FFmpeg4Android：视频文件解码

4 FFmpeg解码

4.1 视频解码流程

a) 视频播放流程
视频播放器播放视频文件，需要经过以下几个步骤：解封装，解码视音频，视音频同步。如果播放本地文件则不需要解协议，为以下几个步骤：解封装，解码视音频，视音频同步。他们的过程如图所示：

（参考雷神博客：[总结]视音频编解码技术零基础学习方法）

其中解码部分是核心，本章主要讲解视频的解码与转码流程。
b) 解码流程

解码者是这固定的几个流程，只要按照这个固定模式来调用函数即可，下面是各函数的作用：

av_register_all()：注册所有组件
avformat_open_input()：打开输入视频文件
avformat_find_stream_info()：获取视频文件信息
avcodec_find_decoder()：查找解码器
avcodec_open2()：打开解码器
av_read_frame()：从输入文件读取一帧压缩数据
avcodec_decode_video2()：解码一帧压缩数据

4.2 解码实例

该实例实现的作用为，将.mp4文件转换成.yuv文件。

项目目录结构：

Java端代码，MainActivity.java：

package com.lzp.sffmpegandroiddecoder;

import android.os.Bundle;
import android.os.Environment;
import android.app.Activity;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button startButton = (Button) this.findViewById(R.id.button_start);
        final EditText urlEdittext_input= (EditText) this.findViewById(R.id.input_url);
        final EditText urlEdittext_output= (EditText) this.findViewById(R.id.output_url);

        startButton.setOnClickListener(new OnClickListener() {
            public void onClick(View arg0){

                String folderurl=Environment.getExternalStorageDirectory().getPath();

                String urltext_input=urlEdittext_input.getText().toString();
                String inputurl=folderurl+"/"+urltext_input;
                Log.e("path", inputurl);

                String urltext_output=urlEdittext_output.getText().toString();
                String outputurl=folderurl+"/"+urltext_output;

                Log.i("inputurl",inputurl);
                Log.i("outputurl",outputurl);

                decode(inputurl,outputurl);

            }
        });
    }

    //JNI
    public native int decode(String inputurl, String outputurl);

    static{
        System.loadLibrary("avutil-54");
        System.loadLibrary("swresample-1");
        System.loadLibrary("avcodec-56");
        System.loadLibrary("avformat-56");
        System.loadLibrary("swscale-3");
        System.loadLibrary("postproc-53");
        System.loadLibrary("avfilter-5");
        System.loadLibrary("avdevice-56");
        System.loadLibrary("sffdecoder");
    }
}

C端源代码位于jni/simplest_ffmpeg_decoder.c，其负责对*.mp4文件的解码操作，根据上节中视频解码流程，实现过的代码如下所示：

#include 
#include 

#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libswscale/swscale.h"
#include "libavutil/log.h"

#ifdef ANDROID
#include 
#include 
#define LOGE(format, ...)  __android_log_print(ANDROID_LOG_ERROR, "(>_<)", format, ##__VA_ARGS__)
#define LOGI(format, ...)  __android_log_print(ANDROID_LOG_INFO,  "(^_^)", format, ##__VA_ARGS__)
#else
#define LOGE(format, ...)  printf("(>_<) " format "\n", ##__VA_ARGS__)
#define LOGI(format, ...)  printf("(^_^) " format "\n", ##__VA_ARGS__)
#endif


//Output FFmpeg's av_log()
void custom_log(void *ptr, int level, const char* fmt, va_list vl)
{
    FILE *fp=fopen("/storage/emulated/0/av_log.txt","a+");
    if(fp){
        vfprintf(fp,fmt,vl);
        fflush(fp);
        fclose(fp);
    }
}

JNIEXPORT jint JNICALL Java_com_lzp_sffmpegandroiddecoder_MainActivity_decode
  (JNIEnv *env, jobject obj, jstring input_jstr, jstring output_jstr)
{
    AVFormatContext *pFormatCtx;
    int             i, videoindex;
    AVCodecContext  *pCodecCtx;
    AVCodec         *pCodec;
    AVFrame *pFrame,*pFrameYUV;
    uint8_t *out_buffer;
    AVPacket *packet;
    int y_size;
    int ret, got_picture;
    struct SwsContext *img_convert_ctx;
    FILE *fp_yuv;
    int frame_cnt;
    clock_t time_start, time_finish;
    double  time_duration = 0.0;

    char input_str[500]={0};
    char output_str[500]={0};
    char info[1000]={0};
    sprintf(input_str,"%s",(*env)->GetStringUTFChars(env,input_jstr, NULL));
    sprintf(output_str,"%s",(*env)->GetStringUTFChars(env,output_jstr, NULL));

    //FFmpeg av_log() callback
    av_log_set_callback(custom_log);

    av_register_all();
    avformat_network_init();
    pFormatCtx = avformat_alloc_context();

    if(avformat_open_input(&pFormatCtx,input_str,NULL,NULL)!=0){
        LOGE("Couldn't open input stream.\n");
        return -1;
    }
    if(avformat_find_stream_info(pFormatCtx,NULL)<0){
        LOGE("Couldn't find stream information.\n");
        return -1;
    }
    videoindex=-1;
    for(i=0; inb_streams; i++)
        if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO){
            videoindex=i;
            break;
        }
    if(videoindex==-1){
        LOGE("Couldn't find a video stream.\n");
        return -1;
    }
    pCodecCtx=pFormatCtx->streams[videoindex]->codec;
    pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
    if(pCodec==NULL){
        LOGE("Couldn't find Codec.\n");
        return -1;
    }
    if(avcodec_open2(pCodecCtx, pCodec,NULL)<0){
        LOGE("Couldn't open codec.\n");
        return -1;
    }

    pFrame=av_frame_alloc();
    pFrameYUV=av_frame_alloc();
    out_buffer=(unsigned char *)av_malloc(av_image_get_buffer_size(AV_PIX_FMT_YUV420P,
        pCodecCtx->width, pCodecCtx->height, 1));
    av_image_fill_arrays(pFrameYUV->data, pFrameYUV->linesize, out_buffer,
        AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1);


    packet=(AVPacket *)av_malloc(sizeof(AVPacket));

    img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, 
        pCodecCtx->pix_fmt,
        pCodecCtx->width, pCodecCtx->height,
        AV_PIX_FMT_YUV420P, SWS_BICUBIC, NULL, NULL, NULL);

    sprintf(info,   "[Input     ]%s\n", input_str);
    sprintf(info, "%s[Output    ]%s\n",info,output_str);
    sprintf(info, "%s[Format    ]%s\n",info, pFormatCtx->iformat->name);
    sprintf(info, "%s[Codec     ]%s\n",info, pCodecCtx->codec->name);
    sprintf(info, "%s[Resolution]%dx%d\n",info, pCodecCtx->width,pCodecCtx->height);

  fp_yuv=fopen(output_str,"wb+");
  if(fp_yuv==NULL){
        printf("Cannot open output file.\n");
        return -1;
    }

    frame_cnt=0;
    time_start = clock();

    while(av_read_frame(pFormatCtx, packet)>=0){
        // 这里只要处理视频帧
        if(packet->stream_index==videoindex){
            ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet);
            if(ret < 0){
                LOGE("Decode Error.\n");
                return -1;
            }
            if(got_picture){
                sws_scale(img_convert_ctx, (const uint8_t* const*)pFrame->data, 
                    pFrame->linesize, 0, pCodecCtx->height,
                    pFrameYUV->data, pFrameYUV->linesize);

                y_size=pCodecCtx->width*pCodecCtx->height;
                fwrite(pFrameYUV->data[0],1,y_size,fp_yuv);    //Y
                fwrite(pFrameYUV->data[1],1,y_size/4,fp_yuv);  //U
                fwrite(pFrameYUV->data[2],1,y_size/4,fp_yuv);  //V
                //Output info
                char pictype_str[10]={0};
                switch(pFrame->pict_type){
                    case AV_PICTURE_TYPE_I:sprintf(pictype_str,"I");break;
                  case AV_PICTURE_TYPE_P:sprintf(pictype_str,"P");break;
                    case AV_PICTURE_TYPE_B:sprintf(pictype_str,"B");break;
                    default:sprintf(pictype_str,"Other");break;
                }
                LOGI("Frame Index: %5d. Type:%s",frame_cnt,pictype_str);
                frame_cnt++;
            }
        }
        av_free_packet(packet);
    }
    //flush decoder
    //FIX: Flush Frames remained in Codec
    while (1) 
    {
        ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet);
        if (ret < 0)
            break;
        if (!got_picture)
            break;
        sws_scale(img_convert_ctx, (const uint8_t* const*)pFrame->data, 
            pFrame->linesize, 0, pCodecCtx->height,
            pFrameYUV->data, pFrameYUV->linesize);
        int y_size=pCodecCtx->width*pCodecCtx->height;
        fwrite(pFrameYUV->data[0],1,y_size,fp_yuv);    //Y
        fwrite(pFrameYUV->data[1],1,y_size/4,fp_yuv);  //U
        fwrite(pFrameYUV->data[2],1,y_size/4,fp_yuv);  //V
        //Output info
        char pictype_str[10]={0};
        switch(pFrame->pict_type){
            case AV_PICTURE_TYPE_I:sprintf(pictype_str,"I");break;
          case AV_PICTURE_TYPE_P:sprintf(pictype_str,"P");break;
            case AV_PICTURE_TYPE_B:sprintf(pictype_str,"B");break;
            default:sprintf(pictype_str,"Other");break;
        }
        LOGI("Frame Index: %5d. Type:%s",frame_cnt,pictype_str);
        frame_cnt++;
    }
    time_finish = clock();
    time_duration=(double)(time_finish - time_start);

    sprintf(info, "%s[Time      ]%fms\n",info,time_duration);
    sprintf(info, "%s[Count     ]%d\n",info,frame_cnt);

    sws_freeContext(img_convert_ctx);

    fclose(fp_yuv);

    av_frame_free(&pFrameYUV);
    av_frame_free(&pFrame);
    avcodec_close(pCodecCtx);
    avformat_close_input(&pFormatCtx);

    return 0;
}

Android.mk文件位于jni/Android.mk，如下所示：

LOCAL_PATH := $(call my-dir)

# FFmpeg library
include $(CLEAR_VARS)
LOCAL_MODULE := avcodec
LOCAL_SRC_FILES := libavcodec-56.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := avdevice
LOCAL_SRC_FILES := libavdevice-56.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := avfilter
LOCAL_SRC_FILES := libavfilter-5.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := avformat
LOCAL_SRC_FILES := libavformat-56.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := avutil
LOCAL_SRC_FILES := libavutil-54.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := postproc
LOCAL_SRC_FILES := libpostproc-53.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := swresample
LOCAL_SRC_FILES := libswresample-1.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := swscale
LOCAL_SRC_FILES := libswscale-3.so
include $(PREBUILT_SHARED_LIBRARY)

# Program
include $(CLEAR_VARS)
LOCAL_MODULE := sffdecoder
LOCAL_SRC_FILES :=simplest_ffmpeg_decoder.c
LOCAL_C_INCLUDES += $(LOCAL_PATH)/include
LOCAL_LDLIBS := -llog -lz
LOCAL_SHARED_LIBRARIES := avcodec avdevice avfilter avformat avutil postproc swresample swscale
include $(BUILD_SHARED_LIBRARY)

build.gradle文件位于，如下所示：

import org.apache.tools.ant.taskdefs.condition.Os

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "com.lzp.sffmpegandroiddecoder"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    //指定动态库路径
    sourceSets {
        main{
            jni.srcDirs = []    // disable automatic ndk-build call, which ignore our Android.mk
            jniLibs.srcDir 'src/main/libs'
        }
    }

    // call regular ndk-build(.cmd) script from app directory
    task ndkBuild(type: Exec) {
        workingDir file('src/main')
        commandLine getNdkBuildCmd()
        //commandLine 'E:/Android/android-ndk-r10e/ndk-build.cmd'   //也可以直接使用绝对路径
    }

    tasks.withType(JavaCompile) {
        compileTask -> compileTask.dependsOn ndkBuild
    }

    task cleanNative(type: Exec) {
        workingDir file('src/main')
        commandLine getNdkBuildCmd(), 'clean'
    }

    clean.dependsOn cleanNative
}

//获取NDK目录路径
def getNdkDir() {
    if (System.env.ANDROID_NDK_ROOT != null)
        return System.env.ANDROID_NDK_ROOT

    Properties properties = new Properties()
    properties.load(project.rootProject.file('local.properties').newDataInputStream())
    def ndkdir = properties.getProperty('ndk.dir', null)
    if (ndkdir == null)
        throw new GradleException("NDK location not found. Define location with ndk.dir in the local.properties file or with an ANDROID_NDK_ROOT environment variable.")

    return ndkdir
}

//根据不同系统获取ndk-build脚本
def getNdkBuildCmd() {
    def ndkbuild = getNdkDir() + "/ndk-build"
    if (Os.isFamily(Os.FAMILY_WINDOWS))
        ndkbuild += ".cmd"

    return ndkbuild
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:26.+'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    testCompile 'junit:junit:4.12'
}

4.3 运行

将sintel.mp4文件拷到手机SDK根目录，运行并点击Star按钮后，在AndroidMonitor中打印如下信息：

Type：I、P、B是各视频帧的类型。

源码+视频下载：http://download.csdn.net/download/itismelzp/10184248