How to Cross-Compile libavif for Android

This guide provides a step-by-step walkthrough for cross-compiling the libavif library for the Android platform using the Android Native Development Kit (NDK) and CMake. You will learn how to set up your environment, compile a compatible AV1 codec dependency (such as aom or dav1d), and configure the final libavif build for Android architectures like arm64-v8a or armeabi-v7a.

Prerequisites

Before beginning, ensure you have the following tools installed on your development host (Linux, macOS, or Windows):

CMake (version 3.10 or higher)
Android NDK (r21 or higher recommended)
Git
Ninja (optional, but highly recommended for faster builds)

Define the path to your Android NDK as an environment variable to simplify the commands:

export ANDROID_NDK=/path/to/your/android-ndk

Step 1: Clone the libavif Repository

Clone the official libavif repository from GitHub and navigate into the directory:

git clone https://github.com/AOMediaCodec/libavif.git
cd libavif

Step 2: Build an AV1 Codec Dependency

libavif requires at least one underlying AV1 encoder or decoder library to function. For Android, dav1d (a highly optimized decoder) or aom (encoder/decoder) are the most common choices.

Here is how to cross-compile aom for Android as a dependency:

Navigate to the external directory and clone aom:

git clone https://aomedia.googlesource.com/aom ext/aom

Create a build directory for aom and configure it using the Android NDK toolchain:

mkdir -p ext/aom/build.android
cd ext/aom/build.android

cmake .. \
  -G Ninja \
  -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
  -DANDROID_ABI=arm64-v8a \
  -DANDROID_PLATFORM=android-21 \
  -DANDROID_STL=c++_static \
  -DENABLE_DOCS=OFF \
  -DENABLE_EXAMPLES=OFF \
  -DENABLE_TESTS=OFF \
  -DENABLE_TOOLS=OFF \
  -DCMAKE_BUILD_TYPE=Release

ninja
cd ../../..

(Note: Replace arm64-v8a with armeabi-v7a, x86, or x86_64 if targeting other architectures).

Step 3: Configure and Build libavif

Now that the AV1 codec is built, you can configure and build libavif using the same Android NDK toolchain and link it to the compiled aom library.

Create and navigate to the libavif build directory:
```
mkdir build.android
cd build.android
```

Run CMake to configure the project. Point the configuration to the Android toolchain and enable the aom library:

cmake .. \
  -G Ninja \
  -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
  -DANDROID_ABI=arm64-v8a \
  -DANDROID_PLATFORM=android-21 \
  -DANDROID_STL=c++_static \
  -DCMAKE_BUILD_TYPE=Release \
  -DAVIF_CODEC_AOM=LOCAL \
  -DAVIF_LOCAL_AOM=ON

Compile the library:
```
ninja
```

Step 4: Verify the Output

Once the build process completes successfully, you will find the compiled libraries inside the build.android directory:

libavif.so (Shared library)
libavif.a (Static library)

You can now copy these binaries and the header files from the include/ directory into your Android Studio project’s cpp folder to utilize AVIF image decoding and encoding via JNI.