接上篇文章…
一般的，一個掃描的檔案是不可能完完全全擺正了的。它多多少少會有些傾斜。
現在有如下圖片：

它逆時針進行了旋轉，那麼想要把多餘的白邊去掉且擺正應該怎麼做呢？
步驟如下：
1、邊緣檢測
2、找出輪廓
3、找出最小外接矩形,獲得旋轉的角度
4、根據旋轉的角度進行仿射變換
5、按切邊的操作進行

程式碼（沒有用到的已註釋）：

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

using namespace cv;
using namespace std;

Mat src, dst, gray_src;

int current_level = 50;
int max_level = 255;
void cut_image(int, void*);
void rotating_image(int, void*);

const char* output_title = "rectminArea";
int main(int argc, char**argv)
{
	src = imread("D:/test/切邊測試圖-旋轉.png", 1);
	if (src.empty())
	{
		cout << "圖片未找到" << endl;
		return -1;
	}
	
	
	//namedWindow(output_title,CV_WINDOW_AUTOSIZE);
	//createTrackbar("Value", output_title, &current_level, max_level, cut_image);
	//cut_image(0,0);  //影象切邊
	rotating_image(0,0); // 將影象旋轉成正的
	imshow("input image", src);

	waitKey(0);
	return 0;

}
//將影象旋轉成正的
void rotating_image(int, void*)
{
  /*1、邊緣檢測
    2、找出輪廓
    3、找出最小外接矩形,獲得旋轉的角度
    4、仿射變換
	5、按切邊的操作進行
  */
	Mat canny_out;
	Canny(src, canny_out, current_level, 2 * current_level, 3, false);
	imshow("canny_out",canny_out);
	float minw = 0;
	float minh = 0;
	double angle = 0;

	vector<vector<Point>> contours;
	vector<Vec4i> hierarchy;
	findContours(canny_out, contours,hierarchy, RETR_TREE,CHAIN_APPROX_SIMPLE, Point(0, 0));
	Mat showImg= Mat::zeros(src.size(), CV_8UC3);;
	RNG rng(12345);
	for (size_t t = 0; t < contours.size(); t++)
	{
		RotatedRect minrect = minAreaRect(contours[t]);
		angle = minrect.angle;
		if (angle != 0)   
		{
			minh = max(minh,minrect.size.height);
	        minw = max(minw,minrect.size.width);
			
		}
	}
	printf("minw:%f\n", minw);
	printf("minh:%f\n", minh);
	printf("angle:%f\n", angle);//值為負數，說明逆時針旋轉，值為正數，說明順時針旋轉
	for (size_t t = 0; t < contours.size(); t++)
	{
		RotatedRect minrect = minAreaRect(contours[t]);
		
		if (minrect.size.height== minh && minrect.size.width == minw)
		{
			
			Point2f pts[4];
			minrect.points(pts);

			for (int i = 0; i < 4; i++)
			{
				Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
				line(showImg, pts[i], pts[(i + 1) % 4], color, 2, 8, 0);
			}
		}
		

	}
	imshow("showImg", showImg);

	Mat dst;
	Point2f center(src.cols/2, src.rows/2);
	Mat rota = getRotationMatrix2D(center, angle, 1.0);
	warpAffine(src, dst, rota, src.size(), INTER_LINEAR,0,Scalar(255,255,255));
	imshow("correct image",dst);
	//影象切邊

}



//影象切邊
void cut_image(int, void*)
{
	Mat canyImg;
	cvtColor(src, gray_src, CV_BGR2GRAY);
	Canny(gray_src, canyImg, current_level, 2 * current_level, 3,false);

	vector<vector<Point>> contours;
	vector<Vec4i>hierachy;
	Mat showImg = Mat::zeros(src.size(), CV_8UC3);
	findContours(canyImg, contours, hierachy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0));
	
	RNG rng(12345);

	int minw = src.cols*0.75;
	int minh = src.rows*0.75;

	Rect box; //獲取roi區域
	Mat contoursimg= Mat::zeros(src.size(), CV_8UC3);
	for (size_t t = 0; t < contours.size(); t++)
	{
		Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
		
		drawContours(contoursimg, contours, int(t), color, 1, 8, hierachy, 0, Point(0, 0));
		
		RotatedRect minrect = minAreaRect(contours[t]); //獲取輪廓的最小外接矩形
		float angle = abs(minrect.angle);
		if (minrect.size.height > minh && minrect.size.width > minw && minrect.size.width < (src.cols - 5))
		{
			Point2f pts[4];
			minrect.points(pts); //獲取最小外接矩形的四個頂點座標
			for(int i=0;i<4;i++)
			{
				line(showImg,pts[i], pts[(i + 1) % 4], color, 2, 8, 0);
				cout << "X座標:" << minrect.center.x <<" "<< "Y座標:" << minrect.center.y << " "<<"偏移角度:" << angle << endl;
			}
			box = minrect.boundingRect();
		}
		
		if (box.width > 0 && box.height > 0)
		{
			Mat roiImg=src(box);//擷取roi區域
			imshow("roiImg", roiImg);
		}
		
	}
	
	imshow(output_title, showImg);
	imshow("contours image", contoursimg);
	

}


/* //做旋轉圖片的程式碼
int main() {
	Mat src = imread("D:/test/切邊測試圖.png");
	imshow("src", src);
	double angle = 45;
	Point2f center(src.cols / 2, src.rows / 2);
	Mat rot = getRotationMatrix2D(center, angle, 1);
	Rect bbox = RotatedRect(center, src.size(), angle).boundingRect();

	rot.at<double>(0, 2) += bbox.width / 2.0 - center.x;
	rot.at<double>(1, 2) += bbox.height / 2.0 - center.y;

	Mat dst;
	warpAffine(src, dst, rot, bbox.size(),1,0,Scalar(255,255,255));
	imshow("dst", dst);
	imwrite("D:/test/切邊測試圖-旋轉.png",dst);
	waitKey(0);
	return 0;

}
*/
 

執行結果：

由此可以知道，該圖片旋轉的角度是-45度，也就是逆時針旋轉了45度。
那麼就可以進行仿射變換：

Mat dst;
	Point2f center(src.cols/2, src.rows/2);
	Mat rota = getRotationMatrix2D(center, angle, 1.0);
	warpAffine(src, dst, rota, src.size(), INTER_LINEAR,0,Scalar(255,255,255));
	imshow("correct image",dst);

最終的結果是：

接下來就是切邊的操作了，這裡參考上一篇文章：