关于OpenCV打开摄像头默认分辨率是640*480的问题

在使用OpenCV打开2k USB摄像头时，发现VideoCapture实例化的对象捕捉的视频帧的分辨率总是$640\times 480$，而我的摄像头硬件分辨率最高可以支持$2560\times 1440$，经研究发现需要重新设置分辨率。

使用OpenCV测试摄像头支持的最高分辨率

#include 
#include 
#include 
#include 

using namespace std;

int main()
{
	cv::Mat frame;
	cv::VideoCapture cap(0);

	//默认分辨率
	double frame_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
	double frame_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT);
	double frame_rate = cap.get(cv::CAP_PROP_FPS);

	cout << "default : " << "( " << frame_width << ", " << frame_height << " )" << ", " << frame_rate << endl;

	//设置分辨率，编码格式
	cap.set(cv::CAP_PROP_FRAME_WIDTH, 5000);
	cap.set(cv::CAP_PROP_FRAME_HEIGHT, 5000);
	cap.set(cv::CAP_PROP_FOURCC, cv::VideoWriter::fourcc('M', 'J', 'P', 'G'));

	double frame_set_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
	double frame_set_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT); 
	double frame_set_rate = cap.get(cv::CAP_PROP_FPS);

	cout << "after Change : " << "( " << frame_set_width << ", " << frame_set_height << " )" << ", " << frame_set_rate << endl;

	return 0;
}
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default : ( 640, 480 ), 30
after Change : ( 2560, 1440 ), 30
[ WARN:0] terminating async callback
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可以看到，欲设置分辨率$5000\times 5000$，但是设置结束后，显示的分辨率为：$2560\times 1440$。因此，$2560\times 1440$就是该摄像头所支持的最高分辨率。

更改分辨率

#include 
#include 
#include 
#include 

using namespace std;

int main()
{
	cv::Mat frame;
	cv::VideoCapture cap(0);

	//默认分辨率
	double frame_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
	double frame_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT);
	double frame_rate = cap.get(cv::CAP_PROP_FPS);

	cout << "default : " << "( " << frame_width << ", " << frame_height << " )" << ", " << frame_rate << endl;

	//设置分辨率，编码格式
	cap.set(cv::CAP_PROP_FRAME_WIDTH, 1920);
	cap.set(cv::CAP_PROP_FRAME_HEIGHT, 1080);
	cap.set(cv::CAP_PROP_FOURCC, cv::VideoWriter::fourcc('M', 'J', 'P', 'G'));

	double frame_set_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
	double frame_set_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT); 
	double frame_set_rate = cap.get(cv::CAP_PROP_FPS);

	cout << "after Change : " << "( " << frame_set_width << ", " << " )" << frame_set_height << ", " << frame_set_rate << endl;

	if (!cap.isOpened())
	{
		return -1;
	}
	cout << "摄像头是否打开：" << cap.isOpened() << endl;

	while (1) {
		cap.read(frame);

		cv::imwrite("./1920-1080-4.jpg", frame);
		cv::namedWindow("frame", 0);
		cv::resizeWindow("frame", 1280, 720);		//设置长宽大小
		cv::moveWindow("frame", 0, 0);              //移动窗口到（0,0）坐标
		cv::imshow("frame", frame);
		cv::waitKey(1);								//间隔时间长一点，保存完整图片
	}
	cap.release();
	cv::destroyAllWindows();

	return 0;
}
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如此，捕获的视频帧分辨率可高达$1920\times 1080$。

namedWindow

void cv::namedWindow(const String& winname,
	int 	flags = WINDOW_AUTOSIZE
)
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resizeWindow

void cv::resizeWindow(const String& winname,
	int 	width,
	int 	height
)
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moveWindow

void cv::moveWindow(const String& winname,
	int 	x,
	int 	y
)
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参考文献：

1. 关于opencv读取摄像头的未解之谜
2. Python+Opencv读取高帧率USB摄像头问题