当纸张本身打印 corners/lines 时，如何找到纸张的角？

Question

我在 Python 中使用 openCV 找到一张 sheet 纸的角来展开它。

img = cv2.imread(images[i])

        corners = cv2.goodFeaturesToTrack(cv2.cvtColor(img,cv2.COLOR_BGR2GRAY),4,.01,1000,useHarrisDetector=True,k=.04)
        corners = np.float32(corners)
        print(corners)
        ratio = 1.6
        cardH = math.sqrt((corners[2][0][0] - corners[1][0][0]) * (corners[2][0][0] - corners[1][0][0]) + (corners[2][0][1] - corners[1][0][1]) * (
                    corners[2][0][1] - corners[1][0][1]))
        cardW = ratio * cardH;
        pts2 = np.float32(
            [[corners[0][0][0], corners[0][0][1]], [corners[0][0][0] + cardW, corners[0][0][1]], [corners[0][0][0] + cardW, corners[0][0][1] + cardH],
             [corners[0][0][0], corners[0][0][1] + cardH]])

        M = cv2.getPerspectiveTransform(corners, pts2)

        offsetSize = 500
        transformed = np.zeros((int(cardW + offsetSize), int(cardH + offsetSize)), dtype=np.uint8);
        dst = cv2.warpPerspective(img, M, transformed.shape)

之前： https://imgur.com/a/H7HjFro

之后： https://imgur.com/a/OA6Iscq

正如您在这些图像中看到的那样，它们检测的是纸张内部的边缘，而不是纸张的边角。我应该考虑完全使用不同的算法吗？我迷路了。

我试过将最小欧几里德距离增加到 1000，但这确实没有任何作用。

请注意，这不是真实信息，这是在 Kaggle 上发现的假数据集。

可以找到kaggle数据集https://www.kaggle.com/mcvishnu1/fake-w2-us-tax-form-dataset

Answer 1

这是 Python/OpenCV 中的一种方法。

请注意，找到的角是从最上角开始逆时针列出的。

读取输入
转换为灰色
高斯模糊
大津门槛
形态学open/close清理阈值
获取最大轮廓
从轮廓近似多边形
获得角落
在输入上绘制多边形
计算边长
计算输出对应角点
从对应的角点得到透视变换矩阵
根据矩阵扭曲输入图像
保存结果

输入：

import cv2
import numpy as np

# read image
img = cv2.imread("efile.jpg")

# convert img to grayscale
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# blur image
blur = cv2.GaussianBlur(gray, (3,3), 0)

# do otsu threshold on gray image
thresh = cv2.threshold(blur, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)[1]

# apply morphology
kernel = np.ones((7,7), np.uint8)
morph = cv2.morphologyEx(thresh, cv2.MORPH_CLOSE, kernel)
morph = cv2.morphologyEx(morph, cv2.MORPH_OPEN, kernel)

# get largest contour
contours = cv2.findContours(morph, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
area_thresh = 0
for c in contours:
    area = cv2.contourArea(c)
    if area > area_thresh:
        area_thresh = area
        big_contour = c

# draw white filled largest contour on black just as a check to see it got the correct region
page = np.zeros_like(img)
cv2.drawContours(page, [big_contour], 0, (255,255,255), -1)

# get perimeter and approximate a polygon
peri = cv2.arcLength(big_contour, True)
corners = cv2.approxPolyDP(big_contour, 0.04 * peri, True)

# draw polygon on input image from detected corners
polygon = img.copy()
cv2.polylines(polygon, [corners], True, (0,0,255), 1, cv2.LINE_AA)
# Alternate: cv2.drawContours(page,[corners],0,(0,0,255),1)

# print the number of found corners and the corner coordinates
# They seem to be listed counter-clockwise from the top most corner
print(len(corners))
print(corners)

# for simplicity get average of top/bottom side widths and average of left/right side heights
# note: probably better to get average of horizontal lengths and of vertical lengths
width = 0.5*( (corners[0][0][0] - corners[1][0][0]) + (corners[3][0][0] - corners[2][0][0]) )
height = 0.5*( (corners[2][0][1] - corners[1][0][1]) + (corners[3][0][1] - corners[0][0][1]) )
width = np.int0(width)
height = np.int0(height)

# reformat input corners to x,y list
icorners = []
for corner in corners:
    pt = [ corner[0][0],corner[0][1] ]
    icorners.append(pt)
icorners = np.float32(icorners)

# get corresponding output corners from width and height
ocorners = [ [width,0], [0,0], [0,height], [width,height] ]
ocorners = np.float32(ocorners)

# get perspective tranformation matrix
M = cv2.getPerspectiveTransform(icorners, ocorners)

# do perspective 
warped = cv2.warpPerspective(img, M, (width, height))

# write results
cv2.imwrite("efile_thresh.jpg", thresh)
cv2.imwrite("efile_morph.jpg", morph)
cv2.imwrite("efile_polygon.jpg", polygon)
cv2.imwrite("efile_warped.jpg", warped)

# display it
cv2.imshow("efile_thresh", thresh)
cv2.imshow("efile_morph", morph)
cv2.imshow("efile_page", page)
cv2.imshow("efile_polygon", polygon)
cv2.imshow("efile_warped", warped)
cv2.waitKey(0)

阈值图像：

形态学清理图像：

在输入上绘制的多边形：

提取角（从右上角逆时针方向）

4

[[[693  67]]
 [[ 23  85]]
 [[ 62 924]]
 [[698 918]]]

扭曲的结果：

当纸张本身打印 corners/lines 时，如何找到纸张的角？

How do I find corners of a paper when there are printed corners/lines on paper itself?

opencv

computer-vision

edge-detection

python-3.x

corner-detection