1. Android FFmpeg开发基本流程

（1）X264/FFmpeg + NDK编译
（2）ffmpeg.so + 编解码C代码
（3）Android.mk 编译
（4）JNI
（5）JAVA代码调用

基本流程如下图所示：

本文涉及FFmpeg，x264编译。

2. NDK配置

首先需要配置NDK开发环境，略
（1）设置$NDK环境变量

# Detect NDK
if [[  -z "$NDK"  ]]; then
    echo "The NDK dir is empty, If the shell can not run normally, you should set the NDK variable to your local ndk.dir"
    exit 1
fi

（2）检查系统类型

# Detect OS
OS=`uname`
HOST_ARCH=`uname -m`
export CCACHE=; type ccache >/dev/null 2>&1 && export CCACHE=ccache
if [ $OS == 'Linux' ]; then
    export HOST_SYSTEM=linux-$HOST_ARCH
elif [ $OS == 'Darwin' ]; then
    export HOST_SYSTEM=darwin-$HOST_ARCH
fi

（3）配置Sysroot和cross_prefix

SYSROOT=$NDK/platforms/android-16/arch-arm
CROSS_PREFIX=$NDK/toolchains/arm-linux-androideabi-4.9/prebuilt/$HOST_SYSTEM/bin/arm-linux-androideabi-

2. x264编译

2.1 下载源代码

git clone http://git.videolan.org/git/x264.git 

2.2 编译脚本

#!/bin/bash

echo "###### 开始编译 x264 ######"

SOURCE=$TARGET_X264_DIR
cd $SOURCE
#PREFIX指定的是编译输出路径,不指定默认是/usr/local/lib和/usr/local/include
#PREFIX=../build

EXTRA_CFLAGS="-march=armv7-a -mfloat-abi=softfp -mfpu=neon -D__ARM_ARCH_7__ -D__ARM_ARCH_7A__"
EXTRA_LDFLAGS="-nostdlib"

./configure \
    --prefix=$PREFIX \
    --cross-prefix=$CROSS_PREFIX \
    --extra-cflags="$EXTRA_CFLAGS" \
    --extra-ldflags="$EXTRA_LDFLAGS" \
    --enable-static \
    --enable-pic \
    --enable-strip \
    --disable-cli \
    --host=arm-linux \
    --sysroot=$SYSROOT

make clean
make && make install

这里面需要注意的是：
（1）PREFIX指定的是编译输出路径,不指定默认是/usr/local/lib和/usr/local/include

这里讲ffmpeg和x264统一放在工程根目录中build文件夹

（2）编译时不能进行多线程，否则汇编优化报错，如下：

make -j4

make: [common/arm/deblock-a.o] Error 127 (ignored)
/Users/guohe/Android/android-ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-gcc -I. -I. -c -DSTACK_ALIGNMENT=4 -DPIC -DHIGH_BIT_DEPTH=0 -DBIT_DEPTH=8 -o common/arm/predict-a.o common/arm/predict-a.S
/bin/sh: j4: command not found

2.3. 结论

Linux中编译x264：

sudo ./configure --enable-shared --prefix=/usr/local
sudo make
sudo make install

通过对比发现：Android中编译x264，最基本的就是配置SYSROOT和CROSS_PREFIX

3. 编译FFmpeg

3.1 下载源代码

git clone git://source.ffmpeg.org/ffmpeg.git $FFMPEG_SOURCE_DIR

3.2 编译脚本

ADD_H264_FEATURE="--enable-encoder=aac \
    --enable-decoder=aac \
    --enable-encoder=libx264 \
    --enable-libx264 \
    --extra-cflags=-I$PREFIX/include \
    --extra-ldflags=-L$PREFIX/lib "

    ./configure \
        --prefix=$PREFIX \
        --enable-pthreads \
        --enable-gpl \
        --enable-version3 \
        --enable-nonfree \
        --enable-static \
        --enable-small \
        --enable-asm \
        --enable-neon \
        --cross-prefix=$CROSS_PREFIX \
        --target-os=linux \
        --arch=arm \
        --enable-cross-compile \
        --sysroot=$SYSROOT \
        $ADD_H264_FEATURE

    make clean
    make -j4
    make install

注：
（1）跨平台编译特殊配置
–sysroot=SYSROOT–cross−prefix=CROSS_PREFIX
–target-os=linux

（2）支持x264
–enable-encoder=libx264 \
–enable-libx264 \
–extra-cflags=-IPREFIX/include –extra−ldflags=−LPREFIX/lib

（3）编译优化
–enable-asm \
–enable-neon \

3.3 编译结果

至此，ffmpeg和x264已经编译完成，编译结果如下：

4. NDK + FFmpeg开发

ffmpeg，x264已经编译完成，下面就聊聊怎么用ffmpeg库进行开发。
下面展示一个基于FFmpeg开发的解码程序，其功能是从一个视频中解码5帧数据。

4.1 复制编译FFmpeg,x264库与头文件

cp $PREFIX/lib/*.a ./jni/lib
cp -r $PREFIX/include/* ./jni/

4.2 编写Android.mk

Android.mk主要包含几下几块：
（1）引入库文件
libavcodec.a libavdevice.a libavfilter.a libavformat.a libavutil.a libpostproc.a libswresample.a libswscale.a libx264.a

#static version of libavutil
include $(CLEAR_VARS)
LOCAL_MODULE:= libavutil_static
LOCAL_SRC_FILES:= lib/libavutil.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

（2）编译源文件
需要设置：
LOCAL_MODULE //库名称
LOCAL_SRC_FILES //源代码文件
LOCAL_LDLIBS //额外链接的库
LOCAL_CFLAGS //CFlags

（3）编译何种类型

include $(BUILD_SHARED_LIBRARY)

（4）详细脚本如下

LOCAL_PATH := $(call my-dir)

#include $(call all-subdir-makefiles)

#static version of libavcodec
include $(CLEAR_VARS)
LOCAL_MODULE:= libavcodec_static
LOCAL_SRC_FILES:= lib/libavcodec.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libavformat
include $(CLEAR_VARS)
LOCAL_MODULE:= libavformat_static
LOCAL_SRC_FILES:= lib/libavformat.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libswscale
include $(CLEAR_VARS)
LOCAL_MODULE:= libswscale_static
LOCAL_SRC_FILES:= lib/libswscale.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libavutil
include $(CLEAR_VARS)
LOCAL_MODULE:= libavutil_static
LOCAL_SRC_FILES:= lib/libavutil.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libavdevice
include $(CLEAR_VARS)
LOCAL_MODULE:= libavdevice_static
LOCAL_SRC_FILES:= lib/libavdevice.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libavfilter
include $(CLEAR_VARS)
LOCAL_MODULE:= libavfilter_static
LOCAL_SRC_FILES:= lib/libavfilter.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libswresample
include $(CLEAR_VARS)
LOCAL_MODULE:= libswresample_static
LOCAL_SRC_FILES:= lib/libswresample.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libpostproc
include $(CLEAR_VARS)
LOCAL_MODULE:= libpostproc_static
LOCAL_SRC_FILES:= lib/libpostproc.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

#static version of libx264
include $(CLEAR_VARS)
LOCAL_MODULE:= libx264_static
LOCAL_SRC_FILES:= lib/libx264.a
LOCAL_CFLAGS := -march=armv7-a -mfloat-abi=softfp -mfpu=neon -O3 -ffast-math -funroll-loops
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)

LOCAL_MODULE := ffmpeg
LOCAL_SRC_FILES := decoder.c encoder.c
LOCAL_LDLIBS := -llog -lz
LOCAL_CFLAGS := -march=armv7-a -mfloat-abi=softfp -mfpu=neon -O3 -ffast-math -funroll-loops
LOCAL_WHOLE_STATIC_LIBRARIES := libavformat_static \
                        libavcodec_static \
                        libavutil_static \
                        libpostproc_static \
                        libswscale_static \
                        libswresample_static \
                        libx264_static \
                        libavfilter_static \
                        libavdevice_static \

include $(BUILD_SHARED_LIBRARY)

4.3 解码程序

Native程序一般分为两块：
（1）JNI调用接口

/*
 * Class:     com_example_jnidemo_MainActivity
 * Method:    info
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_com_example_jnidemo_MainActivity_info
  (JNIEnv *, jobject, jstring);

（2）功能模块
略

详细代码如下：

#include <jni.h>
#include <android/log.h>
#include <stdio.h>

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavformat/avio.h>
#include <libavutil/fifo.h>
#include <libavutil/avutil.h>
#include <libavutil/mem.h>
#include <libswscale/swscale.h>

#include "com_example_jnidemo_MainActivity.h"

#define  LOG_TAG    "FFMPEG INFO"
#define  LOGI(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define  LOGE(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)


void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame) {
  FILE* pFile;
  char szFileName[32];
  int y;

  sprintf(szFileName,"/mnt/sdcard/test/frame%d.ppm",iFrame);

  LOGI("filename : %s",szFileName);

  pFile = fopen(szFileName,"w");
  if(pFile == NULL){
      LOGI("can not open file %s",szFileName);
      return ;
  }

  fprintf(pFile,"P6\n%d %d\n255\n",width,height);

  for(y=0;y<width;++y){
      LOGI("Write file AVFrame");
      fwrite(pFrame->data[0]+y*pFrame->linesize[0],1,width*3,pFile);
  }

  fclose(pFile);
  LOGI("close file %s",szFileName);
}

JNIEXPORT void JNICALL Java_com_example_jnidemo_MainActivity_info(JNIEnv *env, jobject obj, jstring jpath){
    const jbyte* path = (*env)->GetStringUTFChars(env,jpath,NULL);
    AVFormatContext   *pFormatCtx = NULL;
    int               i, videoStream;
    AVCodecContext    *pCodecCtxOrig = NULL;
    AVCodecContext    *pCodecCtx = NULL;
    AVCodec           *pCodec = NULL;
    AVFrame           *pFrame = NULL;
    AVFrame           *pFrameRGB = NULL;
    AVPacket          packet;
    int               frameFinished;
    int               numBytes;
    uint8_t           *buffer = NULL;
    struct SwsContext *sws_ctx = NULL;

    av_register_all();

//      if(avformat_open_input(&pFormatCtx, path, NULL, NULL)!=0)
//      {
//          LOGE("Could not open the file : %s",path);
//          return ;
//    }

    int err_code;

    if(err_code=avformat_open_input(&pFormatCtx, path, NULL, NULL))
    {
        char buf[256];
        av_strerror(err_code, buf, 1024);
        LOGE("Couldn't open file %s: %d(%s)", path, err_code, buf);
        return;
    }

    if(avformat_find_stream_info(pFormatCtx, NULL)<0)
        return ;

    av_dump_format(pFormatCtx, 0, path, 0);

    videoStream=-1;
    for(i=0; i<pFormatCtx->nb_streams; i++)
        if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO)
        {
            videoStream=i;
            break;
        }

    if(videoStream==-1)
        return ;

    pCodecCtxOrig=pFormatCtx->streams[videoStream]->codec;

    pCodec=avcodec_find_decoder(pCodecCtxOrig->codec_id);
    if(pCodec==NULL)
    {
        LOGE("Unsupported codec!\n");
        return ;
    }

    pCodecCtx = avcodec_alloc_context3(pCodec);
    if(avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0)
    {
        LOGE("Couldn't copy codec context");
        return ;
    }

    if(avcodec_open2(pCodecCtx, pCodec, NULL)<0)
        return ;

    pFrame=av_frame_alloc();
    pFrameRGB=av_frame_alloc();
    if(pFrameRGB==NULL)
        return ;

    numBytes=avpicture_get_size(AV_PIX_FMT_RGB24, pCodecCtx->width,pCodecCtx->height);
    buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));

    avpicture_fill((AVPicture *)pFrameRGB, buffer, AV_PIX_FMT_RGB24,pCodecCtx->width, pCodecCtx->height);

    sws_ctx = sws_getContext(pCodecCtx->width,
               pCodecCtx->height,
               pCodecCtx->pix_fmt,
               pCodecCtx->width,
               pCodecCtx->height,
               AV_PIX_FMT_RGB24,
               SWS_BILINEAR,
               NULL,
               NULL,
               NULL
               );

    i=0;
    while(av_read_frame(pFormatCtx, &packet)>=0) {
        if(packet.stream_index==videoStream) {

            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);

            if(frameFinished) {

                sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,
          pFrame->linesize, 0, pCodecCtx->height,
          pFrameRGB->data, pFrameRGB->linesize);

                if(++i<=5)
                    SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
            }
        }
        av_free_packet(&packet);
    }
    av_free(buffer);
    av_frame_free(&pFrameRGB);
    av_frame_free(&pFrame);
    avcodec_close(pCodecCtx);
    avcodec_close(pCodecCtxOrig);
    avformat_close_input(&pFormatCtx);
    (*env)->ReleaseStringUTFChars(env,jpath,path);
}

4.4 编译native程序

进入jni目录，执行：

$NDK_PATH/ndk-build

编译结果：
编译完成后，在lib目录下可以看到编译的so文件，在Android.mk中设置的库文件名为ffmpeg，那么编译的文件应该是”libffmpeg.so”

4.5 JAVA调用JNI

（1）加载库文件

    static {
        System.loadLibrary("ffmpeg");
    }

（2）JNI接口
所有的JNI接口都有native关键字

private native void info(String path);

（3）详细代码

package com.example.jnidemo;

import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        String path = "/sdcard/test/test.mp4";
        info(path);
    }

    private native void info(String path);

    static {
        System.loadLibrary("ffmpeg");
    }

}