FFmpeg通过摄像头实现对视频流进行解码并显示测试代码(旧接口)

这里通过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,然后解码,最后再用opencv显示。用到的模块包括avformat、avcodec和avdevice。libavdevice库是libavformat的一个补充库(complementary library)。主要涉及到的接口函数包括:注:其中一些接口是被废弃的旧接口

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获取到的编码类型和像素格式可能也不同;

7. avcodec_find_decoder:由codec ID查找已注册的解码器;

8. avcodec_open2:初始化AVCodecContext;

9. av_frame_alloc:分配一个AVFrame并设置它的字段为默认值;

10. av_malloc:为一个AVPacket分配内存块;

11. sws_getContext:分配一个SwsContext;

12. av_read_frame:获取流即packet(AVPacket);

13. avcodec_decode_video2:解码视频帧,从流数据到图像数据;

14. sws_scale:转换图像格式;

15. av_free_packet:释放AVPacket;

16. av_frame_free:释放由av_frame_alloc分配的AVFrame;

17. sws_freeContext:释放由sws_getContext分配的SwsContext;

18. av_free:释放由av_malloc分配的AVPacket;

19. avformat_close_input:关闭打开的AVFormatContext并释放。

测试代码(test_ffmpeg_decode_show.cpp):

#include "funset.hpp"
#include 
#include 
#include 
#include 

#ifdef __cplusplus
extern "C" {
#endif

#include 
#include 
#include 
#include 
#include 
#include 

#ifdef __cplusplus
}
#endif

#include 

int test_ffmpeg_decode_show_old()
{
	avdevice_register_all();

#ifdef _MSC_VER
	const char* input_format_name = "vfwcap";
	const char* url = "";
#else
	const char* input_format_name = "video4linux2";
	const char* url = "/dev/video0";
#endif

	AVInputFormat* input_fmt = av_find_input_format(input_format_name);
	AVFormatContext* format_ctx = avformat_alloc_context();
	int ret = avformat_open_input(&format_ctx, url, input_fmt, nullptr);
	if (ret != 0) {
		fprintf(stderr, "fail to open url: %s, return value: %d\n", url, ret);
		return -1;
	}

	ret = avformat_find_stream_info(format_ctx, nullptr);
	if (ret < 0) {
		fprintf(stderr, "fail to get stream information: %d\n", ret);
		return -1;
	}

	int video_stream_index = -1;
	for (int i = 0; i < format_ctx->nb_streams; ++i) {
		const AVStream* stream = format_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, "no video stream\n");
		return -1;
	}

	AVCodecContext* codec_ctx = format_ctx->streams[video_stream_index]->codec;
	AVCodec* codec = avcodec_find_decoder(codec_ctx->codec_id);
	if (!codec) {
		fprintf(stderr, "no decoder was found\n");
		return -1;
	}

	ret = avcodec_open2(codec_ctx, codec, nullptr);
	if (ret != 0) {
		fprintf(stderr, "fail to init AVCodecContext: %d\n", ret);
		return -1;
	}

	AVFrame* frame = av_frame_alloc();
	AVPacket* packet = (AVPacket*)av_malloc(sizeof(AVPacket));
	SwsContext* sws_ctx = sws_getContext(codec_ctx->width, codec_ctx->height, codec_ctx->pix_fmt, codec_ctx->width, codec_ctx->height, AV_PIX_FMT_BGR24, 0, nullptr, nullptr, nullptr);
	if (!frame || !packet || !sws_ctx) {
		fprintf(stderr, "fail to alloc\n");
		return -1;
	}

	int got_picture = -1;
	std::unique_ptr data(new uint8_t[codec_ctx->width * codec_ctx->height * 3]);
	cv::Mat mat(codec_ctx->height, codec_ctx->width, CV_8UC3);
	int width_new = 320, height_new = 240;
	cv::Mat dst(height_new, width_new, CV_8UC3);
	const char* winname = "usb video1";
	cv::namedWindow(winname);

	while (1) {
		ret = av_read_frame(format_ctx, packet);
		if (ret < 0) {
			fprintf(stderr, "fail to av_read_frame: %d\n", ret);
			continue;
		}

		if (packet->stream_index == video_stream_index) {
			ret = avcodec_decode_video2(codec_ctx, frame, &got_picture, packet);
			if (ret < 0) {
				fprintf(stderr, "fail to avcodec_decode_video2: %d\n", ret);
				av_free_packet(packet);
				continue;
			}

			if (got_picture) {
				uint8_t* p[1] = { data.get() };
				int dst_stride[1] = { frame->width * 3 };
				sws_scale(sws_ctx, frame->data, frame->linesize, 0, codec_ctx->height, p, dst_stride);
				mat.data = data.get();
				cv::resize(mat, dst, cv::Size(width_new, height_new));
				cv::imshow(winname, dst);
			}
		}

		av_free_packet(packet);

		int key = cv::waitKey(25);
		if (key == 27) break;
	}

	cv::destroyWindow(winname);
	av_frame_free(&frame);
	sws_freeContext(sws_ctx);
	av_free(packet);
	avformat_close_input(&format_ctx);

	fprintf(stdout, "test finish\n");
	return 0;
}

执行结果如下:

FFmpeg通过摄像头实现对视频流进行解码并显示测试代码(旧接口)_第1张图片

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

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