OpenCV单通道Mat矩阵元素访问方法，及其速度比较

1、OpenCV程序（环境：CPU i5-9500+VS2017+OpenCV4.5.0）
方法1：利用“ Mat.at<存储类型>(行,列) ”访问Mat矩阵元素并赋值

int main()
{
	int n = 10000;
	Mat img = Mat::zeros(Size(n, n), CV_64FC1);
	DWORD startTime = GetTickCount();
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			img.at<double>(i, j) = 6.66;
		}
	}
	DWORD endTime = GetTickCount();
	cout << "The run time is : " << endTime - startTime << " ms" << endl;
	system("pause");
	return 0;
}

方法2-1：利用“ Mat.data ”访问Mat矩阵元素并赋值

int main()
{
	int n = 10000;
	Mat img = Mat::zeros(Size(n, n), CV_64FC1);
	DWORD startTime = GetTickCount();
	double* pData = (double*)img.data;
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			*(pData) = 6.66;
			pData++;
		}
	}
	DWORD endTime = GetTickCount();
	cout << "The run time is : " << endTime - startTime << " ms" << endl;
	system("pause");
	return 0;
}

方法2-2：利用“ Mat.data ”访问Mat矩阵元素并赋值

int main()
{
	int n = 10000;
	Mat img = Mat::zeros(Size(n, n), CV_64FC1);
	DWORD startTime = GetTickCount();
	double* pData = (double*)img.data;
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			pData[n * i + j] = 6.66;
		}
	}
	DWORD endTime = GetTickCount();
	cout << "The run time is : " << endTime - startTime << " ms" << endl;
	system("pause");
	return 0;
}

方法3-1：利用“ Mat.ptr<存储类型>(行)[列] ”访问Mat矩阵元素并赋值

int main()
{
	int n = 10000;
	Mat img = Mat::zeros(Size(n, n), CV_64FC1);
	DWORD startTime = GetTickCount();
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			img.ptr<double>(i)[j] = 6.66;	
		}
	}
	DWORD endTime = GetTickCount();
	cout << "The run time is : " << endTime - startTime << " ms" << endl;
	system("pause");
	return 0;
}

方法3-2：利用“ Mat.ptr<存储类型>(行)[列] ”访问Mat矩阵元素并赋值（速度最快）

int main()
{
	int n = 10000;
	Mat img = Mat::zeros(Size(n, n), CV_64FC1);
	DWORD startTime = GetTickCount();
	double *p = NULL;
	for (int i = 0; i < n; i++)
	{
		p = img.ptr<double>(i);	// p指针指向第i行第0个元素
		for (int j = 0; j < n; j++)
		{
			p[j] = 6.66;		
		}
	}
	DWORD endTime = GetTickCount();
	cout << "The run time is : " << endTime - startTime << " ms" << endl;
	system("pause");
	return 0;
}

2、访问速度比较

OpenCV单通道Mat矩阵元素访问方法	10,000×10,000矩阵元素访问并赋值时间
1：Mat.at<存储类型>(行,列)	4,500 ms
2-1：Mat.data	200 ms
2-2：Mat.data	172 ms
3-1：Mat.ptr<存储类型>(行)[列]	1,700 ms
3-2：Mat.ptr<存储类型>(行)[列]	140 ms