Filament is a real-time physically based rendering engine for Android, iOS, Linux, macOS, Windows, and WebGL. It is designed to be as small as possible and as efficient as possible on Android.
Filament is currently used in the Sceneform library both at runtime on Android devices and as the renderer inside the Android Studio plugin.
Download Filament releases to access stable builds.
Make sure you always use tools from the same release as the runtime library. This is particularly
important for matc
(material compiler).
If you prefer to live on the edge, you can download a continuous build by clicking one of the build badges above.
- Filament, an in-depth explanation of real-time physically based rendering, the graphics capabilities and implementation of Filament. This document explains the math and reasoning behind most of our decisions. This document is a good introduction to PBR for graphics programmers.
- Materials, the full reference
documentation for our material system. This document explains our different material models, how
to use the material compiler
matc
and how to write custom materials. - Material Properties, a reference sheet for the standard material model.
Here are a few sample materials rendered with Filament:
Here are a few screenshots of applications that use Filament in production:
- Native C++ API for Android, iOS, Linux, macOS and Windows
- Java/JNI API for Android, Linux, macOS and Windows
- JavaScript API
- OpenGL 4.1+ for Linux, macOS and Windows
- OpenGL ES 3.0+ for Android and iOS
- Metal for macOS and iOS
- Vulkan 1.0 for Android, Linux, macOS and iOS (with MoltenVk), and Windows
- WebGL 2.0 for all platforms
- Clustered forward renderer
- Cook-Torrance microfacet specular BRDF
- Lambertian diffuse BRDF
- HDR/linear lighting
- Metallic workflow
- Clear coat
- Anisotropic lighting
- Approximated translucent (subsurface) materials
- Cloth shading
- Normal mapping & ambient occlusion mapping
- Image-based lighting
- Physically-based camera (shutter speed, sensitivity and aperture)
- Physical light units
- Point light, spot light and directional light
- SSAO
- ACES-like tone-mapping
- Temporal dithering
- FXAA, MSAA and specular anti-aliasing
- Dynamic resolution (on Android and iOS)
Many other features have been either prototyped or planned:
- IES light profiles/cookies
- Area lights
- Fog
- Color grading
- Bloom
- TAA
- etc.
This repository not only contains the core Filament engine, but also its supporting libraries and tools.
android
: Android libraries and projectsbuild
: Custom Gradle tasks for Android buildsfilamat-android
: Filament material generation library (AAR) for Androidfilament-android
: Filament library (AAR) for Androidsamples
: Android-specific Filament samples
art
: Source for various artworks (logos, PDF manuals, etc.)assets
: 3D assets to use with sample applicationsbuild
: CMake build scriptsdocs
: Documentationmath
: Mathematica notebooks used to explore BRDFs, equations, etc.
filament
: Filament rendering engine (minimal dependencies)ide
: Configuration files for IDEs (CLion, etc.)ios
: Sample projects for iOSjava
: Java bindings for Filament librarieslibs
: Librariesbluegl
: OpenGL bindings for macOS, Linux and Windowsbluevk
: Vulkan bindings for macOS, Linux, Windows and Androidfilabridge
: Library shared by the Filament engine and host toolsfilaflat
: Serialization/deserialization library used for materialsfilagui
: Helper library for Dear ImGuifilamat
: Material generation libraryfilameshio
: Tiny filamesh parsing library (see alsotools/filamesh
)geometry
: Mesh-related utilitiesgltfio
: Loader and optional pipeline for glTF 2.0ibl
: IBL generation toolsimage
: Image filtering and simple transformsimageio
: Image file reading / writing, only intended for internal usematdbg
: DebugServer for inspecting shaders at run-time (debug builds only)math
: Math libraryrays
: Simple path tracer used for baking ambient occlusion, etc.utils
: Utility library (threads, memory, data structures, etc.)
samples
: Sample desktop applicationsshaders
: Shaders used byfilamat
andmatc
third_party
: External libraries and assetsenvironments
: Environment maps under CC0 license that can be used withcmgen
models
: Models under permissive licensestextures
: Textures under CC0 license
tools
: Host toolscmgen
: Image-based lighting asset generatorfilamesh
: Mesh converterglslminifier
: Minifies GLSL source codematc
: Material compilermatinfo
Displays information about materials compiled withmatc
mipgen
Generates a series of miplevels from a source imagenormal-blending
: Tool to blend normal mapsresgen
Aggregates binary blobs into embeddable resourcesroughness-prefilter
: Pre-filters a roughness map from a normal map to reduce aliasingskygen
: Physically-based sky environment texture generatorspecular-color
: Computes the specular color of conductors based on spectral data
web
: JavaScript bindings, documentation, and samples
To build Filament, you must first install the following tools:
- CMake 3.10 (or more recent)
- clang 7.0 (or more recent)
- ninja 1.8 (or more recent)
To build the Java based components of the project you can optionally install (recommended):
- OpenJDK 1.8 (or more recent)
Additional dependencies may be required for your operating system. Please refer to the appropriate section below.
Building the rays
library (used for light baking) is optional and requires the following packages:
- embree 3.0+
- libtbb-dev
To build Filament for Android you must also install the following:
- Android Studio 3.5
- Android SDK
- Android NDK "side-by-side" 20 or higher
Make sure the environment variable ANDROID_HOME
points to the location of your Android SDK.
By default our build system will attempt to compile the Java bindings. To do so, the environment
variable JAVA_HOME
should point to the location of your JDK.
When building for WebGL, you'll also need to set EMSDK
. See WebAssembly.
We recommend using CLion to develop for Filament. Simply open the root directory's CMakeLists.txt in CLion to obtain a usable project.
Once the required OS specific dependencies listed below are installed, you can use the script
located in build.sh
to build Filament easily on macOS and Linux.
This script can be invoked from anywhere and will produce build artifacts in the out/
directory
inside the Filament source tree.
To trigger an incremental debug build:
$ ./build.sh debug
To trigger an incremental release build:
$ ./build.sh release
To trigger both incremental debug and release builds:
$ ./build.sh debug release
To install the libraries and executables in out/debug/
and out/release/
, add the -i
flag.
You can force a clean build by adding the -c
flag. The script offers more features described
by executing build.sh -h
.
By default our build system will attempt to compile the Java bindings. If you wish to skip this
compilation step simply pass the -j
flag to build.sh
:
$ ./build.sh -j release
If you use CMake directly instead of the build script, pass -DENABLE_JAVA=OFF
to CMake instead.
The following CMake options are boolean options specific to Filament:
ENABLE_JAVA
: Compile Java projects: requires a JDK and the JAVA_HOME env varENABLE_LTO
: Enable link-time optimizations if supported by the compilerFILAMENT_BUILD_FILAMAT
: Build filamat and JNI buildingsFILAMENT_SUPPORTS_METAL
: Include the Metal backendFILAMENT_SUPPORTS_VULKAN
: Include the Vulkan backendGENERATE_JS_DOCS
: Build WebGL documentation and tutorialsINSTALL_BACKEND_TEST
: Install the backend test library so it can be consumed on iOSUSE_EXTERNAL_GLES3
: Experimental: Compile Filament against OpenGL ES 3
To turn an option on or off:
$ cd <cmake-build-directory>
$ cmake . -DOPTION=ON # Relace OPTION with the option name, set to ON / OFF
Options can also be set with the CMake GUI.
Make sure you've installed the following dependencies:
clang-7
or higherlibglu1-mesa-dev
libc++-7-dev
(libcxx-devel
andlibcxx-static
on Fedora) or higherlibc++abi-7-dev
(libcxxabi-static
on Fedora) or higherninja-build
libxi-dev
After dependencies have been installed, we highly recommend using the easy build script.
If you'd like to run cmake
directly rather than using the build script, it can be invoked as
follows, with some caveats that are explained further down.
$ mkdir out/cmake-release
$ cd out/cmake-release
$ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../release/filament ../..
Your Linux distribution might default to gcc
instead of clang
, if that's the case invoke
cmake
with the following command:
$ mkdir out/cmake-release
$ cd out/cmake-release
# Or use a specific version of clang, for instance /usr/bin/clang-7
$ CC=/usr/bin/clang CXX=/usr/bin/clang++ CXXFLAGS=-stdlib=libc++ \
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../release/filament ../..
You can also export the CC
and CXX
environment variables to always point to clang
. Another
solution is to use update-alternatives
to both change the default compiler, and point to a
specific version of clang:
$ update-alternatives --install /usr/bin/clang clang /usr/bin/clang-7 100
$ update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-7 100
$ update-alternatives --install /usr/bin/cc cc /usr/bin/clang 100
$ update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++ 100
Finally, invoke ninja
:
$ ninja
This will build Filament, its tests and samples, and various host tools.
To compile Filament you must have the most recent version of Xcode installed and you need to make sure the command line tools are setup by running:
$ xcode-select --install
After installing Java 1.8 you must also ensure that your JAVA_HOME
environment variable is
properly set. If it doesn't already point to the appropriate JDK, you can simply add the following
to your .profile
:
export JAVA_HOME="$(/usr/libexec/java_home)"
Then run cmake
and ninja
to trigger a build:
$ mkdir out/cmake-release
$ cd out/cmake-release
$ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../release/filament ../..
$ ninja
The easiest way to build Filament for iOS is to use build.sh
and the
-p ios
flag. For instance to build the debug target:
$ ./build.sh -p ios debug
See ios/samples/README.md for more information.
Install the following components:
Open the x64 Native Tools Command Prompt for VS 2019
.
Create a working directory, and run cmake in it:
> mkdir out
> cd out
> cmake ..
Then, you should be able to load the generated solution file TNT.sln
in Visual Studio and build the material_sandbox
project.
Run it from the out
directory with:
> samples\Debug\material_sandbox.exe ..\assets\models\monkey\monkey.obj
The following instructions have been tested on a machine running Windows 10. They should take you from a machine with only the operating system to a machine able to build and run Filament.
Google employees require additional steps which can be found here go/filawin.
Install the following components:
If you're using Visual Studio 2017, you'll also need to install the LLVM Compiler Toolchain extension.
Open an appropriate Native Tools terminal for the version of Visual Studio you are using:
- VS 2015: VS2015 x64 Native Tools Command Prompt
- VS 2017: x64 Native Tools Command Prompt for VS 2017
You can find these by clicking the start button and typing "x64 native tools".
Create a working directory:
> mkdir out/cmake-release
> cd out/cmake-release
Create the msBuild project:
# Visual Studio 2015:
> cmake -T"LLVM-vs2014" -G "Visual Studio 14 2015 Win64" ../..
# Visual Studio 2017
> cmake ..\.. -T"LLVM" -G "Visual Studio 15 2017 Win64" ^
-DCMAKE_CXX_COMPILER:PATH="C:\Program Files\LLVM\bin\clang-cl.exe" ^
-DCMAKE_C_COMPILER:PATH="C:\Program Files\LLVM\bin\clang-cl.exe" ^
-DCMAKE_LINKER:PATH="C:\Program Files\LLVM\bin\lld-link.exe"
Check out the output and make sure Clang for Windows frontend was found. You should see a line showing the following output. Note that for Visual Studio 2017 this line may list Microsoft's compiler, but the build will still in fact use Clang and you can proceed.
Clang:C:/Program Files/LLVM/msbuild-bin/cl.exe
You are now ready to build:
> msbuild TNT.sln /t:material_sandbox /m /p:configuration=Release
Run it:
> samples\Release\material_sandbox.exe ..\..\assets\models\monkey\monkey.obj
- To troubleshoot an issue, use verbose mode via
/v:d
flag. - To build a specific project, use
/t:NAME
flag (e.g:/t:material_sandbox
). - To build using more than one core, use parallel build flag:
/m
. - To build a specific profile, use
/p:configuration=
(e.g:/p:configuration=Debug
,/p:configuration=Release
, and/p:configuration=RelWithDebInfo
). - The msBuild project is what is used by Visual Studio behind the scene to build. Building from VS or from the command-line is the same thing.
Alternatively, you can use Ninja to build for Windows. An MSVC installation is still necessary.
First, install the dependencies listed under Windows as well as Ninja. Then open up a Native Tools terminal as before. Create a build directory inside Filament and run the following CMake command:
> cmake .. -G Ninja ^
-DCMAKE_CXX_COMPILER:PATH="C:\Program Files\LLVM\bin\clang-cl.exe" ^
-DCMAKE_C_COMPILER:PATH="C:\Program Files\LLVM\bin\clang-cl.exe" ^
-DCMAKE_LINKER:PATH="C:\Program Files\LLVM\bin\lld-link.exe" ^
-DCMAKE_BUILD_TYPE=Release
You should then be able to build by invoking Ninja:
> ninja
- Before shipping a binary, make sure you used Release profile and make sure you have no libc/libc++ dependencies with Dependency Walker.
- Application Verifier and gflags.exe can be of great help to trackdown heap corruption. Application
Verifier is easy to setup with a GUI. For gflags, use:
gflags /p /enable pheap-buggy.exe
.
To confirm Filament was properly built, run the following command from the build directory:
> samples\material_sandbox.exe --ibl=..\..\samples\envs\pillars ..\..\assets\models\sphere\sphere.obj
Before building Filament for Android, make sure to build Filament for your host. Some of the host tools are required to successfully build for Android.
Filament can be built for the following architectures:
- ARM 64-bit (
arm64-v8a
) - ARM 32-bit (
armeabi-v7a
) - Intel 64-bit (
x86_64
) - Intel 32-bit (
x86
)
Note that the main target is the ARM 64-bit target. Our implementation is optimized first and
foremost for arm64-v8a
.
To build Android on Windows machines, see android/Windows.md.
The easiest way to build Filament for Android is to use build.sh
and the
-p android
flag. For instance to build the release target:
$ ./build.sh -p android release
Run build.sh -h
for more information.
Then invoke CMake in a build directory of your choice, inside of filament's directory:
$ mkdir out/android-build-release-aarch64
$ cd out/android-build-release-aarch64
$ cmake -G Ninja -DCMAKE_TOOLCHAIN_FILE=../../build/toolchain-aarch64-linux-android.cmake \
-DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../android-release/filament ../..
And then invoke ninja
:
$ ninja install
or
$ ninja install/strip
This will generate Filament's Android binaries in out/android-release
. This location is important
to build the Android Studio projects located in filament/android
. After install, the library
binaries should be found in out/android-release/filament/lib/arm64-v8a
.
Then invoke CMake in a build directory of your choice, inside of filament's directory:
$ mkdir out/android-build-release-arm
$ cd out/android-build-release-arm
$ cmake -G Ninja -DCMAKE_TOOLCHAIN_FILE=../../build/toolchain-arm7-linux-android.cmake \
-DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../android-release/filament ../..
And then invoke ninja
:
$ ninja install
or
$ ninja install/strip
This will generate Filament's Android binaries in out/android-release
. This location is important
to build the Android Studio projects located in filament/android
. After install, the library
binaries should be found in out/android-release/filament/lib/armeabi-v7a
.
Then invoke CMake in a build directory of your choice, sibling of filament's directory:
$ mkdir out/android-build-release-x86_64
$ cd out/android-build-release-x86_64
$ cmake -G Ninja -DCMAKE_TOOLCHAIN_FILE=../../filament/build/toolchain-x86_64-linux-android.cmake \
-DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../out/android-release/filament ../..
And then invoke ninja
:
$ ninja install
or
$ ninja install/strip
This will generate Filament's Android binaries in out/android-release
. This location is important
to build the Android Studio projects located in filament/android
. After install, the library
binaries should be found in out/android-release/filament/lib/x86_64
.
Then invoke CMake in a build directory of your choice, sibling of filament's directory:
$ mkdir out/android-build-release-x86
$ cd out/android-build-release-x86
$ cmake -G Ninja -DCMAKE_TOOLCHAIN_FILE=../../filament/build/toolchain-x86-linux-android.cmake \
-DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../out/android-release/filament ../..
And then invoke ninja
:
$ ninja install
or
$ ninja install/strip
This will generate Filament's Android binaries in out/android-release
. This location is important
to build the Android Studio projects located in filament/android
. After install, the library
binaries should be found in out/android-release/filament/lib/x86
.
Before you attempt to build the AAR, make sure you've compiled and installed the native libraries
as explained in the sections above. You must have the following ABIs built in
out/android-release/filament/lib/
:
arm64-v8a
armeabi-v7a
x86_64
x86
To build Filament's AAR simply open the Android Studio project in android/filament-android
. The
AAR is a universal AAR that contains all supported build targets:
arm64-v8a
armeabi-v7a
x86_64
x86
To filter out unneeded ABIs, rely on the abiFilters
of the project that links against Filament's
AAR.
Alternatively you can build the AAR from the command line by executing the following the
android/filament-android
directory:
$ ./gradlew -Pfilament_dist_dir=../../out/android-release/filament assembleRelease
The -Pfilament_dist_dir
can be used to specify a different installation directory (it must match
the CMake install prefix used in the previous steps).
Create a new module in your project and select Import .JAR or .AAR Package when prompted. Make sure to add the newly created module as a dependency to your application.
If you do not wish to include all supported ABIs, make sure to create the appropriate flavors in your Gradle build file. For example:
flavorDimensions 'cpuArch'
productFlavors {
arm8 {
dimension 'cpuArch'
ndk {
abiFilters 'arm64-v8a'
}
}
arm7 {
dimension 'cpuArch'
ndk {
abiFilters 'armeabi-v7a'
}
}
x86_64 {
dimension 'cpuArch'
ndk {
abiFilters 'x86_64'
}
}
x86 {
dimension 'cpuArch'
ndk {
abiFilters 'x86'
}
}
universal {
dimension 'cpuArch'
}
}
The core Filament library can be cross-compiled to WebAssembly from either macOS or Linux. To get started, follow the instructions for building Filament on your platform (macOS or linux), which will ensure you have the proper dependencies installed.
Next, you need to install the Emscripten SDK. The following instructions show how to install the same version that our continuous builds use.
cd <your chosen parent folder for the emscripten SDK>
curl -L https://github.com/emscripten-core/emsdk/archive/a77638d.zip > emsdk.zip
unzip emsdk.zip
mv emsdk-* emsdk
cd emsdk
./emsdk update
./emsdk install sdk-1.38.28-64bit
./emsdk activate sdk-1.38.28-64bit
After this you can invoke the easy build script as follows:
export EMSDK=<your chosen home for the emscripten SDK>
./build.sh -p webgl release
The EMSDK variable is required so that the build script can find the Emscripten SDK. The build
creates a samples
folder that can be used as the root of a simple static web server. Note that you
cannot open the HTML directly from the filesystem due to CORS. One way to deal with this is to
use Python to create a quick localhost server:
cd out/cmake-webgl-release/web/samples
python3 -m http.server # Python 3
python -m SimpleHTTPServer # Python 2.7
You can then open http://localhost:8000/suzanne.html in your web browser.
Alternatively, if you have node installed you can use the live-server package, which automatically refreshes the web page when it detects a change.
Each sample app has its own handwritten html file. Additionally the server folder contains assets such as meshes, textures, and materials.
The samples/
directory contains several examples of how to use Filament with SDL2.
Some of the samples accept FBX/OBJ meshes while others rely on the filamesh
file format. To
generate a filamesh
file from an FBX/OBJ asset, run the filamesh
tool
(./tools/filamesh/filamesh
in your build directory):
filamesh ./assets/models/monkey/monkey.obj monkey.filamesh
Most samples accept an IBL that must be generated using the cmgen
tool (./tools/filamesh/cmgen
in your build directory). These sample apps expect a path to a directory containing the '.rgb32f'
files for the IBL (which are PNGs containing R11F_G11F_B10F
data). To generate an IBL simply use
this command:
cmgen -x ./ibls/ my_ibl.exr
The source environment map can be a PNG (8 or 16 bit), a PSD (16 or 32 bit), an HDR or an OpenEXR file. The environment map can be an equirectangular projection, a horizontal cross, a vertical cross, or a list of cubemap faces (horizontal or vertical).
cmgen
will automatically create a directory based on the name of the source environment map. In
the example above, the final directory will be ./ibls/my_ibl/
. This directory should contain the
pre-filtered environment map (one file per cubemap face and per mip level), the environment map
texture for the skybox and a text file containing the spherical harmonics for indirect diffuse
lighting.
If you prefer a blurred background, run cmgen
with this flag: --extract-blur=0.1
. The numerical
value is the desired roughness between 0 and 1.
You must create an Engine
, a Renderer
and a SwapChain
. The SwapChain
is created from a
native window pointer (an NSView
on macOS or a HWND
on Windows for instance):
Engine* engine = Engine::create();
SwapChain* swapChain = engine->createSwapChain(nativeWindow);
Renderer* renderer = engine->createRenderer();
To render a frame you must then create a View
, a Scene
and a Camera
:
Camera* camera = engine->createCamera();
View* view = engine->createView();
Scene* scene = engine->createScene();
view->setCamera(camera);
view->setScene(scene);
Renderables are added to the scene:
Entity renderable = EntityManager::get().create();
// build a quad
RenderableManager::Builder(1)
.boundingBox({{ -1, -1, -1 }, { 1, 1, 1 }})
.material(0, materialInstance)
.geometry(0, RenderableManager::PrimitiveType::TRIANGLES, vertexBuffer, indexBuffer, 0, 6)
.culling(false)
.build(*engine, renderable);
scene->addEntity(renderable);
The material instance is obtained from a material, itself loaded from a binary blob generated
by matc
:
Material* material = Material::Builder()
.package((void*) BAKED_MATERIAL_PACKAGE, sizeof(BAKED_MATERIAL_PACKAGE))
.build(*engine);
MaterialInstance* materialInstance = material->createInstance();
To learn more about materials and matc
, please refer to the
materials documentation.
To render, simply pass the View
to the Renderer
:
// beginFrame() returns false if we need to skip a frame
if (renderer->beginFrame(swapChain)) {
// for each View
renderer->render(view);
renderer->endFrame();
}
For complete examples of Linux, macOS and Windows Filament applications, look at the source files
in the samples/
directory. These samples are all based on samples/app/
which contains the code
that creates a native window with SDL2 and initializes the Filament engine, renderer and views.
After building Filament, you can use filament-java.jar
and its companion filament-jni
native
library to use Filament in desktop Java applications.
You must always first initialize Filament by calling Filament.init()
.
You can use Filament either with AWT or Swing, using respectively a FilamentCanvas
or a
FilamentPanel
.
Following the steps above (how to use Filament from native code), create an Engine
and a
Renderer
, but instead of calling beginFrame
and endFrame
on the renderer itself, call
these methods on FilamentCanvas
or FilamentPanel
.
See android/samples
for examples of how to use Filament on Android.
You must always first initialize Filament by calling Filament.init()
.
Rendering with Filament on Android is similar to rendering from native code (the APIs are largely
the same across languages). You can render into a Surface
by passing a Surface
to the
createSwapChain
method. This allows you to render to a SurfaceTexture
, a TextureView
or
a SurfaceView
. To make things easier we provide an Android specific API called UiHelper
in the
package com.google.android.filament.android
. All you need to do is set a render callback on the
helper and attach your SurfaceView
or TextureView
to it. You are still responsible for
creating the swap chain in the onNativeWindowChanged()
callback.
See ios/samples
for examples of using Filament on iOS.
Filament on iOS is largely the same as native rendering with C++. A CAEAGLLayer
or CAMetalLayer
is passed to the createSwapChain
method. Filament for iOS supports both OpenGL ES and Vulkan via
MoltenVK.
To generate the documentation you must first install doxygen
and graphviz
, then run the
following commands:
$ cd filament/filament
$ doxygen docs/doxygen/filament.doxygen
Finally simply open docs/html/index.html
in your web browser.
To get started you can use the textures and environment maps found respectively in
third_party/textures
and third_party/environments
. These assets are under CC0 license. Please
refer to their respective URL.txt
files to know more about the original authors.
One of our design goals is that Filament itself should have no dependencies or as few dependencies as possible. The current external dependencies of the runtime library include:
- STL
- robin-map (header only library)
When building with Vulkan enabled, we have a few additional small dependencies:
- vkmemalloc
- smol-v
Host tools (such as matc
or cmgen
) can use external dependencies freely.
Please read and follow the steps in CONTRIBUTING.md. Make sure you are familiar with the code style.
Please see LICENSE.
This is not an officially supported Google product.