OpenCV - Summary "Image Processing-2"

dst = cv2.Sobel(src, ddepth, dx, dy, ksize)

ddepth: The depth of the image. The depth of the output image (can be understood as a data type), -1 means the same depth as the original image.
dx and dy represent the horizontal and vertical directions, respectively. When the combination is dx=1, dy=0, find the first derivative in the x direction, and when the combination is dx=0, dy=1, find the first derivative in the y direction (if it is 1 at the same time, you usually can’t get what you want result)
ksize is the size of the Sobel operator. Optional parameter) The size of the Sobel operator, must be 1, 3, 5 or 7, the default is 3. When calculating the first-order derivatives in the X direction and Y direction, the convolution kernels are:
scale: (optional parameter) the scale factor to enlarge the value obtained by gradient calculation, the effect usually makes the gradient map brighter, the default is 1
delta: (optional parameter) before storing the target image into a multidimensional array, you can increase the value of each pixel by delta, the default is 0
borderType: (optional parameter) determines how the edge pixels are processed when the image is filtered (convoluted), the default is BORDER_DEFAULT

The scharr operator is an improvement of the sobel operator.

It can be understood that the operator is used to find the edge

   使用高斯滤波器，以平滑图像，滤除噪声。

   计算图像中每个像素点的梯度强度和方向。

   应用非极大值（Non-Maximum Suppression）抑制，以消除边缘检测带来的杂散响应。

   应用双阈值（Double-Threshold）检测来确定真实的和潜在的边缘。

   通过抑制孤立的弱边缘最终完成边缘检测。

img=cv2.imread("lena.jpg",cv2.IMREAD_GRAYSCALE)

v1=cv2.Canny(img,80,150)
v2=cv2.Canny(img,50,100)

res = np.hstack((v1,v2))
cv_show(res,'res')

Upsampling is to enlarge the graph, and downsampling is to shrink the graph.

up=cv2.pyrUp(img)
up_down=cv2.pyrDown(up)
cv_show(img-up_down,'img-up_down')

down=cv2.pyrDown(img)
down_up=cv2.pyrUp(down)
l_1=img-down_up
cv_show(l_1,'l_1')

cv2.findContours(img,mode,method)
mode: contour retrieval mode

RETR_EXTERNAL : retrieve only the outermost contours;
RETR_LIST: Retrieve all contours and save them in a linked list;
RETR_CCOMP: Retrieve all contours and organize them into two layers: the top layer is the outer boundary of each part, the second layer is the boundary of the cavity;
RETR_TREE: retrieve all contours, and reconstruct the entire hierarchy of nested contours;

method: contour approximation method

CHAIN_APPROX_NONE: Outlines are output as Freeman chaincodes, all other methods output polygons (sequences of vertices).
CHAIN_APPROX_SIMPLE: Compresses horizontal, vertical and oblique parts, that is, the function keeps only their end parts.