This simple repository contains the files needed to compile the Dlib library for Android development. The repository basically contains two files:
- Android.mk
- Application.mk
They are the files necessary for building the dlib library with NDK, as the dlib libray is written in C++.
For details on how to build and include native C/C++ code in your Android project with NDK, you can start here.
The repositoy does not contain the dlib code, but only links to its repository. In particular, it links to version v19.0 of dlib. The reason for this is that higher versions contain std C++11 entities that are not currently supported by NDK. If one tries to use a higher version of dlib, they will get errors related to several std library members, such as std::round.
LOCAL_MODULE := dlib
This defines the module name, to be referenced by your Android application
LOCAL_C_INCLUDES := ../dlib_v19.7
This defines the subpath where the dlib code lies.
NOTE : As dlib documentation explains, it is important not to include the dlib subfolder in this path. The source code of dlib lies
in the dlib subfolder of the dlib project repository. If the above path were ../dlib_v19.0/dlib/ we would be getting all sorts
of compilation errors as the source code contains header files that may clash with the standard header files, such as string.h. Therefore,
we stop one folder higher.
LOCAL_SRC_FILES :=
This defines the source files that will be compiled. The list contains the basic files that one will need in order to link their Android
project with dlib. If one needs more source files, either add them to your local Dlib4Android clone or in your Android project.
APP_CPPFLAGS := -frtti -fexceptions -std=c++11
It is important to enable C++11 compilation with_-std=c++11_, as dlib uses C++11 features.
For the rest of the flags, feel free to pass your own processor architectures and android SDK versions.
- Download the ndk toolset if you do not already have it installed
- Clone Dlib4Android. It is recommended that you clone the linked dlib repository as well.You can clone both with
git clone --recursive https://github.com/detsikas/Dlib4Android - Build the library with
ndk-build -j2
You need to make sure you have the ndk toolset in your $PATH - Copy the libraries from the libs folder to your Android project
Since Dlib is c++ code, you will be needing your own c++ module in your project that will server as the interface between your Java code and Dlib. Let us call this jni_dlib module. I have not tried to use cmake to build jni_dlib. Instead, I present a way to do it with NDK. In the Android.mk file of jni_dlib, you need to add a section for the prebuilt dlib libraries that you will be importing. In other words, the libraries you copied in step 4 above.
Let us assume that you have a folder called dlib_prebuilt_libs in the root folder of your Android project. Here is a sample of the Android.mk file of jni_dlib module
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := dlib
LOCAL_SRC_FILES := ../../../../dlib_prebuilt_libs/$(TARGET_ARCH_ABI)/libdlib.so
LOCAL_EXPORT_C_INCLUDES := <path to the root folder of your local Dlib4Android clone>
include $(PREBUILT_SHARED_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := jni_dlib
LOCAL_SRC_FILES := <source files of jni_dlib>
LOCAL_SHARED_LIBRARIES := dlib
LOCAL_LDLIBS += -llog -ldl
include $(BUILD_SHARED_LIBRARY)
This repository presents the minimum effort one needs to go through in order to start using dlib with Android. Along the way, there will be several obstacles, such as converting data from the Android standard formats to the formats used by dlib. An excellent reference and starting point for all these exists here.