Direct port of the Bullet physics engine to JavaScript using Emscripten
Example code to give you an idea of the API:
ammo.js is a direct port of the Bullet physics
engine to JavaScript, using Emscripten. The source
code is translated directly to JavaScript, without human rewriting, so
functionality should be identical to the original Bullet.
‘ammo’ stands for “Avoided Making My Own js physics engine by compiling bullet
from C++” 😉
ammo.js is zlib licensed, just like Bullet.
Discussion takes place on IRC at #emscripten on Mozilla’s server
(irc.mozilla.org)
builds/ammo.js contains a prebuilt version of ammo.js. This is probably what you want.
You can also build ammo.js yourself.
The most straightforward thing is if you want to write your code in C++, and
run that on the web. If so, then you can build your C++ code with emscripten
normally and either build and link Bullet using
https://emscripten.org/docs/compiling/Building-Projects.html
or you can use Bullet directly from emscripten-ports, with -s USE_BULLET=1.
In both cases, you don’t need ammo.js, just plain Bullet.
If, on the other hand, you want to write code in JavaScript, you can use the
autogenerated binding code in ammo.js. A complete example appears in
examples/hello_world.js
That is HelloWorld.cpp from Bullet, translated to JavaScript. Other examples
in that directory might be useful as well. In particular see the WebGL
demo code in
examples/webgl_demo/ammo.html
ammo.js autogenerates its API from the Bullet source code, so it should
be basically identical. There are however some differences and things
to be aware of:
See https://github.com/kripken/emscripten/wiki/WebIDL-Binder
for a description of the bindings tool we use here, which includes
instructions for how to use the wrapped objects.
All ammo.js elements should be accessed through Ammo.*. For example,
Ammo.btVector3, etc., as you can see in the example code.
Member variables of structs and classes can be accessed through
setter and getter functions, that are prefixed with |get_| or |set_|.
For example,
rayCallback.get_m_rayToWorld()
will get m_rayToWorld from say a ClosestRayResultCallback. Native
JavaScript getters and setters could give a slightly nicer API here,
however their performance is potentially problematic.
Functions returning or getting float& or btScalar& are converted to
float. The reason is that float& is basically float* with nicer syntax
in C++, but from JavaScript you would need to write to the heap every
time you call such a function, making usage very ugly. With this change,
you can do |new btVector3(5, 6, 7)| and it will work as expected. If
you find a case where you need the float& method, please file an issue.
Not all classes are exposed, as only what is described in ammo.idl is
wrapped. Please submit pull requests with extra stuff that you need
and add.
There is experimental support for binding operator functions. The following
might work:
| Operator | Name in JS |
|---|---|
= |
op_set |
+ |
op_add |
- |
op_sub |
* |
op_mul |
/ |
op_div |
[] |
op_get |
== |
op_eq |
In order to build ammo.js yourself, you will need
Emscripten and cmake.
For more information about setting up Emscripten, see the getting started
guide.
To configure and build ammo into the builds directory, run the following:
$ cmake -B builds
$ cmake --build builds
There are also some key options that can be specified during cmake
configuration, for example:
$ cmake -B builds -DCLOSURE=1 # compile with closure
$ cmake -B builds -DTOTAL_MEMORY=268435456 # allocate a 256MB heap
$ cmake -B builds -DALLOW_MEMORY_GROWTH=1 # enable a resizable heap
On windows, you can build using cmake’s
mingw generator:
> cmake -B builds -G 'MinGW Makefiles'
> cmake --build builds
Note that if you have not installed emscripten via the emsdk, you can configure
its location with -DEMSCRIPTEN_ROOT.
ammo.js can also be built with Docker.
This offers many advantages (keeping its native environment clean, portability, etc.).
To do this, you just have to install Docker and run:
$ docker-compose build # to create the Docker image
$ docker-compose up # to create the Docker container and build ammo.js
$ docker-compose run builder # to build again the ammojs targets after any modification
If you want to add arguments to cmake, you have to edit the docker-compose.yml file.
The size of the ammo.js builds can be reduced in several ways:
Removing uneeded interfaces from ammo.idl. Some good examples of this are btIDebugDraw and DebugDrawer, which are both only needed if visual debug rendering is desired.
Removing methods from the -s EXPORTED_RUNTIME_METHODS=[] argument in make.py. For example, UTF8ToString is only needed if printable error messages are desired from DebugDrawer.
You can run the automatic tests with npm test, which in turn will run ava against both the javascript and WebAssembly builds:
$ npm run test-js # --> AMMO_PATH=builds/ammo.js ava
$ npm run test-wasm # --> AMMO_PATH=builds/ammo.wasm.js ava
It’s also possible to run ava directly for more options:
$ npx ava --verbose
$ npx ava --node-arguments inspect
When no AMMO_PATH is defined, builds/ammo.js is tested by default.
http-server is included as a dev
dependency as an easy way to run the examples. Make sure to serve everything
from the repo root so that the examples can find ammo in the builds
directory:
$ npx http-server -p 3000 .
It’s easy to forget to write |new| when creating an object, for
example
var vec = Ammo.btVector3(1,2,3); // This is wrong! Need 'new'!
This can lead to error messages like the following:
Cannot read property 'a' of undefined
Cannot read property 'ptr' of undefined
This is an annoying aspect of JavaScript, sadly.
If you find a bug in ammo.js and file an issue, please include a script
that reproduces the problem. That way it is easier to debug, and we can
then include that script in our automatic tests.
Pushing a new build in builds/ammo.js should be done only after the
following steps:
Configure with closure
enabled: cmake -B builds -DCLOSURE=1
Build both the asm.js and wasm libraries: cmake --build builds
Make sure they pass all automatic tests: npm test
Run the WebGL demo in examples/webgl_demo and make sure it looks
ok, using something like firefox examples/webgl_demo/ammo.html
(chrome will need a webserver as it doesn’t like file:// urls)
Bullet 2.82 patched with raycast fix from 2.83