fftw3库在Android Studio中的编译和使用

fftw3库是快速傅里叶变换FFT/IFFT的开源实现，可以在多个平台编译。在Android app开发项目中需要做FFT信号分析，优先使用JNI的方式，使用原生语言C/C++实现复杂的科学计算任务。fftw3可以在多个平台编译优化，也可以在Android NDK开发时进行编译调用。本文详细介绍了fftw3在Android studio中的编译和使用方法，欢迎大家留言讨论。

下载fftw3的源码

可以从fftw3的官网下载，另外这里提供了一个下载链接

https://download.csdn.net/download/cyz_2014/87863156

将ffw3文件夹复制到app->src->main->cpp路径中

修改CMakeLists.txt

fftw3中的CMakeLists.txt无需修改，只需要修改外层cpp中的CMakeLists.txt

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.10.2)

# Declares and names the project.

option (ENABLE_FLOAT "single-precision" ON)

SET (ENABLE_FLOAT ON)

project("myapplication")

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_subdirectory(fftw3)

add_library( # Sets the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             native-lib.cpp )

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
                       native-lib

                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

target_link_libraries(native-lib fftw3f)

上面的配置文件只添加了以下4条语句：

option (ENABLE_FLOAT "single-precision" ON)

SET (ENABLE_FLOAT ON)

add_subdirectory(fftw3)

target_link_libraries(native-lib fftw3f)

前两条语句是设置fftw3的编译选项，将其编译为单精度浮点库，生成的库的名称是libfftw3f.so

后两条语句是用于编译ffw3，并将其与native-lib链接。

编译和加载fftw3库

编译以后在app->build->intermediates->merged_native_libs->debug->out->lib中有生成的对应的so库

 在MainActivity 类中加载生成的fftw3f：

static {
    System.loadLibrary("fftw3f");
    System.loadLibrary("native-lib");
}

fftw3库的使用

编写native函数

#include 
#include 

#include 
#include 
#include "fftw3/api/fftw3.h"

#define TAG "Jonathan"

#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, TAG, __VA_ARGS__);

//以下是native函数部分
jfloat a[8] = {1, 0, 2, 0, 3, 0, 4, 0};
fftwf_complex *fftwa = reinterpret_cast((fftwf_complex *) a);

fftwf_plan p;
p = fftwf_plan_dft_1d(4, fftwa, fftwa, FFTW_FORWARD, FFTW_ESTIMATE);
fftwf_execute(p);
fftwf_destroy_plan(p);
LOGD("fftwa: %f + %fi, %f + %fi, %f + %fi, %f + %fi,\n", fftwa[0][0], fftwa[0][1],
         fftwa[1][0], fftwa[1][1], fftwa[2][0], fftwa[2][1], fftwa[3][0], fftwa[3][1]);