# The nphysics testbed§

The nphysics_testbed2d and nphysic_testbed3d crates provide pure-Rust, WASM-compatible, tools for displaying and interacting easily with a physical scene. They are based on the kiss3d graphics engine. Basically, all you have to do is setup your physics World, and then call Testbed::new(world).run() to obtain a fully-functional windowed application with 2D or 3D display of every colliders on your World. This application will allow some controls like starting/stopping/pausing the simulation, and grabbing an object with the mouse.

Note

All the interactive demos from this website have been build using those testbeds. Their source codes can be found on the examples2d and examples3d of the nphysics repository.

In this chapter we will describe an example of the setup of a 3D physical scene with a pile of boxes. This scene will then be displayed and simulated using the nphysics_testbed3d crate. The full code can be found on github. The end-result corresponds to the 3D Stack of boxes demo. The code fore 2D analogous of this demo can be found on github which corresponds to the 2D Stack of boxes demo.

## Initializing the project§

First, initialize a new binary project, e.g., using cargo:

cargo new --bin example_nphysics_testbed_3d
cd example_nphysics_testbed_3d

Then, edit your Cargo.toml file to add nphysics-testbed3d, nphysics3d, ncollide3d (for collider shapes) and nalgebra (for vectors and matrices) as dependencies:

[package]
name    = "examples_nphysics_testbed_3d"
version = "0.1.0"
authors = [ "you" ]

[dependencies]
nalgebra   = "0.16"
ncollide3d = "0.17"
nphysics3d = "0.9"
nphysics-testbed3d = "0.1"

Now modify the src/main.rs to add the corresponding extern crate directives. Here, we use na as an alias for nalgebra:

extern crate nalgebra as na;
extern crate ncollide3d;
extern crate nphysics3d;
extern crate nphysics_testbed3d;

## Setting-up the physics world§

The next step is to setup the physics world. First we will use a few elements to get the tools we need to initialize our World:

use na::{Isometry3, Point3, Vector3};            // For configuring and positioning bodies.
use ncollide3d::shape::{Cuboid, ShapeHandle};    // Shapes for colliders.
use nphysics3d::object::{BodyHandle, Material};  // Body handle and collider material.
use nphysics3d::volumetric::Volumetric;          // To retrieve the center of mass and inertia properties of a shape.
use nphysics3d::world::World;                    // The physics world to be initialized.
use nphysics_testbed3d::Testbed;                 // The testbed to display/run the simulation.

The first thing we want to do is create a new physics world with its default parameters, and change the default gravity (initialized to zero by default) so that is points toward the negative $y$-axis:

let mut world = World::new();
world.set_gravity(Vector3::y() * -9.81);

Then, we initialize the ground represented as a huge box.

const COLLIDER_MARGIN: f32 = 0.01;

let ground_size = 50.0;
let ground_shape =
ShapeHandle::new(Cuboid::new(Vector3::repeat(ground_size - COLLIDER_MARGIN)));
let ground_pos = Isometry3::new(Vector3::y() * -ground_size, na::zero());

COLLIDER_MARGIN,
ground_shape,
BodyHandle::ground(),
ground_pos,
Material::default(),
);

Two elements are notable here:

1. This collider will never move so we don’t need a body and can simply attach it to the BodyHandle::ground().
2. We want our cube to have a half-width of 50 exactly. Therefore, we remove COLLIDER_MARGIN (here equal to 0.01) from the cuboid half-extents. This is then regained by the margin added by nphysics on the collider (and specified as the first parameter of .add_collider(...)). This concept of margin is explained in the section on colliders.

Finally, we can setup our pile of boxes. This time, because our boxes are dynamic, we will have to create one rigid-body for each box, and attach one collider to each rigid-body.

let num = 7; // There will be 7 * 7 * 7 = 343 boxes here.
let rad = 0.1;
let shift = rad * 2.0;
let centerx = shift * (num as f32) / 2.0;
let centery = shift / 2.0;
let centerz = shift * (num as f32) / 2.0;
let height = 2.0;

let geom = ShapeHandle::new(Cuboid::new(Vector3::repeat(rad - COLLIDER_MARGIN)));
let inertia = geom.inertia(1.0);
let center_of_mass = geom.center_of_mass();

for i in 0usize..num {
for j in 0usize..num {
for k in 0usize..num {
let x = i as f32 * shift - centerx;
let y = j as f32 * shift + centery + height;
let z = k as f32 * shift - centerz;

/*
* Create the rigid-body.
*/
let pos = Isometry3::new(Vector3::new(x, y, z), na::zero());
let handle = world.add_rigid_body(pos, inertia, center_of_mass);

/*
* Create the collider and attach it to the body we just created.
*/
COLLIDER_MARGIN,
geom.clone(),
handle,
Isometry3::identity(),
Material::default(),
);
}
}
}

Note

The inertia and center_of_mass properties needed for the rigid-body creation are computed from the collision shape here using geom.inertia(1.0) (with 1.0 the desired density of the solid) and geom.center_of_mass(). This required importing the Volumetric trait with use nphysics3d::volumetric::Volumetric;.

## Running the testbed§

Finally, all that remains to do is set-up the testbed:

let mut testbed = Testbed::new(world);
testbed.look_at(Point3::new(-4.0, 1.0, -4.0), Point3::new(0.0, 1.0, 0.0));
testbed.run();

The .look_at(...) method will position the camera at the coordinates $(-4, 1, -4)$ and orient it such that it points toward the point at $(0, 1, 0)$. The .run() method will open a window and run the simulation until the window is closed (either by pressing the close button or by pressing the key ‘Q’).

## More customizations§

The nphysics testbed provides a few more methods for customizing your simulation. Those methods can be called even if the testbed is created without a World (which can be set later with the .set_world(...) method).

Methods Description
Testbed::new_empty() Create a testbed with no physics world. This is useful to set the color of bodies while you are still initializing the physics world.
.set_world(world) Set the physics world to be managerd by the testbed.
.hide_performance_counters() Disable the output to stdout of physics timings.
.set_body_color(world, body_handle, color) Sets the color of the colliders attached to the specified body. This overrides the default random color. An example can be found on the collision groups demo for setting the dynamic bodies colors to green or blue.
.set_collider_color(world, collider_handle, color) Sets the color of the specified collider. This overrides the default random color. An example can be found on the collision groups demo for setting the static colliders colors to green or blue.
.add_callback(f) Adds a callback to be executed at each render loop, before the next call to world.step(). An example can be found on the sensor demo for handling proximity events to recolor of colliders intersecting the sensor.