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Skia Bindings

This is a supporting package for skia-safe, which provides safe Rust bindings to the Skia Graphics Library.

Organization

This package contains three components.

  • First, full configuration and build support for Skia in build.rs and build_support/.
  • Additional C bindings to help out bindgen with stuff it has problems with or to work around linker errors. These are src/bindings.cpp, and src/shaper.cpp.
  • And a number of functions that are used to download prebuilt binaries.

Skia Build Support

Building Skia is quite exceptional, a number of prerequisites need to be available and configured properly for the target platform.

To configure and build Skia, build_support/skia.rs does all the hard work: it pulls depot_tools/ and skia/ from Google's repositories and a number of additional dependencies by executing skia/tools/git-sync-deps with Python. After that, it configures Skia with Google's GN tool, and finally builds it by giving control to the ninja executable from the depot_tools/ package.

Binding Generation

The files src/*.cpp contain the C++ code that Rust needs to interact with Skia's codebase. These files are processed by the Rust's binding generator that uses libclang for the layout computation and are also compiled by clang.

If both steps went well, the resulting Rust binding code is written to OUT_DIR/skia/bindings.rs, and the skia-bindings library is found in the output directory.

Debug Builds

By default, and for performance reasons, Skia is built in release mode even when cargo creates debug output. Skia debug builds can be enabled only by explicitly setting the environment variable SKIA_DEBUG=1.

Prebuilt Binaries

Because building Skia and creating the bindings is slow and depend on a number of components that lie outside the Rust ecosystem, we decided to experiment with prebuilt binaries.

Whenever a new version of rust-skia is built from the release branch on our CI server, the resulting Skia libraries, skia-bindings library, and bindings.rs are compressed and uploaded to the releases tab of the skia-binaries repository.

And whenever the build script detects that skia-bindings is built from inside a crate and a prebuilt archive is available that matches the repository's hash, platform, and features, it downloads the package, unpacks it, and skips the full build step of Skia and the bindings.

Prebuilt Binaries in an Offline Environment

Some users may not have a stable internet connection or are building skia-bindings in an offline environment. You may download binaries manually from the skia-binaries repository in an environment where you do have internet access.

To use the binaries in an offline build, the environment variable SKIA_BINARIES_URL must be set. This environment variable must point to the tar.gz file where the binaries are located, prepended with file://.

export SKIA_BINARIES_URL='file://path/to/skia-binaries.tar.gz'

Changing the executable used as ninja and gn

On some systems, the bundled ninja and gn executables may not work (as it does on NixOS). To remedy this, the executables used can be set using the following environment variables:

Variable Description Default
SKIA_NINJA_COMMAND The ninja command to run. It can be either a command name or an absolute path. ninja by default, ninja.exe on Windows
SKIA_GN_COMMAND The gn command to run. It can be either a command name or a path that starts at Skia's source directory. bin/gn

Changing the Skia source directory

In some cases, one may wish to provide an alternate Skia source directory. This can be achieved by setting SKIA_SOURCE_DIR, which must be an absolute path to a Skia source directory with all dependencies.

Using system libraries

By default, numerous libraries Skia depends upon are built in addition to Skia itself. In the event that this is not wanted (say, if the crate is being built as part of a package's build routine,) this behavior can be disabled by setting the SKIA_USE_SYSTEM_LIBRARIES environment variable.

Also note that there is one exception here. FreeType is only embedded on Android platforms by default. If your platform does not support a more recent FreeType version, skia-bindings must be built with the feature embed-freetype.

Build Customization

Besides of the features gl, vulkan, metal, and textlayout that can be directly specified when the package is added as a cargo dependency, the Skia build can be customized further in build.rs by adjusting one of two structs that are defined in build_support/skia.rs:

BuildConfiguration

This struct represents the top level build configuration for skia-bindings and contains a number of individual feature flags.

FinalBuildConfiguration

The FinalBuildConfiguration is created from the BuildConfiguration and contains name value pairs used by GN to parameterize the Skia build and preprocessor defines used to create the src/bindings.rs file and the skia-bindings library.

Cross Compiling for Linux

It's possible to cross compile Skia and the Rust bindings for different architectures on Linux. Set the following environment variables and then invoke cargo with the desired --target triple:

  • CLANGCC: Command line to invoke clang to cross-compile C code for the desired target architecure. This command line may include a --target=<triple> option.
  • CLANGCXX: Command line to invoke clang to cross-compile C++ code for the desired target architecure. This command line may include a --target=<triple> option.
  • SDKTARGETSYSROOT: Path to the target sysroot.
  • Either:
    • CC/CXX providing command lines to cross-compile (clang is not required) and HOST_CC providing a command line for build for the host.
    • CC_<target>/CXX_<target> providing command lines to cross-compile (clang is not required).

When using a Yocto SDK for cross-compiling, all of the above environment variables will be set when entering the Yocto SDK environment by sourcing the environment-setup-* script, and CC/CXX are set to cross-compile. That means it is also necessary to set HOST_CC, which usually works when set to just gcc.

For linking your Rust application, you may also need to instruct cargo to use the correct linker and look for native library dependencies (such as Skia's FreeType dependency) in the sysroot. This can for be done via a .cargo/config file or via environment variables. For example if your Rust target platform is aarch64-unknown-linux-gnu and you're Yocto SDK's target is aarch64-poky-linux:

  • CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER=aarch64-poky-linux-g++
  • RUSTFLAGS="-Clink-args=--sysroot=$SDKTARGETSYSROOT"

Dependencies

~0–4MB
~71K SLoC