cv::VideoCapture Class Reference

cv::VideoCapture Class Reference

Class for video capturing from video files, image sequences or cameras.

1. Detailed Description

The class provides C++ API for capturing video from cameras or for reading video files and image sequences.

//============================================================================
// Name        : cv::VideoCapture
// Author      : Yongqiang Cheng
// Version     : Feb 22, 2020
// Copyright   : Copyright (c) 2019 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <opencv2/core.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>

#include <iostream>
#include <stdio.h>

using namespace cv;
using namespace std;

int main(int, char**)
{
	Mat frame;

	//--- INITIALIZE VIDEOCAPTURE
	VideoCapture cap;
	// open the default camera using default API
	// cap.open(0);
	// OR advance usage: select any API backend
	int deviceID = 0;             // 0 = open default camera
	int apiID = cv::CAP_ANY;      // 0 = autodetect default API
	// open selected camera using selected API
	cap.open(deviceID + apiID);
	// check if we succeeded
	if (!cap.isOpened())
	{
		cerr << "ERROR! Unable to open camera\n";
		return -1;
	}

	//--- GRAB AND WRITE LOOP
	cout << "Start grabbing" << endl << "Press any key to terminate" << endl;
	for (;;)
	{
		// wait for a new frame from camera and store it into 'frame'
		cap.read(frame);
		// check if we succeeded
		if (frame.empty())
		{
			cerr << "ERROR! blank frame grabbed\n";
			break;
		}

		// show live and wait for a key with timeout long enough to show images
		imshow("yongqiang", frame);
		if (waitKey(5) >= 0)
		{
			break;
		}
	}

	// the camera will be deinitialized automatically in VideoCapture destructor
	return 0;
}

16:20:11 **** Build of configuration Debug for project DisplayImage ****
make all 
Building file: ../src/DisplayImage.cpp
Invoking: GCC C++ Compiler
g++ -std=c++0x -I/usr/local/include -I/usr/local/include/opencv4 -I/usr/local/include/opencv4/opencv2 -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/DisplayImage.d" -MT"src/DisplayImage.o" -o "src/DisplayImage.o" "../src/DisplayImage.cpp"
Finished building: ../src/DisplayImage.cpp
 
Building target: DisplayImage
Invoking: GCC C++ Linker
g++ -L/usr/local/lib -o "DisplayImage"  ./src/DisplayImage.o   -lopencv_core -lopencv_video -lopencv_ml -lopencv_imgproc -lopencv_img_hash -lopencv_flann -lopencv_features2d -lopencv_calib3d -lopencv_dnn -lopencv_dnn_objdetect -lopencv_cvv -lopencv_text -lopencv_datasets -lopencv_aruco -lopencv_bgsegm -lopencv_shape -lopencv_imgcodecs -lopencv_videoio -lopencv_highgui -lopencv_bioinspired
Finished building target: DisplayImage
 

16:20:13 Build Finished (took 1s.739ms)

2. Note

In C API the black-box structure CvCapture is used instead of VideoCapture.

• (C++) A basic sample on using the VideoCapture interface can be found at OPENCV_SOURCE_CODE/samples/cpp/videocapture_starter.cpp
• (Python) A basic sample on using the VideoCapture interface can be found at OPENCV_SOURCE_CODE/samples/python/video.py
• (Python) A multi threaded video processing sample can be found at OPENCV_SOURCE_CODE/samples/python/video_threaded.py
• (Python) VideoCapture sample showcasing some features of the Video4Linux2 backend OPENCV_SOURCE_CODE/samples/python/video_v4l2.py

backend ['bæk,ɛnd]：n. 后端

3. Enumeration

https://docs.opencv.org/4.2.0/d4/d15/group__videoio__flags__base.html

#include <opencv2/videoio.hpp>
VideoCapture API backends identifier.
VideoCapture API 后端标识符。

enum  	cv::VideoCaptureAPIs { 
  cv::CAP_ANY = 0, 
  cv::CAP_VFW = 200, 
  cv::CAP_V4L = 200, 
  cv::CAP_V4L2 = CAP_V4L, 
  cv::CAP_FIREWIRE = 300, 
  cv::CAP_FIREWARE = CAP_FIREWIRE, 
  cv::CAP_IEEE1394 = CAP_FIREWIRE, 
  cv::CAP_DC1394 = CAP_FIREWIRE, 
  cv::CAP_CMU1394 = CAP_FIREWIRE, 
  cv::CAP_QT = 500, 
  cv::CAP_UNICAP = 600, 
  cv::CAP_DSHOW = 700, 
  cv::CAP_PVAPI = 800, 
  cv::CAP_OPENNI = 900, 
  cv::CAP_OPENNI_ASUS = 910, 
  cv::CAP_ANDROID = 1000, 
  cv::CAP_XIAPI = 1100, 
  cv::CAP_AVFOUNDATION = 1200, 
  cv::CAP_GIGANETIX = 1300, 
  cv::CAP_MSMF = 1400, 
  cv::CAP_WINRT = 1410, 
  cv::CAP_INTELPERC = 1500, 
  cv::CAP_REALSENSE = 1500, 
  cv::CAP_OPENNI2 = 1600, 
  cv::CAP_OPENNI2_ASUS = 1610, 
  cv::CAP_GPHOTO2 = 1700, 
  cv::CAP_GSTREAMER = 1800, 
  cv::CAP_FFMPEG = 1900, 
  cv::CAP_IMAGES = 2000, 
  cv::CAP_ARAVIS = 2100, 
  cv::CAP_OPENCV_MJPEG = 2200, 
  cv::CAP_INTEL_MFX = 2300, 
  cv::CAP_XINE = 2400 
}