前言
以前的文章《C++ OpenCV之透视变换》介绍过透视变换,当时主要是自己固定的变换坐标点,所以在想可不可以做一个通过轮廓检测后自适应的透视变换,实现的思路通过检测主体的轮廓,使用外接矩形和多边形拟合的四个最边的点进行透视变换。
实现效果
# | 实现思路 |
---|---|
1 | 图像灰度图,高斯滤波、二值化 |
2 | 形态学开操作,Canny边缘检测 |
3 | 查找轮廓,遍历轮廓判断周长大于图像宽度的进行多边形拟合 |
4 | 判断拟合的点大于4个的获取到最小旋转矩形 |
5 | 通过多边形拟合的点计算出离最小旋转矩形最近的4个点 |
6 | 找到轮廓最小外接矩形作为透视变换的坐标 |
7 | 将5、6的步骤两个坐标点计算透视变换矩阵 |
8 | 透视变换 |
重点说明
微卡智享
01
排序旋转矩形的坐标点
图片来自网络
获取旋转矩形的函数minAreaRect( )中,四个顶点中y值最大的顶点为p[0],p[0]围着center顺时针旋转,依次经过的顶点为p[1],p[2],p[3]。角度参数angle 是P[0]发出的平行于x轴的射线,逆时针旋转,与碰到的第一个边的夹角,取值范围[-90~0]。注:逆时针旋转角度为负。
在透视变换的4个顶点的顺序为左上,右上,右下,左下,所以根据上面的原理,我们要写一个4点的重新排序,把4个顶点的顺序按透视变换的需要修改过来。
//重新排序旋转矩形坐标点void SortRotatedRectPoints(Point2f vetPoints[], RotatedRect rect){ rect.points(vetPoints);
cout << vetPoints[0] << vetPoints[1] << vetPoints[2] << vetPoints[3] << endl; cout << rect.angle << endl;
Point2f curpoint; //根据Rect的坐标点,Y轴最大的为P[0],p[0]围着center顺时针旋转, //旋转角度为负的话即是P[0]在左下角,为正P[0]是右下角 //重新排序坐标点 if (rect.angle > 0) { curpoint = vetPoints[0]; vetPoints[0] = vetPoints[2]; vetPoints[2] = curpoint; curpoint = vetPoints[1]; vetPoints[1] = vetPoints[3]; vetPoints[3] = curpoint; } else if (rect.angle < 0) { curpoint = vetPoints[0]; vetPoints[0] = vetPoints[1]; vetPoints[1] = vetPoints[2]; vetPoints[2] = vetPoints[3]; vetPoints[3] = curpoint; }
}
02
计算多边形拟合需要透视变换的点
通过多边形拟合出来的点比较多,而使用透视变换也是只要4个点,如果使用最小旋转矩形的4个点没有什么效果,如上图中红色是多边形拟合的点,蓝色框为最小旋转矩形的点,如果用这个点无法实现透视变换的效果,所以通过遍历了多边形拟合的点,计算每个点到最小旋转矩形的距离最近的4个点,形成了上图中的白色框,虽然不完美,但是可以透视变换的效果。
距离的计算用的是欧几里德距离,然后对比找到最近的4个点。
//根据最小矩形点找最近的四边形点//第一参数为输出的点,第二个参数为矩形的4个点,第三个为多边形拟合的点 void GetPointsFromRect(Point2f vetPoints[], Point2f rectPoints[], vector<Point> convex){ //定义最远的4个点,0--左上, 1--右上, 2--右下 3--左下 float ltdist = 99999999.9f; //左上的最大距离 float rtdist = 99999999.9f; //右上的最大距离 float rbdist = 99999999.9f; //右下的最大距离 float lbdist = 99999999.9f; //左下的最大距离 float curdist = 0.0f; //当前点的计算距离
for (auto curpoint : convex) { //计算左上点的距离 curdist = CalcPointDistance(rectPoints[0], curpoint); if (curdist < ltdist) { ltdist = curdist; vetPoints[0] = curpoint; } //计算右上角的点距离 curdist = CalcPointDistance(rectPoints[1], curpoint); if (curdist < rtdist) { rtdist = curdist; vetPoints[1] = curpoint; } //计算右下角点的距离 curdist = CalcPointDistance(rectPoints[2], curpoint); if (curdist < rbdist) { rbdist = curdist; vetPoints[2] = curpoint; } //计算左下角点的距离 curdist = CalcPointDistance(rectPoints[3], curpoint); if (curdist < lbdist) { lbdist = curdist; vetPoints[3] = curpoint; } }}
//计算两点间的距离float CalcPointDistance(Point2f point1, Point2f point2){ //计算两个点的Point差值 Point2f tmppoint = point1 - point2; //利用欧几里德距离计算H return sqrt(pow(tmppoint.x, 2) + pow(tmppoint.y, 2));}
TIPS
距离计算时一开始用的是旋转矩形的中心点离多边形拟合按左上,右上,右下,左下的方向找最远的4个,但是在某些斜的角度比较厉害的时候,这个计算问题不小,所以后来改为离最小旋转矩形的点最近的来找了。按中心点找最远距离的函数代码没删,一并贴上来。
#include<iostream>#include<opencv2/opencv.hpp>
using namespace std;using namespace cv;
//根据中心点找四角最远的点void GetRectPoints(Point2f vetPoints[], Point2f center, vector<Point> convex);//根据最小矩形点找最近的四边形点void GetPointsFromRect(Point2f vetPoints[], Point2f rectPoints[], vector<Point> convex);
//排序旋转矩形坐标点void SortRotatedRectPoints(Point2f vetPoints[], RotatedRect rect);//计算距离float CalcPointDistance(Point2f point1, Point2f point2);
int main(int argc, char** argv) {
Mat src = imread("E:/DCIM/tsnew.jpg"); Mat gray, dst, dst2, result; //图像缩放 resize(src, gray, Size(0, 0), 0.2, 0.2); imshow("src", gray);
//灰度图 cvtColor(gray, dst, COLOR_BGRA2GRAY);
//高斯滤波 GaussianBlur(dst, dst, Size(3, 3), 0.5, 0.5); imshow("gray", dst);
//二值化 threshold(dst, dst2, 0, 255, THRESH_BINARY | THRESH_OTSU); imshow("thresh", dst2);
//形态学开操作 Mat kernel = getStructuringElement(MORPH_RECT, Size(5, 5), Point(-1, -1)); morphologyEx(dst2, dst2, MORPH_OPEN, kernel); imshow("morph", dst2);
//Canny边缘检测 Canny(dst2, result, 127, 255, 7, true); imshow("canny", result);
//查找轮廓 vector<vector<Point>> contours; findContours(result, contours, RETR_LIST, CHAIN_APPROX_SIMPLE);
cout << contours.size() << endl; vector<vector<Point>> contours_poly(contours.size()); Mat tmpgray; gray.copyTo(tmpgray); for (int i = 0; i < contours.size(); ++i) { //计算轮廓周长,大于图像宽度的才算主体 double dlen = arcLength(contours[i], true); if (dlen > gray.cols) { //多边形拟合 approxPolyDP(Mat(contours[i]), contours_poly[i], 10, true);
cout << "当前:" << i << " 点数:" << contours_poly[i].size() << endl; if (contours_poly[i].size() >= 4) { //获取最小旋转矩形 RotatedRect rRect = minAreaRect(contours_poly[i]); Point2f vertices[4]; //重新排序矩形坐标点,按左上,右上,右下,左下顺序 SortRotatedRectPoints(vertices, rRect);
cout << vertices[0] << vertices[1] << vertices[2] << vertices[3] << endl;
//根据获得的4个点画线 for (int k = 0; k < 4; ++k) { line(gray, vertices[k], vertices[(k + 1) % 4], Scalar(255, 0, 0)); }
//多边形拟合的画出轮廓 drawContours(gray, contours_poly, i, Scalar(0, 0, 255));
//计算四边形的四点坐标 Point2f rPoints[4]; //GetRectPoints(rPoints, rRect.center, contours_poly[i]); GetPointsFromRect(rPoints, vertices, contours_poly[i]); for (int k = 0; k < 4; ++k) { line(gray, rPoints[k], rPoints[(k + 1) % 4], Scalar(255, 255, 255)); }
//根据最小矩形和多边形拟合的最大四个点计算透视变换矩阵 //重新排序多边形拟合的4点 Rect rect = rRect.boundingRect(); rectangle(gray, rect, Scalar(0, 0, 0));
Point2f rectPoint[4]; rectPoint[0] = Point2f(rect.x, rect.y); rectPoint[1] = Point2f(rect.x + rect.width, rect.y); rectPoint[2] = Point2f(rect.x + rect.width, rect.y + rect.height); rectPoint[3] = Point2f(rect.x, rect.y + rect.height); //vector<Point> vecpt(vertices, vertices + 4); //GetRectPoints(vertices, rRect.center, vecpt);
//计算透视变换矩阵 Mat warpmatrix = getPerspectiveTransform(rPoints, rectPoint); Mat resultimg; //透视变换 warpPerspective(tmpgray, resultimg, warpmatrix, resultimg.size(), INTER_LINEAR); imshow("resultimg", resultimg);
} } } imshow("src2", gray);
waitKey(0); return 0;}
//根据中心点找最远的四个点void GetRectPoints(Point2f vetPoints[], Point2f center, vector<Point> convex){ //定义最远的4个点,0--左上, 1--右上, 2--右下 3--左下 float ltdist = 0.0f; //左上的最大距离 float rtdist = 0.0f; //右上的最大距离 float rbdist = 0.0f; //右下的最大距离 float lbdist = 0.0f; //左下的最大距离
for (auto curpoint : convex) { //计算点的距离 float curdist = CalcPointDistance(center, curpoint);
if (curpoint.x < center.x && curpoint.y < center.y) { //判断是否在左上 if (curdist > ltdist) { ltdist = curdist; vetPoints[0] = curpoint; } } else if (curpoint.x > center.x && curpoint.y < center.y) { //判断在右上 if (curdist > rtdist) { rtdist = curdist; vetPoints[1] = curpoint; } } else if (curpoint.x > center.x && curpoint.y > center.y) { //判断在右下 if (curdist > rbdist) { rbdist = curdist; vetPoints[2] = curpoint; } } else if (curpoint.x < center.x && curpoint.y > center.y) { //判断在左下 if (curdist > lbdist) { lbdist = curdist; vetPoints[3] = curpoint; } }
} }
//根据最小矩形点找最近的四边形点//第一参数为输出的点,第二个参数为矩形的4个点,第三个为多边形拟合的点 void GetPointsFromRect(Point2f vetPoints[], Point2f rectPoints[], vector<Point> convex){ //定义最远的4个点,0--左上, 1--右上, 2--右下 3--左下 float ltdist = 99999999.9f; //左上的最大距离 float rtdist = 99999999.9f; //右上的最大距离 float rbdist = 99999999.9f; //右下的最大距离 float lbdist = 99999999.9f; //左下的最大距离 float curdist = 0.0f; //当前点的计算距离
for (auto curpoint : convex) { //计算左上点的距离 curdist = CalcPointDistance(rectPoints[0], curpoint); if (curdist < ltdist) { ltdist = curdist; vetPoints[0] = curpoint; } //计算右上角的点距离 curdist = CalcPointDistance(rectPoints[1], curpoint); if (curdist < rtdist) { rtdist = curdist; vetPoints[1] = curpoint; } //计算右下角点的距离 curdist = CalcPointDistance(rectPoints[2], curpoint); if (curdist < rbdist) { rbdist = curdist; vetPoints[2] = curpoint; } //计算左下角点的距离 curdist = CalcPointDistance(rectPoints[3], curpoint); if (curdist < lbdist) { lbdist = curdist; vetPoints[3] = curpoint; } }}
//重新排序旋转矩形坐标点void SortRotatedRectPoints(Point2f vetPoints[], RotatedRect rect){ rect.points(vetPoints);
cout << vetPoints[0] << vetPoints[1] << vetPoints[2] << vetPoints[3] << endl; cout << rect.angle << endl;
Point2f curpoint; //根据Rect的坐标点,Y轴最大的为P[0],p[0]围着center顺时针旋转, //旋转角度为负的话即是P[0]在左下角,为正P[0]是右下角 //重新排序坐标点 if (rect.angle > 0) { curpoint = vetPoints[0]; vetPoints[0] = vetPoints[2]; vetPoints[2] = curpoint; curpoint = vetPoints[1]; vetPoints[1] = vetPoints[3]; vetPoints[3] = curpoint; } else if (rect.angle < 0) { curpoint = vetPoints[0]; vetPoints[0] = vetPoints[1]; vetPoints[1] = vetPoints[2]; vetPoints[2] = vetPoints[3]; vetPoints[3] = curpoint; }
}
//计算两点间的距离float CalcPointDistance(Point2f point1, Point2f point2){ //计算两个点的Point差值 Point2f tmppoint = point1 - point2; //利用欧几里德距离计算H return sqrt(pow(tmppoint.x, 2) + pow(tmppoint.y, 2));}
完