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FFMPEG 解码过程初步学习

news 来源：原创 2024/9/28 17:35:46

1. 视频文件解码过程

解码过程

在这里插入图片描述

步骤如下：

视频文件（封装格式，MP4/FLV/AVI 等）获取视频格式信息等
解复用为Stream 流，准备解码用的Codec
将Stream 流使用解码器解为Raw 格式针

数据结构：
AVFormatContext：整个过程非常重要的数据结果，通过avformat_open_input 从文件获取format 信息，一般封装信息在这一步就都可以获取到了，在通过avformat_find_stream_info 接口，获取读入视频文件中的stream 信息（这里的stream 一般包含视频流、音频流、字幕流（subtitile) 等）
AVCodecContext: 解码过程中需要使用的codec context，在avcodec_send_packet 和avcodec_receive_frame 中都要使用的数据结构，里面包含了解码必要的信息，比如图像width, height, format 等信息
AVPacket: 从文件里面读取出来的包数据
AVFrame: avcodec_send_packet 将AVPacket 数据送给指定的解码器后，通过avcodec_receive_frame 获取到解码后的帧数据封装在AVFrame 中

1.1 音视频格式填充：

int ret = avformat_open_input(&format_ctx, filename, nullptr, nullptr);  // 打开输入文件并将格式上下文赋给 format_ctx
// 填充stream 信息
if (avformat_find_stream_info(format_ctx, NULL) < 0) {std::cout << "no stream in files : " << std::endl;return -1;
}// find the video stream information
ret = av_find_best_stream(format_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, nullptr, 0);
if (ret < 0) {fprintf(stderr, "Cannot find a video stream in the input file\n");return -1;
}
video_stream_index = ret;
video_stream = format_ctx->streams[video_stream_index];ret = av_find_best_stream(format_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, nullptr, 0);
if (ret < 0) {std::cout<< "no audio stream " << ret << std::endl;
} else {audio_stream_index = ret;audio_stream = format_ctx->streams[audio_stream_index];
}

1.2 准备解码用的codec

const AVCodec * video_codec = avcodec_find_decoder(video_stream->codecpar->codec_id);
if (video_codec == nullptr) {std::cout << "we not found the code: " << video_decoder->id << std::endl;
} else {std::cout << "we found the video codec: " << video_codec->name << std::endl;
}
const AVCodec * audio_codec = avcodec_find_decoder(audio_stream->codecpar->codec_id);
if (audio_codec == nullptr) {std::cout << "we not found the code: " << audio_decoder->id << std::endl;
} else {std::cout << "we found the audio codec: " << audio_codec->name << std::endl;
}// codec context for decoder 
AVCodecContext* video_dec_context = avcodec_alloc_context3(video_codec);
if (!video_dec_context) {std::cout << "video dec context alloc failed" << std::endl;
}
if ((ret = avcodec_parameters_to_context(video_dec_context, video_stream->codecpar)) < 0) {std::cerr << "codec copy failed" << std::endl;
}
std::cout<< "video format width: " << video_dec_context->width << std::endl;
std::cout<< "video format height: " << video_dec_context->height << std::endl;
std::cout << "video pixel format: " << video_dec_context->pix_fmt << std::endl;
video_width = video_dec_context->width;
video_height = video_dec_context->height;
video_fmt = video_dec_context->pix_fmt;
// open codec
ret = avcodec_open2(video_dec_context, video_codec, NULL);
if (ret < 0) {std::cerr << "video codec open failed" << std::endl;
}

Audio 的流程和video 的流程相近:
通过stream 信息->AVCodec->AVCodecContext ，再通过AVCodecContext 打开解码器（avcodec_open2)

1.3 解码过程：

在打开解码器后，创建AVPacket AVFrame
AVPacket 是解码前的包， AVFrame 是解码后的帧

AVPacket *packet;
packet = av_packet_alloc();AVFrame* frame = av_frame_alloc();while(1) {// 从里面获取一个packetret1 = av_read_frame(format_ctx, packet);if (ret1 < 0) {break;} else {/*if (packet->stream_index == video_stream_index) {std::cout << "Video: pts: " << packet->pts << " dts: " << packet->dts <<" duration: " << packet->duration << std::endl;} else {if (packet->stream_index != -1 && audio_stream_index == packet->stream_index) {std::cout << "Audio: pts: " << packet->pts << " dts: " << packet->dts <<" duration: " << packet->duration << std::endl;}}*/if (packet->stream_index == video_stream_index) {int result = decode_packet(video_dec_context, packet);if (result == 0) {//video_frame_count++;}} else {decode_packet(audio_dec_context, packet);av_frame_unref(frame);}}av_packet_unref(packet);}
// 解包过程
static int decode_packet(AVCodecContext *dec, const AVPacket *pkt)
{int ret = 0;// submit the packet to the decoderret = avcodec_send_packet(dec, pkt);if (ret < 0) {fprintf(stderr, "Error submitting a packet for decoding (%d)\n", ret);return ret;}// get all the available frames from the decoderwhile (ret >= 0) {ret = avcodec_receive_frame(dec, frame);if (ret < 0) {// those two return values are special and mean there is no output// frame available, but there were no errors during decodingif (ret == AVERROR_EOF || ret == AVERROR(EAGAIN))return 0;// fprintf(stderr, "Error during decoding (%s)\n", av_err2str(ret));std::cerr << "Error during decoding " << ret << std::endl;return ret;}// // write the frame data to output fileif (dec->codec->type == AVMEDIA_TYPE_VIDEO) {static int video_frame_count = 0;/* copy decoded frame to destination buffer:* this is required since rawvideo expects non aligned data */// std::cout << "frame size: " << frame->linesize << std::endl;av_image_copy2(video_dst_data, video_dst_linesize,frame->data, frame->linesize,video_fmt, video_width, video_height);/* write to rawvideo file */size_t size =  fwrite(video_dst_data[0], 1, video_dst_bufsize, video_dst_file);if (size == 0) {std::cerr << "write failed" << std::endl;return -1;} else {video_frame_count++;std::cout << " write to video frame count: " << video_frame_count << std::endl;}}//ret = output_video_frame(frame);// else//     ret = output_audio_frame(frame);av_frame_unref(frame);}return ret;
}