Android + ffmpeg - notes

In order to compile an NDK library, one must target a certain CPU architecture. When NDK rev. 8b was released the following were available:

• ARMv5TE, including Thumb-1 instructions
• ARMv7-A, including Thumb-2 and VFPv3-D16 instructions, with optional support for NEON/VFPv3-D32 instructions
• x86 instructions
• MIPS instructions

ARMv5TE (the default) machine code will run on all ARM-based Android devices. ARMv7-A will run only on devices such as the Verizon Droid or Google Nexus One that have a compatible CPU. The main difference between the two instruction sets is that ARMv7-A supports hardware FPU, Thumb-2, and NEON instructions.

CPU Architecture Used	Compatible Android Platforms
ARM, ARM-NEON	Android 1.5 (API Level 3) and higher
x86	Android 2.3 (API Level 9) and higher
MIPS	Android 2.3 (API Level 9) and higher

Android NDK reference

NEON technology is a 128-bit SIMD (Single Instruction, Multiple Data) architecture extension for the ARM Cortex™-A series processors, designed to provide flexible and powerful acceleration for consumer multimedia applications, delivering a significantly enhanced user experience.

ARM NEON reference

Apparently most Android devices on the market in mid-2012 are ARM design, either ARMv6 (older devices & low-ends) or ARMv7 (Android 3 devices & high-ends)

StackOverflow reference

It would be nice filter out the incompatible terminals based on CPU type by running a check before calling the native library. I can get the CPU type info by reading /proc/cpuinfo (it doesn't require super user privilege). A short example by roman10.

If I want to filter-out the incompatible terminals using Google Play, I may use a database of Android terminals and their features that exists on Wikipedia .

To ensure compatibility, an application using a native library produced with the NDK must declare a <uses-sdk> element in its manifest file, with an android:minSdkVersion attribute value of "3" or higher.

 Android NDK reference

The basic steps of working with NDK:

1. Place your native sources under <project>/jni/...
2. Create <project>/jni/Android.mk to describe your native sources to the NDK build system
3. Optional: Create <project>/jni/Application.mk.
4. Build your native code by running the 'ndk-build' script from your project's directory. It is located in the top-level NDK directory:

   cd <project>
   <ndk>/ndk-build
   

   The build tools copy the stripped, shared libraries needed by your application to the proper location in the application's project directory.
   
5. Finally, compile your application using the SDK tools in the usual way. The SDK build tools will package the shared libraries in the application's deployable .apk file.

The stable native APIs that are guaranteed to be supported in all later releases of the platform:

• libc (C library) headers
• libm (math library) headers
• JNI interface headers
• libz (Zlib compression) headers
• liblog (Android logging) header
• OpenGL ES 1.1 and OpenGL ES 2.0 (3D graphics libraries) headers
• libjnigraphics (Pixel buffer access) header (for Android 2.2 and above).
• A Minimal set of headers for C++ support
• OpenSL ES native audio libraries
• Android native application APIS

 Android NDK reference

Another approach is to compile ffmpeg as a standalone executable like this and then package it with the app. There are a few issues to be aware of:

• In order to run a native linux command we use Runtime.getRuntime.exec("command") - example.
• In order to run our ffmpeg on a non-rooted phone, the safest method (no internet downloads) is to copy the binary inside the assets folder. At runtime we copy it to a writable location and change its file permissions - see other choices. It seems the only folder we can write to and change the file permissions is /data/data/app-package-name (e.g. /data/data/org.vlc.android.run_ffmpeg)
• When reading the binary file from the assets folder, an IOException is thrown if the file size exceeds 1MB. This is because the file is automatically compressed when packaged, and there is a limit set in one of the Android headers - see this thread for details. A quick fix is adding a .mp3 extension to the bundled file - Android will think it's an already compressed mp3 and will not compress it.