以下测试代码实现的功能是：持续从内存块中获取原始数据，然后依次进行解码、编码、最后保存成mp4视频文件。

      可保存成单个视频文件，也可指定每个视频文件的总帧数，保存多个视频文件。

      为了便于查看和修改，这里将可独立的程序段存放在单个函数中：

      1.线程函数set_packet：独立线程，用于用户向指定的内存块中持续写入数据，这里通过调用队列类PacketScaleQueue，可参考：https://blog.csdn.net/fengbingchun/article/details/132128885 中说明。这里填充的数据参考FFmpeg源码中的doc/examples/encode_video.c：每次向队列中写入一帧数据

void set_packet(PacketScaleQueue& packet_encode)
{while (packet_encode_flag) {Buffer buffer;packet_encode.popPacket(buffer);static int i = 0;// refrence: ffmpeg/doc/examples/encode_video.c// prepare a dummy image: Yunsigned char* p1 = buffer.data;for (auto y = 0; y < height; ++y) {for (auto x = 0; x < width; ++x) {p1[y * width + x] = x + y + i * 3;}}// Cb and Crunsigned char* p2 = buffer.data + width * height;unsigned char* p3 = buffer.data + width * height + width * height / 4;for (auto y = 0; y < height / 2; ++y) {for (auto x = 0; x < width / 2; ++x) {p2[y * width / 2 + x] = 128 + y + i * 2;p3[y * width / 2 + x] = 64 + x + i * 5;}}packet_encode.pushScale(buffer);if (++i > 25) i = 0;std::this_thread::sleep_for(std::chrono::milliseconds(40));}
}

      2.回调函数read_packet：avio_alloc_context中使用，不断从队列中获取数据，av_read_frame中也会从这里获取数据：每次从队列中获取一帧数据

int read_packet(void* opaque, uint8_t* buf, int buf_size)
{PacketScaleQueue* packet_encode = static_cast<PacketScaleQueue*>(opaque);Buffer buffer;packet_encode->popScale(buffer);memcpy(buf, buffer.data, buf_size);packet_encode->pushPacket(buffer);return buf_size;
}

      3.函数get_input_format_context：创建输入AVFormatContext，需要调用av_dict_set设置video_size和pixel_format

AVFormatContext* get_input_format_context(AVIOContext* avio_ctx)
{AVFormatContext* ifmt_ctx = avformat_alloc_context();if (!ifmt_ctx) {print_error_string(AVERROR(ENOMEM));return nullptr;}ifmt_ctx->pb = avio_ctx;AVDictionary* dict = nullptr;av_dict_set(&dict, "video_size", video_size, 0);av_dict_set(&dict, "pixel_format", pixel_format, 0);auto ret = avformat_open_input(&ifmt_ctx, nullptr, av_find_input_format("rawvideo"), &dict);if (ret < 0) {fprintf(stderr, "Could not open input\n");print_error_string(ret);return nullptr;}av_dict_free(&dict);ret = avformat_find_stream_info(ifmt_ctx, nullptr);if (ret < 0) {fprintf(stderr, "Could not find stream information\n");print_error_string(ret);return nullptr;}for (unsigned int i = 0; i < ifmt_ctx->nb_streams; ++i) {const AVStream* stream = ifmt_ctx->streams[i];if (stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {video_stream_index = i;fprintf(stdout, "type of the encoded data: %d, dimensions of the video frame in pixels: width: %d, height: %d, pixel format: %d\n",stream->codecpar->codec_id, stream->codecpar->width, stream->codecpar->height, stream->codecpar->format);}}if (video_stream_index == -1) {fprintf(stderr, "error: no video stream\n");return nullptr;}if (ifmt_ctx->streams[video_stream_index]->codecpar->codec_id != AV_CODEC_ID_RAWVIDEO) {fprintf(stderr, "error: this test code only support rawvideo encode: %d\n", ifmt_ctx->streams[video_stream_index]->codecpar->codec_id);return nullptr;}av_dump_format(ifmt_ctx, 0, "nothing", 0);return ifmt_ctx;
}

      4.函数get_decode_context：创建解码AVCodecContext

AVCodecContext* get_decode_context(AVFormatContext* ifmt_ctx)
{AVCodec* decoder = nullptr;auto ret = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &decoder, 0);if (ret < 0) {fprintf(stderr, "fail to av_find_best_stream: %d\n", ret);print_error_string(ret);return nullptr;}AVCodecContext* dec_ctx = avcodec_alloc_context3(decoder);if (!dec_ctx) {fprintf(stderr, "fail to avcodec_alloc_context3\n");return nullptr;}ret = avcodec_parameters_to_context(dec_ctx, ifmt_ctx->streams[video_stream_index]->codecpar);if (ret < 0) {fprintf(stderr, "fail to avcodec_parameters_to_context: %d\n", ret);print_error_string(ret);return nullptr;}dec_ctx->framerate = av_guess_frame_rate(ifmt_ctx, ifmt_ctx->streams[video_stream_index], nullptr);ret = avcodec_open2(dec_ctx, decoder, nullptr);if (ret != 0) {fprintf(stderr, "fail to avcodec_open2: %d\n", ret);print_error_string(ret);return nullptr;}return dec_ctx;
}

      5.函数get_encode_context：创建编码AVCodecContext，注意对enc_ctx的相关成员的赋值

AVCodecContext* get_encode_context(AVFormatContext* ifmt_ctx)
{AVCodec* encodec = avcodec_find_encoder_by_name("mpeg1video"); // ffmpeg.exe -encodersif (!encodec) {fprintf(stderr, "fail to avcodec_find_encoder_by_name\n");return nullptr;}AVCodecContext* enc_ctx = avcodec_alloc_context3(encodec);if (!enc_ctx) {fprintf(stderr, "fail to avcodec_alloc_context3\n");return nullptr;}enc_ctx->bit_rate = 400000;enc_ctx->framerate = ifmt_ctx->streams[video_stream_index]->r_frame_rate;enc_ctx->pix_fmt = AV_PIX_FMT_YUV420P;enc_ctx->height = ifmt_ctx->streams[video_stream_index]->codecpar->height;enc_ctx->width = ifmt_ctx->streams[video_stream_index]->codecpar->width;enc_ctx->time_base = av_inv_q(av_d2q(frame_rate, 4096));enc_ctx->max_b_frames = 1;//if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER) // trueenc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;auto ret = avcodec_open2(enc_ctx, encodec, nullptr);if (ret != 0) {fprintf(stderr, "fail to avcodec_open2: %d\n", ret);print_error_string(ret);return nullptr;}return enc_ctx;
}

      6.函数get_output_format_context：创建输出AVFormatContext

AVFormatContext* get_output_format_context(const AVCodecContext* enc_ctx, const char* filename)
{AVFormatContext* ofmt_ctx = nullptr;auto ret = avformat_alloc_output_context2(&ofmt_ctx, nullptr, nullptr, filename);if (ret < 0 || !ofmt_ctx) {fprintf(stderr, "fail to avformat_alloc_output_context2: %d\n", ret);print_error_string(ret);return nullptr;}AVStream* out_stream = avformat_new_stream(ofmt_ctx, nullptr);if (!out_stream) {fprintf(stderr, "fail to avformat_new_stream\n");return nullptr;}ret = avcodec_parameters_from_context(out_stream->codecpar, enc_ctx);if (ret < 0) {fprintf(stderr, "fail to avcodec_parameters_from_context: %d\n", ret);print_error_string(ret);return nullptr;}out_stream->time_base = enc_ctx->time_base;if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) { // trueret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);if (ret < 0) {fprintf(stderr, "fail to avio_open: %d\n", ret);print_error_string(ret);return nullptr;}}ret = avformat_write_header(ofmt_ctx, nullptr);if (ret < 0) {fprintf(stderr, "fail to avformat_write_header: %d\n", ret);print_error_string(ret);return nullptr;}av_dump_format(ofmt_ctx, 0, filename, 1);return ofmt_ctx;
}

      7.函数decode: 注意对packet的判断，它有可能为nullptr；调用一次avcodec_send_packet，可能需调用多次avcodec_receive_frame，因此需要将avcodec_receive_frame放在while中；需要对dec_ctx->time_base进行设置

int decode(AVPacket* packet, AVFormatContext* ifmt_ctx, AVCodecContext* dec_ctx, AVFrame* frame)
{if (packet)av_packet_rescale_ts(packet, ifmt_ctx->streams[video_stream_index]->time_base, dec_ctx->time_base);auto ret = avcodec_send_packet(dec_ctx, packet);if (ret < 0) {fprintf(stderr, "fail to avcodec_send_packet: %d\n", ret);print_error_string(ret);return -1;}while (1) {ret = avcodec_receive_frame(dec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) { // AVERROR(EAGAIN): decode is not yet complete//fprintf(stderr, "Warning: avcodec_receive_frame: %d\n", ret);//print_error_string(ret);break;}else if (ret < 0) {fprintf(stderr, "fail to avcodec_receive_frame: %d\n", ret);print_error_string(ret);return ret;}frame->pts = frame->best_effort_timestamp;break;}if (packet)av_packet_unref(packet);return 0;
}

      8.函数encode：注意：调用一次avcodec_send_frame，可能需要调用多次avcodec_receive_packet，因此需要将avcodec_receive_packet放在while中；需要对ofmt_ctx->streams[0]->time_base进行设置，否则生成的mp4中无法获取帧速率；也可在调用avformat_write_header时设置video_track_timescale指定

int encode(AVCodecContext* enc_ctx, AVFrame* frame, AVPacket* packet, AVFormatContext* ifmt_ctx, AVFormatContext* ofmt_ctx)
{auto ret = avcodec_send_frame(enc_ctx, frame);if (ret < 0) {if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {fprintf(stderr, "Warning: avcodec_send_frame: %d\n", ret);print_error_string(ret);ret = 0;}else {fprintf(stderr, "fail to avcodec_send_frame: %d\n", ret);print_error_string(ret);return ret;}}while (1) {ret = avcodec_receive_packet(enc_ctx, packet);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) { // AVERROR(EAGAIN): encode is not yet complete//fprintf(stderr, "Warning: avcodec_receive_packet: %d\n", ret);//print_error_string(ret);break;}if (ret < 0) {fprintf(stderr, "fail to avcodec_receive_packet: %d\n", ret);print_error_string(ret);return ret;}packet->stream_index = 0;av_packet_rescale_ts(packet, ifmt_ctx->streams[video_stream_index]->time_base, ofmt_ctx->streams[0]->time_base);//packet2->pts = packet2->dts = frame->pts *//    ofmt_ctx->streams[0]->time_base.den / ofmt_ctx->streams[0]->time_base.num / //    (enc_ctx->framerate.num / enc_ctx->framerate.den);ret = av_interleaved_write_frame(ofmt_ctx, packet);if (ret < 0) {print_error_string(ret);return ret;}av_packet_unref(packet);}return 0;
}

      9.主函数test_ffmpeg_save_video_slice：注意：在退出前需要flush decoder和encoder；写入视频文件需先调用avformat_write_header，然后持续调用av_interleaved_write_frame，退出前再需调用av_write_trailer；每次新文件的写入，需重新调用get_decode_context、get_encode_context、get_output_format_context并在之前作相应free

int test_ffmpeg_save_video_slice()
{PacketScaleQueue packet_encode;packet_encode.init(queue_size, block_size);std::thread thread_packet(set_packet, std::ref(packet_encode));uint8_t* avio_ctx_buffer = static_cast<uint8_t*>(av_malloc(block_size));if (!avio_ctx_buffer) {print_error_string(AVERROR(ENOMEM));return -1;}AVIOContext* avio_ctx = avio_alloc_context(avio_ctx_buffer, block_size, 0, &packet_encode, &read_packet, nullptr, nullptr);if (!avio_ctx) {print_error_string(AVERROR(ENOMEM));return -1;}AVFormatContext* ifmt_ctx = get_input_format_context(avio_ctx);if (!ifmt_ctx) {fprintf(stderr, "fail to get_input_format_context\n");return -1;}AVPacket *packet = av_packet_alloc(), *packet2 = av_packet_alloc();if (!packet || !packet2) {fprintf(stderr, "fail to av_packet_alloc\n");return -1;}AVFrame* frame = av_frame_alloc();if (!frame) {fprintf(stderr, "fail to av_frame_alloc\n");return -1;}int count = 0, name = 1;AVCodecContext* enc_ctx = nullptr;AVFormatContext* ofmt_ctx = nullptr;AVCodecContext* dec_ctx = nullptr;while (1) {auto ret = av_read_frame(ifmt_ctx, packet);if (ret < 0) {break;}if (packet->stream_index != video_stream_index) {av_packet_unref(packet);continue;}if (count % slice_size == 0) {enc_ctx = get_encode_context(ifmt_ctx);if (!enc_ctx) {fprintf(stderr, "fail to avcodec_alloc_context3\n");return -1;}std::string filename = std::to_string(name) + ".mp4";filename = std::string(path) + filename;ofmt_ctx = get_output_format_context(enc_ctx, filename.c_str());if (!ofmt_ctx) {fprintf(stderr, "fail to get_output_format_context\n");return -1;}dec_ctx = get_decode_context(ifmt_ctx);if (!dec_ctx) {fprintf(stderr, "fail to get_decode_context\n");return -1;}++name;}if (decode(packet, ifmt_ctx, dec_ctx, frame) != 0) return -1;if (encode(enc_ctx, frame, packet2, ifmt_ctx, ofmt_ctx) != 0) return -1;//fprintf(stdout, "count: %d\n", count);if (count + 1 == total_frames) { // terminate looppacket_encode_flag = false;// flush the decoderdecode(nullptr, ifmt_ctx, dec_ctx, frame);if (frame->data[0])encode(enc_ctx, frame, packet2, ifmt_ctx, ofmt_ctx);// flush the encoderencode(enc_ctx, nullptr, packet2, ifmt_ctx, ofmt_ctx);av_write_trailer(ofmt_ctx);break;}++count;if (count > 1 && count % slice_size == 0) {// flush the decoderdecode(nullptr, ifmt_ctx, dec_ctx, frame);if (frame->data[0])encode(enc_ctx, frame, packet2, ifmt_ctx, ofmt_ctx);// flush the encoderencode(enc_ctx, nullptr, packet2, ifmt_ctx, ofmt_ctx);av_write_trailer(ofmt_ctx);avcodec_free_context(&dec_ctx);avcodec_free_context(&enc_ctx);avio_closep(&ofmt_ctx->pb);avformat_free_context(ofmt_ctx);}}av_packet_free(&packet);av_packet_free(&packet2);av_frame_free(&frame);avcodec_free_context(&dec_ctx);avcodec_free_context(&enc_ctx);avio_closep(&ofmt_ctx->pb);avformat_close_input(&ofmt_ctx);avformat_close_input(&ifmt_ctx);// note: the internal buffer could have changed, and be != avio_ctx_bufferif (avio_ctx) {av_freep(&avio_ctx->buffer);av_freep(&avio_ctx);}thread_packet.join();fprintf(stdout, "test finish\n");return 0;
}

      执行结果如下图所示：

      可调用ffprobe.exe验证每个生成的视频文件的总帧数，如1.mp4,执行如下命令：

ffprobe.exe -v error -select_streams v:0 -show_entries stream=nb_frames -of default=nokey=1:noprint_wrappers=1 1.mp4

      GitHub：https://github.com/fengbingchun/OpenCV_Test