C++ opencv设置视频的捕获方式为 MJPG设置失败

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我有一款4k摄像头，在设置分辨率为4k的时候总是出现帧率不够的情况，
使用命令查看

v4l2-ctl --device=/dev/video0 --list-formats-ext

发现

v4l2-ctl --device=/dev/video0 --list-formats-ext
ioctl: VIDIOC_ENUM_FMTType: Video Capture[0]: 'MJPG' (Motion-JPEG, compressed)Size: Discrete 3840x2160Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 640x480Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 2592x1944Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 2688x1512Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 2048x1536Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 1920x1080Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 1600x1200Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 1280x960Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 1280x720Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 960x540Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 800x600Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 640x360Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 720x1280Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)[1]: 'YUYV' (YUYV 4:2:2)Size: Discrete 3840x2160Interval: Discrete 1.000s (1.000 fps)Size: Discrete 640x480Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 2592x1944Interval: Discrete 0.500s (2.000 fps)Size: Discrete 2688x1512Interval: Discrete 0.500s (2.000 fps)Size: Discrete 2048x1536Interval: Discrete 0.333s (3.000 fps)Size: Discrete 1920x1080Interval: Discrete 0.200s (5.000 fps)Size: Discrete 1600x1200Interval: Discrete 0.200s (5.000 fps)Size: Discrete 1280x960Interval: Discrete 0.133s (7.500 fps)Size: Discrete 1280x720Interval: Discrete 0.100s (10.000 fps)Size: Discrete 960x540Interval: Discrete 0.050s (20.000 fps)Size: Discrete 800x600Interval: Discrete 0.050s (20.000 fps)Size: Discrete 640x360Interval: Discrete 0.033s (30.000 fps)Interval: Discrete 0.040s (25.000 fps)Interval: Discrete 0.050s (20.000 fps)Interval: Discrete 0.100s (10.000 fps)Size: Discrete 720x1280Interval: Discrete 0.200s (5.000 fps)

因此我们需要设置视频捕获格式为 MJPG

C++ opencv中设置视频捕获格式为 MJPG

报错如下：

[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (1186) setProperty OpenCV | GStreamer warning: GStreamer: unhandled property
[ WARN:0] global ../modules/videoio/src/cap_gstreamer.cpp (1036) getProperty OpenCV | GStreamer warning: unhandled property: 6

这些警告可能是由于使用 GStreamer 后端而不是 V4L2（Video4Linux2）后端引起的。在某些系统上，默认情况下，OpenCV 使用 GStreamer 进行视频捕获。

要在 C++ 中使用 V4L2 后端设置视频捕获格式为 MJPG，您可以尝试使用以下代码：

#include <opencv2/opencv.hpp>int main()
{// 打开视频捕获设备cv::VideoCapture cap(0, cv::CAP_V4L2);// 设置视频捕获格式为 MJPGcap.set(cv::CAP_PROP_FOURCC, cv::VideoWriter::fourcc('M', 'J', 'P', 'G'));// 检查是否成功设置格式int fourcc = cap.get(cv::CAP_PROP_FOURCC);if (fourcc == cv::VideoWriter::fourcc('M', 'J', 'P', 'G')){std::cout << "视频捕获格式已成功设置为 MJPG" << std::endl;}else{std::cout << "视频捕获格式设置失败" << std::endl;}// 释放视频捕获设备cap.release();return 0;
}

在上述代码中，我们通过将 cv::CAP_V4L2 作为第二个参数传递给 cv::VideoCapture 构造函数来明确使用 V4L2 后端。这样可以确保使用 V4L2 后端进行视频捕获，而不是默认的 GStreamer 后端。

请注意，V4L2 后端的可用性取决于您的系统和安装的 OpenCV 版本。如果您的系统不支持 V4L2 后端，您可能无法通过设置 FourCC 来更改视频捕获格式。

下面是一个设置（Video Capture） ioctl: VIDIOC_ENUM_FMT 类型: 为MJPG
并且设置帧率和分辨率并显示视频的代码

/*****************************************************
2023.8.5： test_opencv.cpp
******************************************************/
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/calib3d/calib3d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <iostream>
#include <fstream>
using namespace cv;
using namespace std;int main()
{// 测试视频// VideoCapture cap;cv::VideoCapture cap(0, cv::CAP_V4L2);if (!cap.isOpened()){printf("camera 打开失败\n");}else{printf("camera 打开 ok\n");}// 设置视频捕获格式为 MJPGcap.set(cv::CAP_PROP_FOURCC, cv::VideoWriter::fourcc('M', 'J', 'P', 'G'));// 检查是否成功设置格式int fourcc = cap.get(cv::CAP_PROP_FOURCC);if (fourcc == cv::VideoWriter::fourcc('M', 'J', 'P', 'G')){std::cout << "视频捕获格式已成功设置为 MJPG" << std::endl;}else{std::cout << "视频捕获格式设置失败" << std::endl;}// 设置视频分辨率int width = 3840;int height = 2160;cap.set(cv::CAP_PROP_FRAME_WIDTH, width);cap.set(cv::CAP_PROP_FRAME_HEIGHT, height);// 设置视频帧率double frameRate = 30.0;cap.set(cv::CAP_PROP_FPS, frameRate);// 检查是否成功设置分辨率和帧率int setWidth = cap.get(cv::CAP_PROP_FRAME_WIDTH);int setHeight = cap.get(cv::CAP_PROP_FRAME_HEIGHT);double setFrameRate = cap.get(cv::CAP_PROP_FPS);if (setWidth == width && setHeight == height && setFrameRate == frameRate){std::cout << "视频分辨率和帧率已成功设置" << std::endl;printf("width=%d\n", frameWidth);printf("height=%d\n", frameHeight);printf("width=%d\n", frameWidth);}else{std::cout << "视频分辨率和帧率设置失败" << std::endl;}Mat frame;while (true){cap >> frame;if (frame.data == NULL){printf("Image is empty\n");break;}namedWindow("111", 0); // 参数为零，则可以自由拖动imshow("111", frame);waitKey(1);}return 0;
}

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