Opencv基础 （二）：使用OpenCV读取和写入视频

在OpenCV中读写视频与读写图像非常相似。视频就是一系列通常被称为帧的图像。所以，你需要做的就是循环播放视频序列中的所有帧，然后一次处理一帧。在这篇文章中，我们将演示如何从一个文件、一个图像序列和一个网络摄像头读取、显示和写入视频。我们还将研究流程中可能发生的一些错误，并帮助理解如何解决这些错误。

让我们先看一下读取视频文件的代码示例。它本质上包含了从磁盘读取视频并显示它的功能。随着您的深入，我们将详细讨论这个实现中使用的函数。

Python

import cv2 

# Create a video capture object, in this case we are reading the video from a file
vid_capture = cv2.VideoCapture('Resources/Cars.mp4')

if (vid_capture.isOpened() == False):
	print("Error opening the video file")
# Read fps and frame count
else:
	# Get frame rate information
	# You can replace 5 with CAP_PROP_FPS as well, they are enumerations
	fps = vid_capture.get(5)
	print('Frames per second : ', fps,'FPS')

	# Get frame count
	# You can replace 7 with CAP_PROP_FRAME_COUNT as well, they are enumerations
	frame_count = vid_capture.get(7)
	print('Frame count : ', frame_count)

while(vid_capture.isOpened()):
	# vid_capture.read() methods returns a tuple, first element is a bool 
	# and the second is frame
	ret, frame = vid_capture.read()
	if ret == True:
		cv2.imshow('Frame',frame)
		# 20 is in milliseconds, try to increase the value, say 50 and observe
		key = cv2.waitKey(20)
		
		if key == ord('q'):
			break
	else:
		break

# Release the video capture object
vid_capture.release()
cv2.destroyAllWindows()

C++

// Include Libraries
#include
#include

// Namespace to nullify use of cv::function(); syntax
using namespace std;
using namespace cv;

int main()
{
	// initialize a video capture object
	VideoCapture vid_capture("Resources/Cars.mp4");

	// Print error message if the stream is invalid
	if (!vid_capture.isOpened())
	{
		cout << "Error opening video stream or file" << endl;
	}

	else
	{
		// Obtain fps and frame count by get() method and print
		// You can replace 5 with CAP_PROP_FPS as well, they are enumerations
		int fps = vid_capture.get(5);
		cout << "Frames per second :" << fps;

		// Obtain frame_count using opencv built in frame count reading method
		// You can replace 7 with CAP_PROP_FRAME_COUNT as well, they are enumerations
		int frame_count = vid_capture.get(7);
		cout << "  Frame count :" << frame_count;
	}


	// Read the frames to the last frame
	while (vid_capture.isOpened())
	{
		// Initialise frame matrix
		Mat frame;

	    // Initialize a boolean to check if frames are there or not
		bool isSuccess = vid_capture.read(frame);

		// If frames are present, show it
		if(isSuccess == true)
		{
			//display frames
			imshow("Frame", frame);
		}

		// If frames are not there, close it
		if (isSuccess == false)
		{
			cout << "Video camera is disconnected" << endl;
			break;
		}
		
		//wait 20 ms between successive frames and break the loop if key q is pressed
		int key = waitKey(20);
		if (key == 'q')
		{
			cout << "q key is pressed by the user. Stopping the video" << endl;
			break;
		}


	}
	// Release the video capture object
	vid_capture.release();
	destroyAllWindows();
	return 0;
}

以下是我们将在这篇博文中讨论的OpenCV视频I/O的主要功能:

• cv2.videoccapture—创建一个视频捕获对象，它将帮助流或显示视频。
• cv2.VideoWriter -将输出的视频保存到一个目录中。
• 此外，我们还讨论了其他需要的函数，如cv2.imshow()， cv2.waitKey()和get()方法，用于读取视频元数据，如帧高度，宽度，帧数等。