opencv实践::透视变换

问题描述 
拍摄或者扫描图像不是规则的矩形，会对后期处理产生不 好影响，需要通过透视变换校正得到正确形状。

解决思路 
通过二值分割 + 形态学方法 + Hough直线 +透视变换

#include <opencv2/opencv.hpp>
#include <iostream>
#include <math.h>

using namespace cv;
using namespace std;

int main(int argc, char** argv) {
    Mat src = imread("D:/case6.png");
    if (src.empty()) {
        printf("could not load image...\n");
        return 0;
    }
    namedWindow("input image", CV_WINDOW_AUTOSIZE);
    imshow("input image", src);

    // 二值处理 取反
    Mat gray_src, binary, dst;
    cvtColor(src, gray_src, COLOR_BGR2GRAY);
    threshold(gray_src, binary, 0, 255, THRESH_BINARY_INV | THRESH_OTSU);
    //imshow("binary image", binary);

    // 形态学操作
    Mat kernel = getStructuringElement(MORPH_RECT, Size(5, 5), Point(-1, -1));
    morphologyEx(binary, dst, MORPH_CLOSE, kernel, Point(-1, -1), 3);
    //imshow("morphology", dst);

    // 轮廓发现
    bitwise_not(dst, dst, Mat());
    vector<vector<Point>> contours;
    vector<Vec4i> hireachy;
    findContours(dst, contours, hireachy, CV_RETR_TREE, CHAIN_APPROX_SIMPLE, Point());

    // 轮廓绘t制
    int width = src.cols;
    int height = src.rows;
    Mat drawImage = Mat::zeros(src.size(), CV_8UC3);
    for (size_t t = 0; t < contours.size(); t++) {
        Rect rect = boundingRect(contours[t]);
        if (rect.width > width / 2 && rect.width < width - 5) {
            drawContours(drawImage, contours, static_cast<int>(t), Scalar(0, 0, 255), 2, 8, hireachy, 0, Point());
        }
    }
    //imshow("contours", drawImage);

    vector<Vec4i> lines;
    Mat contoursImg;
    int accu = min(width*0.5, height*0.5);
    cvtColor(drawImage, contoursImg, COLOR_BGR2GRAY);
    HoughLinesP(contoursImg, lines, CV_HOUGH_PROBABILISTIC, CV_PI / 200.0, accu, accu, 0);
    Mat linesImage = Mat::zeros(src.size(), CV_8UC3);
    for (size_t t = 0; t < lines.size(); t++) {
        Vec4i ln = lines[t];
        line(linesImage, Point(ln[0], ln[1]), Point(ln[2], ln[3]), Scalar(0, 0, 255), 2, 8, 0);
    }
    printf("number of lines : %d\n", lines.size());
    //imshow("lines image", linesImage);


    // 寻找与定位上下左右四条直线
    int deltah = 0;
    Vec4i topLine, bottomLine;
    Vec4i leftLine, rightLine;
    for (int i = 0; i < lines.size(); i++) {
        Vec4i ln = lines[i];
        deltah = abs(ln[3] - ln[1]);
        if (ln[3] < height / 2.0 && ln[1] < height / 2.0 && deltah < accu - 1) {
            if (topLine[3] > ln[3] && topLine[3] > 0) {
                topLine = lines[i];
            }
            else {
                topLine = lines[i];
            }
        }
        if (ln[3] > height / 2.0 && ln[1] > height / 2.0 && deltah < accu - 1) {
            bottomLine = lines[i];
        }
        if (ln[0] < width / 2.0 && ln[2] < width / 2.0) {
            leftLine = lines[i];
        }
        if (ln[0] > width / 2.0 && ln[2] > width / 2.0) {
            rightLine = lines[i];
        }
    }
    cout << "top line : p1(x, y) = " << topLine[0] << "," << topLine[1] << " p2(x, y) = " << topLine[2] << "," << topLine[3] << endl;
    cout << "bottom line : p1(x, y) = " << bottomLine[0] << "," << bottomLine[1] << " p2(x, y) = " << bottomLine[2] << "," << bottomLine[3] << endl;
    cout << "left line : p1(x, y) = " << leftLine[0] << "," << leftLine[1] << " p2(x, y) = " << leftLine[2] << "," << leftLine[3] << endl;
    cout << "right line : p1(x, y) = " << rightLine[0] << "," << rightLine[1] << " p2(x, y) = " << rightLine[2] << "," << rightLine[3] << endl;
    
    // 拟合四条直线方程，求直线相交的点
    float k1, c1;
    k1 = float(topLine[3] - topLine[1]) / float(topLine[2] - topLine[0]);
    c1 = topLine[1] - k1 * topLine[0];
    float k2, c2;
    k2 = float(bottomLine[3] - bottomLine[1]) / float(bottomLine[2] - bottomLine[0]);
    c2 = bottomLine[1] - k2 * bottomLine[0];
    float k3, c3;
    k3 = float(leftLine[3] - leftLine[1]) / float(leftLine[2] - leftLine[0]);
    c3 = leftLine[1] - k3 * leftLine[0];
    float k4, c4;
    k4 = float(rightLine[3] - rightLine[1]) / float(rightLine[2] - rightLine[0]);
    c4 = rightLine[1] - k4 * rightLine[0];

    // 四条直线交点
    Point p1; // 左上角
    p1.x = static_cast<int>((c1 - c3) / (k3 - k1));
    p1.y = static_cast<int>(k1*p1.x + c1);
    Point p2; // 右上角
    p2.x = static_cast<int>((c1 - c4) / (k4 - k1));
    p2.y = static_cast<int>(k1*p2.x + c1);
    Point p3; // 左下角
    p3.x = static_cast<int>((c2 - c3) / (k3 - k2));
    p3.y = static_cast<int>(k2*p3.x + c2);
    Point p4; // 右下角
    p4.x = static_cast<int>((c2 - c4) / (k4 - k2));
    p4.y = static_cast<int>(k2*p4.x + c2);
    cout << "p1(x, y)=" << p1.x << "," << p1.y << endl;
    cout << "p2(x, y)=" << p2.x << "," << p2.y << endl;
    cout << "p3(x, y)=" << p3.x << "," << p3.y << endl;
    cout << "p4(x, y)=" << p4.x << "," << p4.y << endl;

    // 显示四个点坐标
    circle(linesImage, p1, 2, Scalar(255, 0, 0), 2, 8, 0);
    circle(linesImage, p2, 2, Scalar(255, 0, 0), 2, 8, 0);
    circle(linesImage, p3, 2, Scalar(255, 0, 0), 2, 8, 0);
    circle(linesImage, p4, 2, Scalar(255, 0, 0), 2, 8, 0);
    line(linesImage, Point(topLine[0], topLine[1]), Point(topLine[2], topLine[3]), Scalar(0, 255, 0), 2, 8, 0);
    //imshow("four corners", linesImage);

    // 透视变换
    vector<Point2f> src_corners(4);
    src_corners[0] = p1;
    src_corners[1] = p2;
    src_corners[2] = p3;
    src_corners[3] = p4;

    vector<Point2f> dst_corners(4);
    dst_corners[0] = Point(0, 0);
    dst_corners[1] = Point(width, 0);
    dst_corners[2] = Point(0, height);
    dst_corners[3] = Point(width, height);

    // 获取透视变换矩阵
    Mat resultImage;
    Mat warpmatrix = getPerspectiveTransform(src_corners, dst_corners);
    warpPerspective(src, resultImage, warpmatrix, resultImage.size(), INTER_LINEAR);
    namedWindow("Final Result", CV_WINDOW_AUTOSIZE);
    imshow("Final Result", resultImage);

    waitKey(0);
    return 0;
}