ubuntu22.04@laptop OpenCV Get Started: 002_reading_writing_videos

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：

1. Read/Display/Write视频文件
2. Read/Display/Write系列图片
3. 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.cpp

1 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.py

2 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 file
VideoCapture vid_capture("Resources/Cars.mp4");

Python:

# Create a video capture object, in this case we are reading the video from a file
vid_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.cpp

2 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.py

2 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.cpp

0 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.py

2 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.cpp

0 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.py

2 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() method
Int 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() method
frame_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 file
• apiPreference: API backends identifier
• fourcc: 4-character code of codec, used to compress the frames fourcc
• fps: Frame rate of the created video stream
• frame_size: Size of the video frames
• isColor: 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 object
VideoWriter output("Resources/output.avi", VideoWriter::fourcc('M', 'J', 'P', 'G'),frames_per_second, frame_size);

Python:

# Initialize video writer object
output = 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.cpp

1 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.py

2 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：

• 2. video_read_from_file
• 6. video_write_from_webcam

8. 总结

主要通过以下三个函数API实现：

1. videoCapture()：获取数据源
2. read()：读取数据
3. imshow()：显示图像
4. 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