步骤： 
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API:
功能：近似于曲线的或与另一曲线/多边形的多边形用更少的顶点，使得它们之间的距离是小于或等于指定的精度
approxCurve=cv.approxPolyDP(curve, epsilon, closed[, approxCurve])
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矩形：
BoundingRect是说，用一个最小的矩形，把找到的形状包起来。还有一个带旋转的矩形，面积会更小
minAreaRect函数返回矩形的中心点坐标，长宽，旋转角度[-90,0)，当矩形水平或竖直时均返回-90
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圆：
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#include
#include
#include using namespace std;
using namespace cv;
Mat src, gray_src, dst;
const char* output_title = "绘制结果";
const char* gray_title = "灰度图像";
int threshold_value = https://www.it610.com/article/50;
int threshold_max = 255;
void threshold_Demo(int, void*);
//在轮廓周围绘制矩形和圆形
int main(int argc, char ** argv)
{
src = imread("E://VS-pro//images//text.bmp");
if (!src.data)
{
printf("can not find .. \n");
return -1;
}
imshow("原图", src);
//变灰度
cvtColor(src, gray_src, COLOR_BGR2GRAY);
blur(gray_src, gray_src, Size(3, 3), Point(-1, -1));
imshow(gray_title, gray_src);
createTrackbar("threshold value: ", gray_title, &threshold_value, threshold_max, threshold_Demo);
threshold_Demo(0, 0);
waitKey(0);
return 0;
}void threshold_Demo(int, void*)
{
Mat bin_src;
//转换为二值图像
threshold(gray_src, bin_src, threshold_value, threshold_max, THRESH_BINARY);
imshow("二值图像", bin_src);
vector contours;
//放轮廓点
vector hierachy;
//图像的拓扑结构
//找轮廓
findContours(bin_src, contours, hierachy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(-1, -1));
//根据轮廓点找最小矩形和圆
vector contours_ploy(contours.size());
//存放绘制矩形的点
vector ploy_rects(contours.size());
vector【OpenCV-轮廓周围绘制矩形框以及圆形框】 ccs(contours.size());
//放圆心
vector radius(contours.size());
//放半径 vector minRects(contours.size());
vector myellipse(contours.size());
for (size_t i = 0;
i < contours.size();
i++)
{
approxPolyDP(Mat(contours[i]), contours_ploy[i], 3, true);
//减少多边形轮廓点数
ploy_rects[i] = boundingRect(contours_ploy[i]);
//取矩形
minEnclosingCircle(contours_ploy[i], ccs[i], radius[i]);
//圆//另一种方法
if (contours_ploy[i].size() > 5) {
myellipse[i] = fitEllipse(contours_ploy[i]);
minRects[i] = minAreaRect(contours_ploy[i]);
} } //绘制
Mat drawImg;
RNG rng(12345);
// draw it
drawImg = Mat::zeros(src.size(), src.type());
Point2f pts[4];
for (size_t t = 0;
t < contours.size();
t++) {
Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
//rectangle(drawImg, ploy_rects[t], color, 2, 8);
//circle(drawImg, ccs[t], radius[t], color, 2, 8);
if (contours_ploy[t].size() > 5) {
ellipse(drawImg, myellipse[t], color, 1, 8);
//画圆
minRects[t].points(pts);
for (int r = 0;
r < 4;
r++) {
line(drawImg, pts[r], pts[(r + 1) % 4], color, 1, 8);
//画矩形四条边
}
}
} imshow(output_title, drawImg);
}
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