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通过FFmpeg对视频进行编解码时，如果输入文件存在本机或通过USB摄像头、笔记本内置摄像头获取数据时，可通过avformat_open_input接口中的第二个参数直接指定即可。但如果待处理的视频数据存在于内存块中时，该如何指定，可通过FFmpeg中的结构体AVIOContext实现，此时avformat_open_input中的第二个参数传nullptr。
涉及到FFmpeg中的主要函数是avio_alloc_context,声明如下：
AVIOContext *avio_alloc_context(unsigned char *buffer,int buffer_size,int write_flag,void *opaque,int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),int64_t (*seek)(void *opaque, int64_t offset, int whence)) (1).buffer：通过AVIOContext进行输入/输出操作的内存块，由av_malloc分配，av_free释放。av_read_frame会持续从此处取数据。
(2).buffer_size: 内存块大小。
(3).write_flag: 如果buffer作为输出即写入则为1(FFmpeg将处理后的数据写入buffer)，如果buffer作为输入则设置为0(FFmpeg从buffer获取数据).
(4).opaque: 指向用户特定数据的不透明指针。
(5).read_packet: 回调函数，当buffer作为输入时必须指定，否则可为nullptr。此回调函数的参数依次为avio_alloc_context中的opaque、buffer、buffer_size。
(6).write_packet:回调函数，当buffer作为输出时必须指定，否则可为nullptr。此回调函数的参数依次为avio_alloc_context中的opaque、buffer、buffer_size。
(7).seek：回调函数，用于查找指定字节位置的函数，可为nullptr。
调用完此接口后，需要将此接口返回的指针赋值给AVFormatContext的pb即I/O context。
以下为测试代码段：
(1).主线程用于实时显示内存块内容。另有一个单独线程用于创建数据。这里使用队列：线程set_packet持续向队列中push数据；回调函数read_packet持续从队列中pop数据
typedef struct Buffer {unsigned char* data;unsigned int length;
} Buffer;class BufferQueue {
public:BufferQueue() = default;~BufferQueue() {}void push(Buffer& buffer) {std::unique_lock<std::mutex> lck(mtx);queue.push(buffer);cv.notify_all();}void pop(Buffer& buffer) {std::unique_lock<std::mutex> lck(mtx);while (queue.empty()) {cv.wait(lck);}buffer = queue.front();queue.pop();}unsigned int size() {return queue.size();}private:std::queue<Buffer> queue;std::mutex mtx;std::condition_variable cv;
};class PacketScaleQueue {
public:PacketScaleQueue() = default;~PacketScaleQueue() {Buffer buffer;while (getPacketSize() > 0) {popPacket(buffer);delete[] buffer.data;}while (getScaleSize() > 0) {popScale(buffer);delete[] buffer.data;}}void init(unsigned int buffer_num = 16, unsigned int buffer_size = 1024 * 1024 * 4) {for (unsigned int i = 0; i < buffer_num; ++i) {Buffer buffer = { new unsigned char[buffer_size], buffer_num};pushPacket(buffer);}}void pushPacket(Buffer& buffer) { packet_queue.push(buffer); }void popPacket(Buffer& buffer) { packet_queue.pop(buffer); }unsigned int getPacketSize() { return packet_queue.size(); }void pushScale(Buffer& buffer) { scale_queue.push(buffer); }void popScale(Buffer& buffer) { scale_queue.pop(buffer); }unsigned int getScaleSize() { return scale_queue.size(); }private:BufferQueue packet_queue, scale_queue;
};(2).线程函数set_packet内容如下：类PacketScaleQueue中有两个BufferQueue: packet_queue：未被使用的；scale_queue：已被使用的
void set_packet(PacketScaleQueue& packet_encode)
{while (packet_encode_flag) {static unsigned char v1 = 0, v2 = 0, v3 = 255;static const size_t size = height * width;Buffer buffer;packet_encode.popPacket(buffer);memset(buffer.data, v1, size);memset(buffer.data + size, v2, size);memset(buffer.data + size * 2, v3, size);packet_encode.pushScale(buffer);++v1;++v2;--v3;if (v1 == 255) v1 = 0;if (v2 == 255) v2 = 0;if (v3 == 0) v3 = 255;std::this_thread::sleep_for(std::chrono::milliseconds(40));}
}(3).回调函数read_packet内容如下：
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;
}(4).主函数test_ffmpeg_avio_show内容如下：
int test_ffmpeg_avio_show()
{PacketScaleQueue packet_encode;packet_encode.init(30, 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 = avformat_alloc_context();if (!ifmt_ctx) {print_error_string(AVERROR(ENOMEM));return -1;}ifmt_ctx->pb = avio_ctx;AVDictionary* dict = nullptr;av_dict_set(&dict, "video_size", "640x480", 0);av_dict_set(&dict, "pixel_format", "bgr24", 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 ret;}ret = avformat_find_stream_info(ifmt_ctx, nullptr);if (ret < 0) {fprintf(stderr, "Could not find stream information\n");print_error_string(ret);return ret;}av_dump_format(ifmt_ctx, 0, "nothing", 0);int video_stream_index = -1;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 -1;}AVCodecParameters* codecpar = ifmt_ctx->streams[video_stream_index]->codecpar;if (codecpar->codec_id != AV_CODEC_ID_RAWVIDEO) {fprintf(stderr, "error: this test code only support rawvideo encode: %d\n", codecpar->codec_id);return -1;}AVPacket* packet = static_cast<AVPacket*>(av_malloc(sizeof(AVPacket)));if (!packet) {fprintf(stderr, "fail to av_malloc\n");return -1;}cv::Mat mat(height, width, CV_8UC3);const char* winname = "show video";cv::namedWindow(winname);while (1) {ret = av_read_frame(ifmt_ctx, packet);if (ret >= 0 && packet->stream_index == video_stream_index && packet->size > 0) {mat.data = packet->data;cv::imshow(winname, mat);av_packet_unref(packet);int key = cv::waitKey(30);if (key == 27) {packet_encode_flag = false;break;}}}av_freep(packet);cv::destroyWindow(winname);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);}//avio_context_free(&avio_ctx); ==> av_freep(&avio_ctx);av_dict_free(&dict);thread_packet.join();fprintf(stdout, "test finish\n");return 0;
}执行结果如下图所示：
GitHub：https://github.com/fengbingchun/OpenCV_Test