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# Softie 2D

Andrew Luo

A simple soft-body physics simulation

No support for HTML canvas

The following is a Javascript implementation of the particle spring-mass model, which can be used to investigate the dynamics of a soft-body and soft-body interactions with obstacles..

$$F = -kx$$

This simulation is based off of Gonkee's excellent [But How DO Soft Body Simulations Work?] video.

Click to stimulate some velocity on a particle of the soft-body

Press 'e' to randomly simulate a mass particle

# Parameters

Number of Masses (Horitzontal)
Number of Masses (Vertical)

# Obstacles

Floor
Triangle
Rectangle
Start Init

# Refresher on Basic Geometry

• Closest Point to a line
• Simple Raycasting Algorithm
• Point Line-Segment Distance

# How It Works

1. Similar to how atoms form a lattice structure in nature, the structure of a soft-body is composed of point masses attached to other point masses by springs according to 8-adjacency
2. The triangles in the soft-body create a stable structure.
3. All the mass points of the system are stored in a sequential array as objects.
4. In each frame, the velocities and positions are calculated using the simple Euler's method.
• $$velocity += \frac{F * dt}{m}$$
• $$position += velocity * dt$$
5. To test collisions with polygon obstacles, a technique called ray casting is used.
• A beam (arbitrarily chosen from the right) is sent to the point, and the number of intersections with polygon edges is counted.
• If the number of intersections is even, then the point is not inside the polygon
• If the number of intersections is odd, then the point is inside the polygon
• If point is inside polygon, push point out to closest point on edge, then reflect velocity off the normal.

# Reflection

• Still some glaring issues
• Very large damping and spring coefficients cause the simulation to explode
• Have to tone down acceleration due to gravity to prevent the whole structure from collapsing
• Same physics used in game engines! i.e. Godot