Python Opencv实践 - 车牌定位（纯练手，存在失败场景，可以继续优化）

使用传统的计算机视觉方法定位图像中的车牌，参考了部分网上的文章，实际定位效果对于我目前使用的网上的图片来说还可以。实测发现对于车身本身是蓝色、或是车牌本身上方有明显边缘的情况这类图片定位效果较差。纯练手项目，仅供参考。代码中imagePreProcess对某些图片定位率相比于imagePreProcess2做预处理的效果要好。后续可以尝试做一个如果imagePreProcess2识别无效后使用imagePreProcess再处理，或者加上阈值自适应打分的机制优化。目前对于我做的练手项目来说足够了。

注意：以下代码是参考了网上的一些文章后，按照自己的思路写的，定位效果尚可。参考的文章有：python-opencv实战：车牌识别（一）：精度还不错的车牌定位_基于阈值分割的车牌定位识别-CSDN博客

https://www.cnblogs.com/fyunaru/p/12083856.html

import cv2 as cv
import numpy as np
import matplotlib.pyplot as plt

#过滤矩形的参数
minRectW = 100
minRectH = 50
#判断车牌颜色的参数
#一般情况下，蓝色车牌H分量的值通常都在115附近徘徊
# S分量和V分量因光照不同而差异较大(opencv中H分量的取值范围是0到179，而不是图像学中的0到360；S分量和V分量的取值范围是到255)
deltaH = 15
hsvLower = np.array([115 - deltaH,60,60])
hsvUpper = np.array([115 + deltaH,255,255])

#灰度拉伸
def grayScaleStretch(img):
    maxGray = float(img.max())
    minGray = float(img.min())
    for i in range(img.shape[0]):
        for j in range(img.shape[1]):
            img[i,j] = 255 / (maxGray - minGray) * (img[i,j] - minGray)
    return img

#图像二值化
def image2Binary(img):
    #选取灰度最大最小值的中间值
    maxGray = float(img.max())
    minGray = float(img.min())
    threshold = (minGray + maxGray) / 2
    ret,bin = cv.threshold(img, threshold, 255, cv.THRESH_BINARY)
    return bin

#图像预处理
def imagePreProcess(img):
    #转换为灰度图
    imgGray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)
    #灰度拉伸
    imgGray = grayScaleStretch(imgGray)
    #plt.imshow(imgGray, cmap='gray')
    kernel = cv.getStructuringElement(cv.MORPH_ELLIPSE, (3,3))
    #做开运算
    imgOpen = cv.morphologyEx(imgGray, cv.MORPH_OPEN, kernel)
    #plt.imshow(imgOpen, cmap='gray')
    #获得差分图
    imgDiff = cv.absdiff(imgGray, imgOpen)
    #plt.imshow(imgDiff, cmap='gray')
    imgDiff = cv.GaussianBlur(imgDiff, (3,3), 5)
    #plt.imshow(imgDiff, cmap='gray')
    #图像二值化
    imgBinary = image2Binary(imgDiff)
    #plt.imshow(imgBinary, cmap='gray')
    cannyEdges = cv.Canny(imgBinary, 127, 200)
    #plt.imshow(cannyEdges, cmap='gray')
    #对Canny检测边缘结果做处理
    kernel = np.ones((3,3), np.uint8)
    imgOut = cv.morphologyEx(cannyEdges, cv.MORPH_CLOSE, kernel)
    imgOut = cv.dilate(imgOut, kernel, iterations=1)
    imgOut = cv.morphologyEx(imgOut, cv.MORPH_OPEN, kernel)
    #imgOut = cv.erode(imgOut, kernel, iterations=1)
    imgOut = cv.morphologyEx(imgOut, cv.MORPH_CLOSE, kernel)
    #plt.imshow(imgOut, cmap='gray')
    return imgOut

#图像预处理2 - 对于某些
def imagePreProcess2(img):
    imgGray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)
    #灰度拉伸
    imgGray = grayScaleStretch(imgGray)
    imgGray = cv.GaussianBlur(imgGray, (3,3), 5)
    #进行边缘检测
    cannyEdges = cv.Canny(imgGray, 180, 230)
    #二值化
    imgBinary = image2Binary(cannyEdges)
    #plt.imshow(imgBinary, cmap='gray')
    #先做闭运算再做开运算
    kernel = np.ones((3,3), np.uint8)
    imgOut = cv.morphologyEx(imgBinary, cv.MORPH_CLOSE, kernel)
    imgOut = cv.morphologyEx(imgOut, cv.MORPH_OPEN, kernel)
    imgOut = cv.absdiff(imgBinary, imgOut)
    imgOut = cv.morphologyEx(imgOut, cv.MORPH_CLOSE, kernel)
    imgOut = cv.dilate(imgOut, kernel, iterations=1)
    plt.imshow(imgOut, cmap='gray')
    return imgOut

#debug
def printHSV(hsvSrc):
    for i in range(hsvSrc.shape[0]):
        for j in range(hsvSrc.shape[1]):
            (h,s,v) = hsvSrc[i][j]
            print(h,s,v)

#定位车牌
def locate_plate(imgProcessing, imgOriginal):
    contours,hierarchy = cv.findContours(imgProcessing, cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE)
    carPlateCandidates = []
    for contour in contours:
        (x,y,w,h) = cv.boundingRect(contour)
        #过滤掉一些小的矩形
        if (w < minRectW or h < minRectH):
            continue
        #cv.rectangle(imgOriginal, (int(x), int(y)), (int(x + w),int(y + h)), (0,255,0), 2)
        carPlateCandidates.append([int(x),int(y),int(x + w),int(y + h)])
    
    #plt.imshow(imgOriginal[:,:,::-1])
    maxMean = 0
    target = []
    target_mask = []
    #依次检查候选车牌列表，用HSV颜色空间判别是否是车牌
    for candidate in carPlateCandidates:
        (x0,y0,x1,y1) = candidate
        candidateROI = imgOriginal[y0:y1,x0:x1]
        hsvROI = cv.cvtColor(candidateROI, cv.COLOR_BGR2HSV)
        mask = cv.inRange(hsvROI, hsvLower, hsvUpper)
        #print(mask)
        #plt.imshow(mask, cmap='gray')
        #使用均值找出蓝色最多的区域
        mean = cv.mean(mask)
        #print(mean)
        if mean[0] > maxMean:
            maxMean = mean[0]
            target = candidate
            target_mask = mask
    #对target的范围进行缩小，找出蓝色刚开始和结束的坐标
    print(target_mask)
    nonZeroPoints = cv.findNonZero(target_mask)
    #print(nonZeroPoints)
    sortByX = np.sort(nonZeroPoints, axis=0)
    xMin = sortByX[0][0][0]
    xMax = sortByX[-1][0][0]
    print(sortByX)
    sortByY = np.sort(nonZeroPoints, axis=1)
    yMin = sortByY[0][0][1]
    yMax = sortByY[-1][0][1]
    print(sortByY)


    print("X min:" + str(xMin) + " X max:" + str(xMax) + " Y min:" + str(yMin) + " Y max:" + str(yMax))
    (x0,y0,x1,y1) = target
    print("Original:" + str(x0) + "," + str(y0) + "," + str(x1) + "," + str(y1))
    #target = (x0 + xMin, y0 + yMin, x0 + (xMax - xMin), y0 + yMax - yMin)
    target = [x0 + xMin, y0 + yMin, x0 + xMax, y0 + yMax]
    return target

#读取图像
imgCarPlate = cv.imread("../../SampleImages/carplate/carplate_chongqing.jpg", cv.IMREAD_COLOR)
#plt.imshow(imgCarPlate[:,:,::-1])
img4locate = imagePreProcess2(imgCarPlate)
target = locate_plate(img4locate, imgCarPlate)
(x0,y0,x1,y1) = target
cv.rectangle(imgCarPlate, (x0,y0), (x1,y1), (0,255,0), 2)
plt.imshow(imgCarPlate[:,:,::-1])

成功的例子：

不太成功的例子（轮廓检测的不太好，并且轮廓中蓝色的值过早出现，可以优化判断为连续的蓝色而不是零散的蓝色）

失败的例子（没能检测出小轮廓，车身本身为蓝色，替换为imagePreProcess后能够成功）：