python + opencv图像处理——对象测量

1、弧长与面积
①轮廓发现；
②计算每个轮廓的弧长与面积，像素单位；
2、多边形拟合
①获取轮廓的多边形拟合结果（approxPolyDP）
espilon越小折线越逼近真实形状
close是否为闭合区域
3、几何矩计算
①原点矩；
②中心矩；
③图像重心坐标；
注：OTSU二值化可能会丢失数据

# 对象测量
from matplotlib import pyplot as plt 
from cv2 import cv2 as cv
import numpy as np 

# 采用canny边缘提取
def edge_demo(image):
    blurred = cv.GaussianBlur(image,(3,3),0)  #高斯降噪，适度
    gray = cv.cvtColor(blurred,cv.COLOR_BGR2GRAY)
    #求梯度
    xgrd = cv.Sobel(gray,cv.CV_16SC1,1,0)
    ygrd = cv.Sobel(gray,cv.CV_16SC1,0,1)

    egde_output = cv.Canny(xgrd,ygrd,50,150)  #50低阈值，150高阈值
    #egde_output = cv.Canny(gray,50,150)   #都可使用
    cv.imshow('canny_edge',egde_output)
    return egde_output

def measure_object(image):
    
    # dst = cv.cvtColor(image,cv.COLOR_BGR2GRAY)
    # ret,binary = cv.threshold(dst,0,255,cv.THRESH_BINARY_INV|cv.THRESH_OTSU)
    # print('threshold valus: %s'%ret)
    # cv.imshow('binary image',binary)
    
    binary = edge_demo(image)
    dst = cv.cvtColor(binary,cv.COLOR_GRAY2BGR)
    contours, heriachy = cv.findContours(binary, cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE)#注意 我的版本只有两个返回参数，具体看函数说明是返回几个参数
    for i,contour in enumerate(contours):
        area = cv.contourArea(contour)
        rect = cv.boundingRect(contour)#得到轮廓的外接矩形
        x,y,w,h = cv.boundingRect(contour)
        rate = min(w,h)/max(w,h)#宽高比
        print('%d rectangle rate:%s'%(i,rate))
        mm = cv.moments(contour)#得到几何矩
        #print(mm)  #为啥不一样？为啥黑色的背景就没有问题
        type(mm)
        
        cx = mm['m10']/mm['m00']
        cy = mm['m01']/mm['m00']    #得到图像重心（中心）位置
        cv.circle(dst,(np.int(cx),np.int(cy)),2,(0,255,255),-1)#标记中心点
        #cv.rectangle(dst,(x,y),(x+w,y+h),(0,0,255),2)#绘制外接矩形
        print('contour area:%s'%area)
        #多边形拟合
        approxCurve = cv.approxPolyDP(contour,4,True)
        print(approxCurve.shape)
        if approxCurve.shape[0] > 6:#画圆、椭圆
            cv.drawContours(dst,contours,i,(0,255,0),2)
        if approxCurve.shape[0] == 4:#矩形恒等于4
            cv.drawContours(dst,contours,i,(0,0,255),2)
        if approxCurve.shape[0] == 3:#三角形恒等于3
            cv.drawContours(dst,contours,i,(255,0,0),2)
        if approxCurve.shape[0] == 6:#六边形恒等于6
            cv.drawContours(dst,contours,i,(255,0,0),2)
    cv.imshow('measure_contour',dst)
    cv.imwrite("C:\\pictures\\others\\012345678.jpg",dst)

if __name__ == "__main__":
    filepath = "C:\\pictures\\others\\measure1.png"
    img = cv.imread(filepath)       # blue green red
    cv.namedWindow("input image",cv.WINDOW_AUTOSIZE)
    cv.imshow("input image",img)

    measure_object(img)

    cv.waitKey(0)
    cv.destroyAllWindows()

结果如下：