一个使用FFmpeg库读取3gp视频的例子-Android中使用FFmpeg媒体库(三)

本文中的代码来自于https://github.com/churnlabs/android-ffmpeg-sample，更多的可以参考这个项目代码。我会在代码中加一些自己的注释。感谢作者churnlabs给我们提供这么好的例子以供我们学习。
在Android的一些系统层应用开发大多数是采用jni的方式调用，另外对于一些比较吃CPU或者处理逻辑比较复杂的程序，也可以考虑使用jni方式来封装。可以提高程序的执行效率。

本文涉及到以下几个方面：
1 将3gp文件push到模拟机器的sdcard中
2 写jni代码，内部调用ffmpeg库的方法，编译jni库
3 loadLibrary生成的库，然后撰写相应的java代码
4 执行程序，并查看最终运行结果。
最终程序的显示效果如下：


1 使用eclipse的DDMS工具，将vid.3pg push到sdcard中

2 撰写相应的jni文件

/*
 * Copyright 2011 - Churn Labs, LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * This is mostly based off of the FFMPEG tutorial:
 * http://dranger.com/ffmpeg/
 * With a few updates to support Android output mechanisms and to update
 * places where the APIs have shifted.
 */

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

//包含ffmpeg库头文件，这些文件都直接方案jni目录下
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>

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

/* Cheat to keep things simple and just use some globals. */
//全局对象
AVFormatContext *pFormatCtx;
AVCodecContext *pCodecCtx;
AVFrame *pFrame;
AVFrame *pFrameRGB;
int videoStream;

/*
 * Write a frame worth of video (in pFrame) into the Android bitmap
 * described by info using the raw pixel buffer.  It's a very inefficient
 * draw routine, but it's easy to read. Relies on the format of the
 * bitmap being 8bits per color component plus an 8bit alpha channel.
 */

//定义的静态方法，将某帧AVFrame在Android的Bitmap中绘制
static void fill_bitmap(AndroidBitmapInfo*  info, void *pixels, AVFrame *pFrame)
{
    uint8_t *frameLine;

    int  yy;
    for (yy = 0; yy < info->height; yy++) {
        uint8_t*  line = (uint8_t*)pixels;
        frameLine = (uint8_t *)pFrame->data[0] + (yy * pFrame->linesize[0]);

        int xx;
        for (xx = 0; xx < info->width; xx++) {
            int out_offset = xx * 4;
            int in_offset = xx * 3;

            line[out_offset] = frameLine[in_offset];
            line[out_offset+1] = frameLine[in_offset+1];
            line[out_offset+2] = frameLine[in_offset+2];
            line[out_offset+3] = 0;
        }
        pixels = ( char *)pixels + info->stride;
    }
}

//定义java回调函数，相当与 com.churnlabs中的ffmpegsample中的MainActivity类中的openFile方法。
void Java_com_churnlabs_ffmpegsample_MainActivity_openFile(JNIEnv * env, jobject this )
{
    int ret;
    int err;
    int i;
    AVCodec *pCodec;
    uint8_t *buffer;
    int numBytes;
    //注册所有的函数
    av_register_all();
    LOGE( "Registered formats" );
    //打开sdcard中的vid.3gp文件
    err = av_open_input_file(&pFormatCtx, "file:/sdcard/vid.3gp" , NULL, 0, NULL);
    LOGE( "Called open file" );
    if (err!=0) {
        LOGE( "Couldn't open file" );
        return ;
    }
    LOGE( "Opened file" );

    if (av_find_stream_info(pFormatCtx)<0) {
        LOGE( "Unable to get stream info" );
        return ;
    }

    videoStream = -1;
    //定义设置videoStream
    for (i=0; i<pFormatCtx->nb_streams; i++) {
        if (pFormatCtx->streams[i]->codec->codec_type==CODEC_TYPE_VIDEO) {
            videoStream = i;
            break ;
        }
    }
    if (videoStream==-1) {
        LOGE( "Unable to find video stream" );
        return ;
    }

    LOGI( "Video stream is [%d]" , videoStream);
    //定义编码类型
    pCodecCtx=pFormatCtx->streams[videoStream]->codec;
    //获取解码器
    pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
    if (pCodec==NULL) {
        LOGE( "Unsupported codec" );
        return ;
    }
    //使用特定的解码器打开
    if (avcodec_open(pCodecCtx, pCodec)<0) {
        LOGE( "Unable to open codec" );
        return ;
    }
    //分配帧空间
    pFrame=avcodec_alloc_frame();
    //分配RGB帧空间
    pFrameRGB=avcodec_alloc_frame();
    LOGI( "Video size is [%d x %d]" , pCodecCtx->width, pCodecCtx->height);
    //获取大小
    numBytes=avpicture_get_size(PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height);
    分配空间
    buffer=(uint8_t *)av_malloc(numBytes* sizeof (uint8_t));

    avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24,
                            pCodecCtx->width, pCodecCtx->height);
}
//定义java回调函数，相当与 com.churnlabs中的ffmpegsample中的MainActivity类中的drawFrame方法。
void Java_com_churnlabs_ffmpegsample_MainActivity_drawFrame(JNIEnv * env, jobject this , jstring bitmap)
{
    AndroidBitmapInfo  info;
    void *              pixels;
    int                ret;

    int