Rope Simulation with Position Based Dynamics

This post explains how a simple rope simulator works using OpenGL, Position-Based Dynamics (PBD), and compute shaders. The project is written in C++ and runs completely on the GPU.

The goal is to simulate a rope that reacts to forces like gravity, stretches under tension, and can break when pulled too far.

Features

Feature	Included
Position-Based Dynamics	Yes
Compute Shader Acceleration	Yes
Object Dragging/Interaction	No
Rope Breaking	Yes
Simple Demo	Yes
Tension Visualization	Yes
Advanced Demo (Raylib Integration)	No
Multiple Rope Interaction	No

What is Position-Based Dynamics?

Position-Based Dynamics (PBD) is a method for simulating soft bodies. Instead of using forces and solving for acceleration, PBD directly updates positions to meet certain constraints.

In this case, each rope segment is a constraint that tries to keep a fixed distance between two particles.

struct Particle {
  glm::vec4 pos;   // xyz = position, w = mass
  glm::vec4 prev;  // previous position
};

Compute Shaders

This project uses OpenGL compute shaders to run both the simulation and constraint solving on the GPU. There are two main compute shaders:

Integration Shader

vec3 velocity = p.position.xyz - p.prevPosition.xyz;
vec3 newPos = p.position.xyz + velocity + gravity * dt * dt;

If a particle is selected and dragging is enabled, its position is fixed to the mouse.

Constraint Solver Shader

if (dist > segmentLength * breakThreshold) {
    pairs[id] = ivec2(-1, -1); // mark as broken
}

Rope Breaking

When the distance between two connected particles becomes too long, the rope segment breaks.

The break condition is:

#define BREAK_THRESHOLD 1.5f

If the distance becomes more than 1.5 times the original length, the connection is removed.

Rendering with Geometry Shader

The rope is drawn using a geometry shader that creates a thick colored line from each segment. The color depends on how much tension the segment is under.

float tension = abs(dist - segmentLength) / segmentLength * tensionScale;
fColor = mix(blue, red, tension);

• Blue = low tension
• Red = high tension

Interaction

The program lets you interact with the rope through the keyboard:

• J / K – Move selection to next/previous particle
• W / S – Increase or decrease mass
• P – Pin or unpin the selected particle
• Space – Toggle dragging (not fully implemented)

Dragging with the mouse is not finished, but the selected particle is highlighted.

Updating Rope Segments

When a segment breaks, the rope structure changes. The program updates the index buffer to reflect these changes and re-renders only the connected parts.

How It Works

1. A number of particles are initialized in a straight line.
2. Constraints are added between each pair of particles.
3. On each frame:
   • The compute shader updates particle positions.
   • The constraint shader corrects distances and breaks segments if needed.
   • The index buffer is rebuilt to reflect which segments are still connected.
4. The rope is rendered using geometry and fragment shaders.

Summary

This rope simulation project shows how to combine GPU compute shaders and modern OpenGL to simulate and render a dynamic rope. It includes features like:

• Position-Based Dynamics for stable simulation
• GPU-based constraint solving
• Rope breaking under stress
• Visual tension feedback

Although dragging and multiple ropes are not yet implemented, the system is flexible and can be extended further.