FFmpeg实现获取USB摄像头视频流测试代码

矫情吗；* 2022-10-03 00:33 982阅读 0赞

通过USB摄像头(注：windows7/10下使用内置摄像头，linux下接普通的usb摄像头(Logitech))获取视频流用到的模块包括avformat和avdevice。头文件仅include avdevice.h即可，因为avdevice.h中会include avformat.h。libavdevice库是libavformat的一个补充库(complementary library)。主要涉及到的接口函数包括：

1. avdevice\_register\_all：初始化libavdevice库并注册所有输入输出设备；

2. av\_find\_input\_format：根据输入格式的名字查找AVInputFormat，在测试代码中，windows平台使用”vfwcap”(video for windows capture)，linux平台使用”v4l2”(Video4Linux2)；

3. avformat\_alloc\_context：分配AVFormatContext；

4. avformat\_open\_input：打开输入流并读取header；

5. avformat\_find\_stream\_info：读取媒体文件的数据包以获取流信息；

6. 通过AVFormatContext中AVStream查找视频/音频流索引，这里在windows10下获取到的编码类型为mjpeg即AV\_CODEC\_ID\_MJPEG，像素格式为yuv422p即AV\_PIX\_FMT\_YUVJ422P；在linux或windows7下获取到的编码类型为rawvideo即AV\_CODEC\_ID\_RAWVIDEO，像素格式yuyv422即AV\_PIX\_FMT\_YUYV422；可见摄像头类型不同、系统不同获取到的编码类型和像素格式可能也不同；

7. av\_read\_frame：获取流即packet(AVPacket)；

8. av\_packet\_unref：释放packet引用的缓冲区；

9. avformat\_close\_input：关闭打开的输入AVFormatContext。

测试代码如下(test\_ffmpeg\_usb\_stream.cpp)：

#include "funset.hpp"
    #include <iostream>
    
    #ifdef __cplusplus
    extern "C" {
    #endif
    
    #include <libavdevice/avdevice.h>
    
    #ifdef __cplusplus
    }
    #endif
    
    int test_ffmpeg_usb_stream()
    {
    	avdevice_register_all();
    
    #ifdef _MSC_VER
    	const char* input_format_name = "vfwcap";
    	const char* url = "";
    #else
    	const char* input_format_name = "v4l2";
    	const char* url = "/dev/video0";
    #endif
    	AVInputFormat* fmt = av_find_input_format(input_format_name);
    
    	AVFormatContext* ctx = avformat_alloc_context();
    	int ret = avformat_open_input(&ctx, url, fmt, nullptr);
    	if (ret != 0) {
    		fprintf(stderr, "fail to open input stream: %d\n", ret);
    		return -1;
    	}
    
    	// read packets of a media file to get stream information
    	ret = avformat_find_stream_info(ctx, nullptr);
    	if (ret < 0) {
    		fprintf(stderr, "fail to get stream information: %d\n", ret);
    		return -1;
    	}
    
    	// find audio/video stream index
    	int video_stream_index = -1;
    	int audio_stream_index = -1;
    	fprintf(stdout, "Number of elements in AVFormatContext.streams: %d\n", ctx->nb_streams);
    	for (int i = 0; i < ctx->nb_streams; ++i) {
    		const AVStream* stream = ctx->streams[i];
    		fprintf(stdout, "type of the encoded data: %d\n", stream->codecpar->codec_id);
    		if (stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
    			video_stream_index = i;
    			fprintf(stdout, "dimensions of the video frame in pixels: width: %d, height: %d, pixel format: %d\n",
    				stream->codecpar->width, stream->codecpar->height, stream->codecpar->format);
    		}
    		else if (stream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
    			audio_stream_index = i;
    			fprintf(stdout, "audio sample format: %d\n", stream->codecpar->format);
    		}
    	}
    
    	if (video_stream_index == -1) {
    		fprintf(stderr, "Error: no video stream\n");
    		return -1;
    	}
    
    	if (audio_stream_index == -1) {
    		fprintf(stdout, "Warning: no audio stream\n");
    	}
    
    	int cnt = 0;
    	AVPacket pkt;
    	while (1) {
    		if (++cnt > 100) break;
    
    		ret = av_read_frame(ctx, &pkt);
    		if (ret < 0) {
    			fprintf(stderr, "error or end of file: %d\n", ret);
    			continue;
    		}
    
    		if (pkt.stream_index == video_stream_index) {
    			fprintf(stdout, "video stream, packet size: %d\n", pkt.size);
    		}
    
    		if (pkt.stream_index == audio_stream_index) {
    			fprintf(stdout, "audio stream, packet size: %d\n", pkt.size);
    		}
    
    		av_packet_unref(&pkt);
    	}
    
    	avformat_close_input(&ctx);
    
    	fprintf(stdout, "test finish\n");
    	return 0;
    }

Linux下执行结果如下：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlbmdiaW5nY2h1bg_size_16_color_FFFFFF_t_70][]

**GitHub**：[https://github.com/fengbingchun/OpenCV\_Test][https_github.com_fengbingchun_OpenCV_Test]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlbmdiaW5nY2h1bg_size_16_color_FFFFFF_t_70]: /images/20220109/1fac1de1f3a74e8a84e36b7946ee7ebc.png
[https_github.com_fengbingchun_OpenCV_Test]: https://github.com/fengbingchun/OpenCV_Test