ubuntu22.04@laptop OpenCV Get Started: 002_reading_writing_videos

ubuntu22.04@laptop OpenCV Get Started: 002_reading_writing_videos

1. 源由2. Read/Display/Write应用Demo3 video_read_from_file3.1 C++应用Demo3.2 Python应用Demo3.3 重点过程分析3.3.1 读取视频文件3.3.2 读取文件信息3.3.3 帧读取&显示 4 video_read_from_image_sequence4.1 C++应用Demo4.2 Python应用Demo4.3 重点过程分析 5 video_read_from_webcam5.1 C++应用Demo5.2 Python应用Demo5.3 重点过程分析 6 video_write_from_webcam6.1 C++应用Demo6.2 Python应用Demo6.3 重点过程分析6.3.1 获取视频参数6.3.2 设置保存视频参数6.3.3 保存视频文件 7 video_write_to_file7.1 C++应用Demo7.2 Python应用Demo7.3 重点过程分析 8. 总结9. 参考资料

1. 源由

在OpenCV中对视频的读写操作与图像的读写操作非常相似。视频不过是一系列通常被称为帧的图像。所以，所需要做的就是在视频序列中的所有帧上循环，然后一次处理一帧。

接下来研读下：

Read/Display/Write视频文件Read/Display/Write系列图片Read/Display/Write网络摄像头

2. Read/Display/Write应用Demo

002_reading_writing_videos是OpenCV读写、显示视频文件例程。

确认OpenCV安装路径：

$ find /home/daniel/ -name "OpenCVConfig.cmake"/home/daniel/OpenCV/installation/opencv-4.9.0/lib/cmake/opencv4//home/daniel/OpenCV/opencv/build/OpenCVConfig.cmake/home/daniel/OpenCV/opencv/build/unix-install/OpenCVConfig.cmake$ export OpenCV_DIR=/home/daniel/OpenCV/installation/opencv-4.9.0/lib/cmake/opencv4/

3 video_read_from_file

3.1 C++应用Demo

C++应用Demo工程结构：

002_reading_writing_videos/CPP/video_read_from_file$ tree ..├── CMakeLists.txt├── Resources│   └── Cars.mp4└── video_read_from_file.cpp1 directory, 3 files

C++应用Demo工程编译执行：

$ cd video_read_from_file$ mkdir build$ cd build$ cmake ..$ cmake --build . --config Release$ cd ..$ ./build/video_read_from_file

3.2 Python应用Demo

Python应用Demo工程结构：

002_reading_writing_videos/Python$ tree . -L 2.├── requirements.txt├── Resources│   ├── Cars.mp4│   └── Image_sequence├── video_read_from_file.py├── video_read_from_image_sequence.py├── video_read_from_webcam.py├── video_write_from_webcam.py└── video_write_to_file.py2 directories, 7 files

Python应用Demo工程执行：

$ workoncv-4.9.0$ python video_read_from_file.py

3.3 重点过程分析

3.3.1 读取视频文件

VideoCapture(path, apiPreference)

C++:

# Create a video capture object, in this case we are reading the video from a fileVideoCapture vid_capture("Resources/Cars.mp4");

Python:

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

3.3.2 读取文件信息

vid_capture.isOpened()vid_capture.get()

C++:

if (!vid_capture.isOpened())  {    cout << "Error opening video stream or file" << endl;  }else  {            // Obtain fps and frame count by get() method and print    int fps = vid_capture.get(5):    cout << "Frames per second :" << fps;    frame_count = vid_capture.get(7);    cout << "Frame count :" << frame_count;  }

Python:

if (vid_capture.isOpened() == False):  print("Error opening the video file")else:  # Get frame rate information   fps = int(vid_capture.get(5))  print("Frame Rate : ",fps,"frames per second")     # Get frame count  frame_count = vid_capture.get(7)  print("Frame count : ", frame_count)

3.3.3 帧读取&显示

vid_capture.read()cv2.imshow()

C++:

while (vid_capture.isOpened()){        // Initialize 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;        }    }

Python:

while(vid_capture.isOpened()):  # vCapture.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)    k = cv2.waitKey(20)    # 113 is ASCII code for q key    if k == 113:      break  else:    break

4 video_read_from_image_sequence

4.1 C++应用Demo

C++应用Demo工程结构：

002_reading_writing_videos/CPP/video_read_from_image_sequence$ tree . -L 2.├── CMakeLists.txt├── Resources│   └── Image_Sequence└── video_read_from_image_sequence.cpp2 directories, 2 files

C++应用Demo工程编译执行：

$ cd video_read_from_image_sequence$ mkdir build$ cd build$ cmake ..$ cmake --build . --config Release$ cd ..$ ./build/video_read_is

4.2 Python应用Demo

Python应用Demo工程结构：

002_reading_writing_videos/Python$ tree . -L 2.├── requirements.txt├── Resources│   ├── Cars.mp4│   └── Image_sequence├── video_read_from_file.py├── video_read_from_image_sequence.py├── video_read_from_webcam.py├── video_write_from_webcam.py└── video_write_to_file.py2 directories, 7 files

Python应用Demo工程执行：

$ workoncv-4.9.0$ python video_read_from_image_sequence.py

4.3 重点过程分析

读取系列照片文件

VideoCapture(path, apiPreference)

C++:

VideoCapture vid_capture("Resources/Image_sequence/Cars%04d.jpg");

Python:

vid_capture = cv2.VideoCapture('Resources/Image_sequence/Cars%04d.jpg')

注：Cars%04d.jpg: Cars0001.jpg, Cars0002.jpg, Cars0003.jpg, etc

5 video_read_from_webcam

5.1 C++应用Demo

C++应用Demo工程结构：

002_reading_writing_videos/CPP/video_read_from_webcam$ tree ..├── CMakeLists.txt└── video_read_from_webcam.cpp0 directories, 2 files

C++应用Demo工程编译执行：

$ cd video_read_from_webcam$ mkdir build$ cd build$ cmake ..$ cmake --build . --config Release$ cd ..$ ./build/video_read_from_webcam

5.2 Python应用Demo

Python应用Demo工程结构：

002_reading_writing_videos/Python$ tree . -L 2.├── requirements.txt├── Resources│   ├── Cars.mp4│   └── Image_sequence├── video_read_from_file.py├── video_read_from_image_sequence.py├── video_read_from_webcam.py├── video_write_from_webcam.py└── video_write_to_file.py2 directories, 7 files

Python应用Demo工程执行：

$ workoncv-4.9.0$ python video_read_from_webcam.py

5.3 重点过程分析

VideoCapture(path, apiPreference)

C++:

VideoCapture vid_capture(0);

Python:

vid_capture = cv2.VideoCapture(0)

注：cv2.CAP_DSHOW不要使用，有时会导致vid_capture.isOpened()返回false。

6 video_write_from_webcam

6.1 C++应用Demo

C++应用Demo工程结构：

002_reading_writing_videos/CPP/video_write_from_webcam$ tree ..├── CMakeLists.txt└── video_write_from_webcam.cpp0 directories, 2 files

C++应用Demo工程编译执行：

$ cd video_write_from_webcam$ mkdir build$ cd build$ cmake ..$ cmake --build . --config Release$ cd ..$ ./build/video_write_from_webcam

6.2 Python应用Demo

Python应用Demo工程结构：

002_reading_writing_videos/Python$ tree . -L 2.├── requirements.txt├── Resources│   ├── Cars.mp4│   └── Image_sequence├── video_read_from_file.py├── video_read_from_image_sequence.py├── video_read_from_webcam.py├── video_write_from_webcam.py└── video_write_to_file.py2 directories, 7 files

Python应用Demo工程执行：

$ workoncv-4.9.0$ python video_write_from_webcam.py

6.3 重点过程分析

6.3.1 获取视频参数

vid_capture.get()

C++:

// Obtain frame size information using get() methodInt frame_width = static_cast<int>(vid_capture.get(3));int frame_height = static_cast<int>(vid_capture.get(4));Size frame_size(frame_width, frame_height);int fps = 20;

Python:

# Obtain frame size information using get() methodframe_width = int(vid_capture.get(3))frame_height = int(vid_capture.get(4))frame_size = (frame_width,frame_height)fps = 20

6.3.2 设置保存视频参数

VideoWriter(filename, apiPreference, fourcc, fps, frameSize[, isColor]) filename: pathname for the output video fileapiPreference: API backends identifierfourcc: 4-character code of codec, used to compress the frames fourccfps: Frame rate of the created video streamframe_size: Size of the video framesisColor: If not zero, the encoder will expect and encode color frames. Else it will work with grayscale frames (the flag is currently supported on Windows only).

C++:

//Initialize video writer objectVideoWriter output("Resources/output.avi", VideoWriter::fourcc('M', 'J', 'P', 'G'),frames_per_second, frame_size);

Python:

# Initialize video writer objectoutput = cv2.VideoWriter('Resources/output_video_from_file.avi', cv2.VideoWriter_fourcc('M','J','P','G'), 20, frame_size)

保存视频文件格式可以选择：

AVI: cv2.VideoWriter_fourcc(‘M’,‘J’,‘P’,‘G’)MP4: cv2.VideoWriter_fourcc(*‘XVID’)

6.3.3 保存视频文件

output.write()

C++:

while (vid_capture.isOpened()){        // Initialize frame matrix        Mat frame;           // Initialize a boolean to check if frames are there or not        bool isSuccess = vid_capture.read(frame);         // If frames are not there, close it        if (isSuccess == false)        {            cout << "Stream disconnected" << endl;            break;        }          // If frames are present        if(isSuccess == true)        {            //display frames            output.write(frame);                  // display frames                  imshow("Frame", frame);                   // wait for 20 ms between successive frames and break                          // the loop if key q is pressed                  int key = waitKey(20);                  if (key == ‘q’)                  {                      cout << "Key q key is pressed by the user.                       Stopping the video" << endl;                      break;                  }        } }

Python:

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:           # Write the frame to the output files           output.write(frame)    else:         print(‘Stream disconnected’)           break

7 video_write_to_file

7.1 C++应用Demo

C++应用Demo工程结构：

002_reading_writing_videos/CPP/video_write_to_file$ tree -L 2.├── CMakeLists.txt├── Resources│   └── Cars.mp4└── video_write_to_file.cpp1 directory, 4 files

C++应用Demo工程编译执行：

$ cd video_write_to_file$ mkdir build$ cd build$ cmake ..$ cmake --build . --config Release$ cd ..$ ./build/video_write_to_file

7.2 Python应用Demo

Python应用Demo工程结构：

002_reading_writing_videos/Python$ tree . -L 2.├── requirements.txt├── Resources│   ├── Cars.mp4│   └── Image_sequence├── video_read_from_file.py├── video_read_from_image_sequence.py├── video_read_from_webcam.py├── video_write_from_webcam.py└── video_write_to_file.py2 directories, 7 files

Python应用Demo工程执行：

$ workoncv-4.9.0$ python video_write_to_file.py

7.3 重点过程分析

整合了以下章节重点过程：

video_read_from_file video_write_from_webcam

8. 总结

主要通过以下三个函数API实现：

videoCapture()：获取数据源read()：读取数据imshow()：显示图像write()：保存数据

其他API函数：

isOpened() - 数据源打开是否成功过get() - 获取数据源相关信息

9. 参考资料

【1】ubuntu22.04@laptop OpenCV Get Started
【2】ubuntu22.04@laptop OpenCV安装
【3】ubuntu22.04@laptop OpenCV定制化安装