Windows上默認的內置攝像頭一般支持兩種編解碼格式：rawvideo和mjpeg。在調用FFmpeg接口時默認的采用rawvideo。這里通過DirectShow實現為mjpeg進行編解碼。

通過命令行調用FFmpeg可執行文件：

(1). 可獲取Windows上連接的視頻設備，命令如下：

ffmpeg.exe -list_devices true -f dshow -i dummy

(2). 可獲取指定視頻設備支持的編解碼格式和video size，命令如下：

ffmpeg.exe -f dshow -list_options true -i video="Integrated Webcam"

這里通過C++實現獲取連接的視頻設備，代碼如下：

AVFormatContext* format_context = avformat_alloc_context();
AVDictionary* dict = nullptr;
av_dict_set(&dict, "list_devices", "true", 0);
AVInputFormat* input_format = av_find_input_format("dshow");
avformat_open_input(&format_context, "", input_format, &dict);
avformat_close_input(&format_context);
av_dict_free(&dict);

這里通過C++實現支持的編解碼格式和video size，代碼如下：

AVFormatContext* format_context = avformat_alloc_context();
AVDictionary* dict = nullptr;
av_dict_set(&dict, "list_options", "true", 0);
AVInputFormat* input_format = av_find_input_format("dshow");
avformat_open_input(&format_context, "video=Integrated Webcam", input_format, &dict); // video=video device name
avformat_close_input(&format_context);
av_dict_free(&dict);

設置編解碼格式通過av_dict_set好像不能成功，這里是通過AVFormatContext的video_codec_id進行設置，通過av_dict_set設置幀率好像也不能成功。通過調用OpenCV的接口進行實時顯示，代碼如下：

#include "funset.hpp"
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <memory>
#include <fstream>
#include <thread>
#include "common.hpp"#ifdef __cplusplus
extern "C" {
#endif#include <libavdevice/avdevice.h>
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libswscale/swscale.h>
#include <libavutil/mem.h>
#include <libavutil/imgutils.h>#ifdef __cplusplus
}
#endif#include <opencv2/opencv.hpp>#ifdef _MSC_VER
int test_ffmpeg_decode_dshow()
{avdevice_register_all();AVCodecID id = AV_CODEC_ID_MJPEG;AVCodec* encoder_id = avcodec_find_encoder(id);AVCodec* decoder_id = avcodec_find_decoder(id);if (!encoder_id || !decoder_id) {fprintf(stderr, "codec not found: %d\n", id);return -1;}AVFormatContext* format_context = avformat_alloc_context();format_context->video_codec_id = id; // 指定編解碼格式AVInputFormat* input_format = av_find_input_format("dshow");AVDictionary* dict = nullptr;//if (av_dict_set(&dict, "vcodec"/*"input_format"*/, "mjpeg", 0) < 0) fprintf(stderr, "fail to av_dict_set: line: %d\n", __LINE__); // 通過av_dict_set設置編解碼格式好像不起作用if (av_dict_set(&dict, "video_size", "320x240", 0) < 0) fprintf(stderr, "fail to av_dict_set: line: %d\n", __LINE__);//if (av_dict_set(&dict, "r", "25", 0) < 0) fprintf(stderr, "fail to av_dict_set: line: %d\n", __LINE__); // 通過av_dict_set設置幀率好像不起作用int ret = avformat_open_input(&format_context, "video=Integrated Webcam", input_format, &dict);if (ret != 0) {fprintf(stderr, "fail to avformat_open_input: %d\n", ret);return -1;}ret = avformat_find_stream_info(format_context, nullptr);if (ret < 0) {fprintf(stderr, "fail to get stream information: %d\n", ret);return -1;}int video_stream_index = -1;for (unsigned int i = 0; i < format_context->nb_streams; ++i) {const AVStream* stream = format_context->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, "no video stream\n");return -1;}fprintf(stdout, "frame rate: %f\n", av_q2d(format_context->streams[video_stream_index]->r_frame_rate));AVCodecParameters* codecpar = format_context->streams[video_stream_index]->codecpar;const AVCodec* codec = avcodec_find_decoder(codecpar->codec_id);if (!codec) {fprintf(stderr, "fail to avcodec_find_decoder\n");return -1;}if (codecpar->codec_id != id) {fprintf(stderr, "this test code only support mjpeg encode: %d\n", codecpar->codec_id);return -1;}AVCodecContext* codec_context = avcodec_alloc_context3(codec);if (!codec_context) {fprintf(stderr, "fail to avcodec_alloc_context3\n");return -1;}codec_context->pix_fmt = AVPixelFormat(codecpar->format);codec_context->height = codecpar->height;codec_context->width = codecpar->width;codec_context->thread_count = 16;ret = avcodec_open2(codec_context, codec, nullptr);if (ret != 0) {fprintf(stderr, "fail to avcodec_open2: %d\n", ret);return -1;}AVPixelFormat dst_pixel_format = AV_PIX_FMT_BGR24;AVFrame* frame = av_frame_alloc();AVPacket* packet = (AVPacket*)av_malloc(sizeof(AVPacket));SwsContext* sws_context = sws_getContext(codec_context->width, codec_context->height, codec_context->pix_fmt, codec_context->width, codec_context->height, dst_pixel_format, 0, nullptr, nullptr, nullptr);if (!frame || !packet || !sws_context) {fprintf(stderr, "fail to alloc\n");return -1;}uint8_t *bgr_data[4];int bgr_linesize[4];av_image_alloc(bgr_data, bgr_linesize, codec_context->width, codec_context->height, dst_pixel_format, 1);cv::Mat mat(codec_context->height, codec_context->width, CV_8UC3);const char* winname = "dshow mjpeg video";cv::namedWindow(winname);while (1) {ret = av_read_frame(format_context, packet);if (ret >= 0 && packet->stream_index == video_stream_index && packet->size > 0) {ret = avcodec_send_packet(codec_context, packet);if (ret < 0) {fprintf(stderr, "##### fail to avcodec_send_packet: %d\n", ret);av_packet_unref(packet);continue;}ret = avcodec_receive_frame(codec_context, frame);if (ret < 0) {fprintf(stderr, "##### fail to avcodec_receive_frame: %d\n", ret);av_packet_unref(packet);continue;}sws_scale(sws_context, frame->data, frame->linesize, 0, codec_context->height, bgr_data, bgr_linesize);mat.data = bgr_data[0];cv::imshow(winname, mat);} else if (ret < 0 || packet->size <= 0) {fprintf(stderr, "##### fail to av_read_frame: %d, packet size: %d\n", ret, packet->size);continue;}av_packet_unref(packet);int key = cv::waitKey(30);if (key == 27) break;}cv::destroyWindow(winname);sws_freeContext(sws_context);av_frame_free(&frame);av_freep(packet);av_freep(&bgr_data[0]);avformat_close_input(&format_context);av_dict_free(&dict);
}fprintf(stdout, "test finish\n");return 0;
}
#else
int test_ffmpeg_decode_dshow()
{fprintf(stderr, "Error: this test code only support windows platform\n");return -1;
}
#endif

執行結果如下：

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

總結

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