opencv videocapture

OpenCV 的 VideoCapture 类可以用来获取视频的帧。可以从文件、摄像头或网络源捕获视频。使用时只需要实例化一个 VideoCapture 对象，并传入视频源的地址或摄像头编号即可。然后可以使用 read() 方法来获取视频的下一帧，或是使用 grab() 和 retrieve() 方法来获取视频帧。

相关问题

opencv VideoCapture

OpenCV VideoCapture is a class that provides an interface for capturing video from cameras or files. It is a part of the OpenCV library that is used for computer vision applications. VideoCapture can be used to capture live video from a webcam or to read video files.

To use VideoCapture, you need to create an instance of the class and specify the device or file you want to capture video from. The device is identified by its index number, which starts from 0. For example, to capture video from the default camera, you can use the following code:

import cv2

cap = cv2.VideoCapture(0)

This creates a VideoCapture object and sets it to capture video from the first camera device available on the system.

You can also specify a video file to read using the constructor. For example, to read a video file named "test.mp4", you can use the following code:

import cv2

cap = cv2.VideoCapture("test.mp4")

Once you have created a VideoCapture object, you can use the read() method to read frames from the video stream. The read() method returns a tuple containing a boolean value and a frame. The boolean value indicates whether the frame was successfully read or not, and the frame is a NumPy array containing the image data.

import cv2

cap = cv2.VideoCapture(0)

while True:
    ret, frame = cap.read()

    if ret:
        cv2.imshow("Frame", frame)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

This code captures video from the default camera and displays each frame in a window. The loop continues until the user presses the 'q' key to quit. Finally, the VideoCapture object is released and the display window is destroyed.

Android的 OpenCV VideoCapture环境搭建

要在 Android 上使用 OpenCV 的 VideoCapture，需要先在 Android Studio 中进行以下步骤：

1. 下载 OpenCV for Android，并解压到指定目录。

2. 在 Android Studio 中创建一个新的项目。

3. 在项目的 build.gradle 文件中添加 OpenCV 的依赖项：

dependencies {
    implementation project(':openCVLibrary')
}

4. 在项目的 AndroidManifest.xml 文件中添加以下权限：

<uses-permission android:name="android.permission.CAMERA" />

5. 在项目的 MainActivity.java 文件中添加以下代码：

private BaseLoaderCallback mLoaderCallback = new BaseLoaderCallback(this) {
    @Override
    public void onManagerConnected(int status) {
        switch (status) {
            case LoaderCallbackInterface.SUCCESS: {
                // OpenCV loaded successfully
                // Now we can use OpenCV functions
                VideoCapture videoCapture = new VideoCapture();
                videoCapture.open(0); // 0 for back camera, 1 for front camera
                break;
            }
            default: {
                super.onManagerConnected(status);
                break;
            }
        }
    }
};

@Override
protected void onResume() {
    super.onResume();
    OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION, this, mLoaderCallback);
}

这段代码首先创建了一个 BaseLoaderCallback 对象，然后在 onResume() 方法中调用了 OpenCVLoader.initAsync() 方法，以在后台加载 OpenCV 库。当加载完成后，会回调 BaseLoaderCallback 的 onManagerConnected() 方法，在该方法中可以使用 OpenCV 的函数。

在这里，我们创建了一个 VideoCapture 对象，并通过 open() 方法打开了相机。通过传递参数 0 或 1，可以选择使用后置摄像头或前置摄像头。

以上就是在 Android Studio 中搭建 OpenCV VideoCapture 环境的步骤。需要注意的是，由于 OpenCV for Android 本身比较大，所以在编译项目时可能会比较耗时。如果遇到编译失败的情况，可以尝试增加 Gradle 的内存限制（在项目的 gradle.properties 文件中添加 org.gradle.jvmargs=-Xmx3g）。