ffmpeg example演示教程 -AudioDecode

本人刚开始学习ffmpeg，写此博客作为笔记，也希望能帮助其他刚入门的童鞋！ffmpeg源码下载和编译本文就直接跳过，源码doc/example路径下有很多demo，可以为我们演示ffmpeg的api如何使用，对我们理解ffmpeg的api有很大帮助。本文将介绍decode_audio.c如何编译使用。

代码主体框架：
1、根据Codec ID查找编码器（Codec）
2、根据编码器 ID查找解析器（Parse）。注：解析器用于解封装，解封装后的数据放到AVPacket中
3、根据Codec创建AVCodecContext。
4、循环读取输入文件，送入解析器解封装出AVPacket，然后再解码出AVFrame数据。
inputFile ----(解封装)–> AVPacket —(解码)—>AVFrame —（保存pcm数据）–> File.pcm

注：音频中一个AVFrame可能包含多个音频帧。而视频则一个AVFrame对应一帧图像。

因测试方便，我修改了部分代码用于适配FLAC格式的音频文件。代码如下

/** Copyright (c) 2001 Fabrice Bellard** Permission is hereby granted, free of charge, to any person obtaining a copy* of this software and associated documentation files (the "Software"), to deal* in the Software without restriction, including without limitation the rights* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell* copies of the Software, and to permit persons to whom the Software is* furnished to do so, subject to the following conditions:** The above copyright notice and this permission notice shall be included in* all copies or substantial portions of the Software.** THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN* THE SOFTWARE.*//*** @file* audio decoding with libavcodec API example** @example decode_audio.c*/#include <stdio.h>
#include <stdlib.h>
#include <string.h>#include <libavutil/frame.h>
#include <libavutil/mem.h>#include <libavcodec/avcodec.h>#define AUDIO_INBUF_SIZE 20480
#define AUDIO_REFILL_THRESH 4096static void decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame,FILE *outfile)
{int i, ch;int ret, data_size;/* send the packet with the compressed data to the decoder */ret = avcodec_send_packet(dec_ctx, pkt);if (ret < 0) {fprintf(stderr, "Error submitting the packet to the decoder\n");exit(1);}/* read all the output frames (in general there may be any number of them */while (ret >= 0) {ret = avcodec_receive_frame(dec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)return;else if (ret < 0) {fprintf(stderr, "Error during decoding\n");exit(1);}data_size = av_get_bytes_per_sample(dec_ctx->sample_fmt);if (data_size < 0) {/* This should not occur, checking just for paranoia */fprintf(stderr, "Failed to calculate data size\n");exit(1);}printf("+++++ FLC-DBG: data_size = %d +++++\n", data_size); //单通道样本（sample）大小printf("+++++ FLC-DBG: frame->nb_samples = %d +++++\n", frame->nb_samples); //帧个数printf("+++++ FLC-DBG: dec_ctx->channels = %d +++++\n", dec_ctx->channels); //通道数printf("+++++ FLC-DBG: frame->linesize[0] = %d +++++\n", frame->linesize[0]); //frame->data[0]数据长度（Bytes）printf("+++++ FLC-DBG: frame->linesize[1] = %d +++++\n", frame->linesize[1]);//FLAC格式的文件，解码后数据都放在frame->data[0]中，长度可根据frame->linesize[0]来获取。//原始代码不同声道数据存在不同的frame->data[i]中。
//        for (i = 0; i < frame->nb_samples; i++)
//            for (ch = 0; ch < dec_ctx->channels; ch++)
//                fwrite(frame->data[ch] + data_size*i, 1, data_size, outfile);//仅保存单通道数据。//for (i = 0; i < frame->nb_samples; i++)//  fwrite(frame->data[0] + i * data_size*dec_ctx->channels, 1, data_size, outfile);//保存全部通道fwrite(frame->data[0], 1, frame->linesize[0], outfile);}
}int main(int argc, char **argv)
{const char *outfilename, *filename;const AVCodec *codec;AVCodecContext *c= NULL;AVCodecParserContext *parser = NULL;int len, ret;FILE *f, *outfile;uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];uint8_t *data;size_t   data_size;AVPacket *pkt;AVFrame *decoded_frame = NULL;if (argc <= 2) {fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);exit(0);}filename    = argv[1];outfilename = argv[2];pkt = av_packet_alloc();/* find the MPEG audio decoder *///此处修改为FLAC的codec id。//codec = avcodec_find_decoder(AV_CODEC_ID_MP2);codec = avcodec_find_decoder(AV_CODEC_ID_FLAC);if (!codec) {fprintf(stderr, "Codec not found\n");exit(1);}parser = av_parser_init(codec->id);if (!parser) {fprintf(stderr, "Parser not found\n");exit(1);}c = avcodec_alloc_context3(codec);if (!c) {fprintf(stderr, "Could not allocate audio codec context\n");exit(1);}/* open it */if (avcodec_open2(c, codec, NULL) < 0) {fprintf(stderr, "Could not open codec\n");exit(1);}f = fopen(filename, "rb");if (!f) {fprintf(stderr, "Could not open %s\n", filename);exit(1);}outfile = fopen(outfilename, "wb");if (!outfile) {av_free(c);exit(1);}/* decode until eof */data      = inbuf;data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, f);while (data_size > 0) {if (!decoded_frame) {if (!(decoded_frame = av_frame_alloc())) {fprintf(stderr, "Could not allocate audio frame\n");exit(1);}}ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size,data, data_size,AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);if (ret < 0) {fprintf(stderr, "Error while parsing\n");exit(1);}data      += ret;data_size -= ret;printf("\n+++++ FLC-DBG: pkt->data:%p, pkt->size:%d +++++\n",pkt->data, pkt->size);if (pkt->size)decode(c, pkt, decoded_frame, outfile);if (data_size < AUDIO_REFILL_THRESH) {memmove(inbuf, data, data_size);data = inbuf;len = fread(data + data_size, 1,AUDIO_INBUF_SIZE - data_size, f);if (len > 0)data_size += len;}}/* flush the decoder */pkt->data = NULL;pkt->size = 0;decode(c, pkt, decoded_frame, outfile);fclose(outfile);fclose(f);avcodec_free_context(&c);av_parser_close(parser);av_frame_free(&decoded_frame);av_packet_free(&pkt);return 0;
}

使用方法

./decode_audio ~/Music/yesterdayOnceMore.flac ~/Music/output.pcm

查看编码后的数据，使用Audacity，选择导入原始数据。

配置原始数据的采样率、位宽、声道数。

点击播放按钮，试听我们解码出来的文件吧

整个工程的下载地址：
https://download.csdn.net/download/lyy901135/11296755
注：Makefile中需要对头文件和库的路径进行适配。

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