[opencv] morphological filtering (2): open operation, morphological gradient, top hat, black hat

1、 Open operation

Open operation （Opening Operation), In fact, it is a process of corrosion before expansion , Its mathematical expression is as follows ：
$$dst=open(src,element)=dilate(erode(src,element))$$
effect ： Open operation can eliminate small objects , Separating objects at the finer points , And it can smooth the boundary of larger objects without significantly changing its area .

Sample code ：

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<iostream>
using namespace cv;
using namespace std;
int main()
{
    
	// Load the original drawing 
	Mat image = imread("E:\\Pec\\ Captain America .jpg");
	// create a window 
	namedWindow("【 The original picture 】");
	namedWindow("【 design sketch 】");
	// Show the original image 
	imshow("【 The original picture 】", image);
	// Define the kernel 
	Mat element = getStructuringElement(MORPH_RECT, Size(15, 15));
	// Carry out morphological operation 
	morphologyEx(image, image, MORPH_OPEN, element);
	// Show the original image 
	imshow("【 design sketch 】", image);
	waitKey(0);
}

2、 Closed operation

The process of expansion before corrosion is called closed operation （Closing Operation), Its mathematical expression is as follows ：
$$dst=close(src,element)=erode(dilate(src,element))$$
effect ： Closed operation can eliminate small black holes （ Black areas ）

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<iostream>
using namespace cv;
using namespace std;
int main()
{
    
	// Load the original drawing 
	Mat image = imread("E:\\Pec\\ Captain America .jpg");
	// create a window 
	namedWindow("【 The original picture 】");
	namedWindow("【 design sketch 】");
	// Show the original image 
	imshow("【 The original picture 】", image);
	// Define the kernel 
	Mat element = getStructuringElement(MORPH_RECT, Size(15, 15));
	// Carry out morphological operation 
	morphologyEx(image, image, MORPH_CLOSE, element);
	// Show the original image 
	imshow("【 design sketch 】", image);
	waitKey(0);
}

3、 Morphological gradients

Morphological gradients （Morphological Gradient) It is the difference between expansive soil and corrosion diagram , The mathematical expression is as follows ：
$$dst=morph-grad(src,element)=dilate(src,element)-erode(src,element)$$
effect ： The binary image can be lumped （blob) The edge of the protrudes , Using morphological gradient to preserve the edge contour of the object .

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<iostream>
using namespace cv;
using namespace std;
int main()
{
    
	// Load the original drawing 
	Mat image = imread("E:\\Pec\\ Captain America .jpg");
	// create a window 
	namedWindow("【 The original picture 】");
	namedWindow("【 design sketch 】");
	// Show the original image 
	imshow("【 The original picture 】", image);
	// Define the kernel 
	Mat element = getStructuringElement(MORPH_RECT, Size(15, 15));
	// Carry out morphological operation 
	morphologyEx(image, image, MORPH_GRADIENT, element);
	// Show the original image 
	imshow("【 design sketch 】", image);
	waitKey(0);
}

4、 Top hat

Top hat operation （Top Hat) It is often translated into “ formal hat ” operation , It's the original image and the one just introduced above “ Open operation ” The difference between the results of .
$$dst=tophat(src,element)=src-open(src,element)$$
Because the result of the open operation is to enlarge the crack or local low brightness area . therefore , Subtract the calculated graph from the original graph , The resulting rendering highlights a brighter area than the area around the outline of the original image , And this operation is related to the size of the selected core .

effect ： Top hat operations are often used to separate patches that are lit up near neighbors . In an image with a large background , When small objects are more regular , You can use the top hat operation to advance the background .

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<iostream>
using namespace cv;
using namespace std;
int main()
{
    
	// Load the original drawing 
	Mat image = imread("E:\\Pec\\ Captain America .jpg");
	// create a window 
	namedWindow("【 The original picture 】");
	namedWindow("【 design sketch 】");
	// Show the original image 
	imshow("【 The original picture 】", image);
	// Define the kernel 
	Mat element = getStructuringElement(MORPH_RECT, Size(15, 15));
	// Carry out morphological operation 
	morphologyEx(image, image, MORPH_TOPHAT, element);
	// Show the original image 
	imshow("【 design sketch 】", image);
	waitKey(0);
}

5、 Black hat

Black hat （Black Hat) Operation is the difference between the result of closed operation and the original image , The mathematical expression is ：
$$dst=blackhat(src,element)=close(src,element)-src$$
The effect image after black hat operation highlights the darker area than the area around the outline of the original image , And this operation is related to the size of the selected core .

effect ： Black hat operation is used to separate patches darker than adjacent points , The rendering has a very perfect outline .

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<iostream>
using namespace cv;
using namespace std;
int main()
{
    
	// Load the original drawing 
	Mat image = imread("E:\\Pec\\ Captain America .jpg");
	// create a window 
	namedWindow("【 The original picture 】");
	namedWindow("【 design sketch 】");
	// Show the original image 
	imshow("【 The original picture 】", image);
	// Define the kernel 
	Mat element = getStructuringElement(MORPH_RECT, Size(15, 15));
	// Carry out morphological operation 
	morphologyEx(image, image, MORPH_BLACKHAT, element);
	// Show the original image 
	imshow("【 design sketch 】", image);
	waitKey(0);
}

6、 The core API function ：morphologyEx()

void morphologyEx(
	InputArray src,
    OutputArray dst,
    int op,
    InputArray kernel,
    Pointanchor=Point(-1,-1),
    int iteration=1,
    int borderType=BORDER_CONSTANT,
    const Scalar& borderValue=morphologyDefaultBorderValue()
);

• The first parameter ： The input image , Source image , fill Mat Class . The image bit depth should be 5 One of the species ：CV_8U、CV_16U、CV_16S、CV_32F and CV_64F

• The second parameter ： The target image , The output parameters of the function , Need the same size and type as the source image

• The third parameter ：int Type of op, Indicates the type of morphological operation , It can be any identifier in the following table

  [ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-G93BBVtQ-1656987857859)(C:\Users\lenovo\AppData\Roaming\Typora\typora-user-images\image-20220705095322440.png)]

• Fourth parameter ：InputArray Type of kernel, The kernel of morphological operation . if NULL, It means that the reference point is located at the center 3x3 The core of . Generally use functions getStructuringElement With the use of this parameter .getStructuringElement The function returns the structural elements of the specified shape and size （ Kernel matrix ）

  getStructuringElement The first parameter of the function represents the shape of the kernel ：

  • （1） rectangular ——MORPH_RECT
  • （2） Crisscross ——MORPH_CROSS
  • （3） ellipse ——MORPH_ELLIPSE

  and getStructuringElement The second and third parameters of the function are the size of the kernel and the location of the anchor point respectively

  It's called erode and dilate Function before , First define a Mat Type to get getStructuringElement The return value of the function , For the location of the anchor , Have default values (-1,-1), Indicates that the anchor point is in the center . Also note ： Cross shaped element The shape only depends on the position of the anchor . And in other cases , Anchor points only affect the offset of morphological operation results

  //getStructuringElement Function related calls are as follows ：
  int g_nStructuringElement(MORPH_RECT,Size(2*g_nStructuringElement+1,                              2*g_nStructuringElement+1),Point(g_nStructuringElement,g_nStructuringElement));
  

  Then you can call erode、dilate or morphologyEx Function time , from kernel Fill in and save the parameters getStructuringElement The return value of Mat Type variable .

• Fifth parameter ： The position of the anchor , The default value is (-1,-1), The anchor point is in the center

• Sixth parameter ： Number of iterations using the function , The default value is 1.

• Seventh parameter ： Some kind of boundary pattern used to infer pixels outside the image . Have default values BORDER_CONSTANT

• Eighth parameter ： The boundary value when the boundary is constant , Have default values morphologyDEfaultBorderValue(), Generally, it does not need to be considered . These operations can be carried out locally （in-place) operation , And for multi-channel images , Each channel operates separately .

7、 Comprehensive examples ： Morphological filtering

explain ： The following program running examples will appear 4 A window displaying images , Contains the original graph 、 open \ A closed operation , inflation \ Corrode one , Top hat \ Black hat operation one . They use scroll bars... Respectively , To control the morphological effect , And the iteration value is 10 It's the middle point when . Besides , You can also use the keyboard 1、2、3 And the space bar to adjust the structure of different elements

#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<iostream>
using namespace cv;
using namespace std;
//-----------------
// Global variable declaration 
//---------------------
Mat g_srcImage, g_dstImage;
int g_nElementShape = MORPH_RECT;// The shape of the element result 

//-----------------------------------------
// Variables accept TrackBar Positional arguments 
//------------------------------------------
int g_nMaxIterationNum = 10;
int g_nOpenCloseNum = 0;
int g_nErodeDilateNum = 0;
int g_nTopBlackHatNum = 0;

//--------------------------------------------
// Global function declaration 
//-------------------------------------------
static void on_OpenClose(int, void*);
static void on_ErodeDilate(int, void*);
static void on_TopBlackHat(int ,void*);
static void ShowHelpText();// Help me with text display 

int main()
{
    
	system("color 5E");
	// Load the original drawing 
	g_srcImage = imread("E:\\Pec\\ Iron man .jpg");
	if (!g_srcImage.data) {
    
		printf(" Picture reading error \n");
		return false;
	}
	// Show the original image 
	namedWindow("【 The original picture 】");
	imshow("【 The original picture 】", g_srcImage);
	// Create three windows 
	namedWindow("【 Open operation / Closed operation 】", 1);
	namedWindow("【 corrosion / inflation 】", 1);
	namedWindow("【 Top hat / Black hat 】", 1);
	// parameter assignment 
	g_nOpenCloseNum = 9;
	g_nErodeDilateNum = 9;
	g_nTopBlackHatNum = 9;
	// Create track bars for three windows respectively 
	createTrackbar(" Iteration value ", "【 Open operation / Closed operation 】", &g_nOpenCloseNum, g_nMaxIterationNum * 2 + 1, on_OpenClose);
	createTrackbar(" Iteration value ", "【 corrosion / inflation 】", &g_nErodeDilateNum, g_nErodeDilateNum * 2 + 1, on_ErodeDilate);
	createTrackbar(" Iteration value ", "【 Top hat / Black hat 】", &g_nTopBlackHatNum, g_nTopBlackHatNum * 2 + 1, on_TopBlackHat);
	printf("----------------------------------------------------------------------\n");
	printf("|  Operation instructions  |\n");
	printf("|  Press the keyboard Q perhaps ESC, Program exit  |\n");
	printf("|  Press the keyboard 1, Use ellipses (Elliptic) Structure original structure elements MORPH_ELLIPSE| |\n");
	printf("|  Press the keyboard 2, Use rectangles (Rectangle) Structure original structure elements MORPH_RECT |\n");
	printf("|  Press the keyboard 3, Use a cross (Cross-shaped) Structure original structure elements MORPH_CROSS |\n");
	printf("|  Press the keyboard space, In the rectangle 、 Cycle between ellipse and cross  |\n");
	printf("-------------------------------------------------------------------\n");
	// Polling for key information 
	while (1)
	{
    
		int c;
		// Execute callback function 
		on_OpenClose(g_nOpenCloseNum, 0);
		on_ErodeDilate(g_nErodeDilateNum, 0);
		on_TopBlackHat(g_nTopBlackHatNum, 0);
		c = waitKey(0);
		// Press the keyboard Q perhaps ESC, Program exit 
		if ((char)c == 'q' || (char)c == 27)
			break;
		// Press the keyboard 1, Use ellipses (Elliptic) Structure original structure elements MORPH_ELLIPSE
		else if ((char)c == 49)//1 Of ASII Code for 49
			g_nElementShape = MORPH_ELLIPSE;
		// Press the keyboard 2, Use rectangles (Rectangle) Structure original structure elements MORPH_RECT
		else if ((char)c == 50)
			g_nElementShape = MORPH_RECT;
		// Press the keyboard 3, Use a cross (Cross-shaped) Structure original structure elements MORPH_CROSS
		else if ((char)c == 51)
			g_nElementShape = MORPH_CROSS;
		// Press the keyboard space, In the rectangle 、 Cycle between ellipse and cross 
		else if ((char)c == ' ')
			g_nElementShape = (g_nElementShape + 1) % 3;
	}
	return 0;
}
static void on_OpenClose(int, void*)
{
    
	// Definition of offset 
	int offset = g_nOpenCloseNum - g_nMaxIterationNum;// Offset 
	int Absolute_offset = offset > 0 ? offset : -offset;// Take a positive value 
	// Custom core 
	Mat element = getStructuringElement(g_nElementShape, Size(Absolute_offset * 2 + 1, Absolute_offset * 2 + 1), Point(Absolute_offset, Absolute_offset));
	// To operate 
	if (offset < 0)
		morphologyEx(g_srcImage, g_dstImage, CV_MOP_OPEN, element);
	else
		morphologyEx(g_srcImage, g_dstImage,MORPH_CLOSE, element);
	imshow("【 Open operation / Closed operation 】", g_dstImage);
}
static void on_ErodeDilate(int, void*)
{
    
	// Definition of offset 
	int offset = g_nErodeDilateNum - g_nMaxIterationNum;// Offset 
	int Absolute_offset = offset > 0 ? offset : -offset;// Take a positive value 
	// Custom core 
	Mat element = getStructuringElement(g_nElementShape, Size(Absolute_offset * 2 + 1, Absolute_offset * 2 + 1), Point(Absolute_offset, Absolute_offset));
	// To operate 
	if (offset < 0)
		erode(g_srcImage, g_dstImage,element);
	else
		dilate(g_srcImage, g_dstImage, element);
	imshow("【 corrosion / inflation 】", g_dstImage);
}
static void on_TopBlackHat(int, void*)
{
    
	// Definition of offset 
	int offset = g_nTopBlackHatNum - g_nMaxIterationNum;// Offset 
	int Absolute_offset = offset > 0 ? offset : -offset;// Take a positive value 
	// Custom core 
	Mat element = getStructuringElement(g_nElementShape, Size(Absolute_offset * 2 + 1, Absolute_offset * 2 + 1), Point(Absolute_offset, Absolute_offset));
	// To operate 
	if (offset < 0)
		morphologyEx(g_srcImage, g_dstImage, MORPH_TOPHAT, element);
	else
		morphologyEx(g_srcImage, g_dstImage, MORPH_BLACKHAT, element);
	imshow("【 Top hat / Black hat 】", g_dstImage);
}

Original image ：

Open operation 、 corrosion 、 Top hat image

Closed operation 、 inflation 、 Black hat image