a quaternion based on the angle and axis
p@orient = quaternion(angleBetween, rotationAxis);

// Get the random angle based on the point number and remap it between the user-defined min and max angles
float randomRotationAngle = fit(rand(@ptnum * 79123012265), 0, 1, chf("min_angle"), chf("max_angle"));

// Create a quaternion based on the random angle and the point normal
vector4 randomRotationQuat = quaternion(randomRotationAngle, @N);

// Combine the quaternions to get the final rotation
p@orient = qmultiply(randomRotationQuat, @orient);
Finally, we can use a sweep node or a "polywire" node to produce rope geometry. We can also copy a custom frame geometry on the rope's endpoints, making the overall setup completely modular.
Since we are creating rope's geometry post simulation, we can't use the default UV unwrap method because we will get different UV coordinates on different frames which will lead to jittery textures. To setup consistent UV coordinates, I have setup "template" UVs for each unique object and then copied the UV attribute onto our simulated geometry. And since the topology is consistent throughout each frame we can achieve consistent UVs.


The rope will use a tileable texture so we don't need to dedicate a lot of texture space for it. The whole bridge geometry fits well in one UV tile, even leaving space for multiple plank texture variations.
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