利用OpenCV对图像旋转

做车牌识别时，要对定位出来的车牌进行字符校正，字符校正可采用如下方法：

对图像进行灰度化，然后对其进行边缘提取，再在1到180度角内对图像进行旋转，记录下边缘提取后的图像在x轴方向上的投影，当x轴方向上的投影最小的时候即表示图像中字符平行于y轴，已经完成矫正，此时记录下旋转的倾斜角。然后再对图像旋转即可。

以下为对图像旋转代码：

#include "cv.h"
#include "highgui.h"
#include "math.h"

// clockwise 为true则顺时针旋转，否则为逆时针旋转
IplImage* rotateImage(IplImage* src, int angle, bool clockwise)
{
	angle = abs(angle) % 180;
	if (angle > 90)
	{
		angle = 90 - (angle % 90);
	}
	IplImage* dst = NULL;
	int width =
		(double)(src->height * sin(angle * CV_PI / 180.0)) +
		(double)(src->width * cos(angle * CV_PI / 180.0 )) + 1;
	int height =
		(double)(src->height * cos(angle * CV_PI / 180.0)) +
		(double)(src->width * sin(angle * CV_PI / 180.0 )) + 1;
	int tempLength = sqrt((double)src->width * src->width + src->height * src->height) + 10;
	int tempX = (tempLength + 1) / 2 - src->width / 2;
	int tempY = (tempLength + 1) / 2 - src->height / 2;
	int flag = -1;

	dst = cvCreateImage(cvSize(width, height), src->depth, src->nChannels);
	cvZero(dst);
	IplImage* temp = cvCreateImage(cvSize(tempLength, tempLength), src->depth, src->nChannels);
	cvZero(temp);

	cvSetImageROI(temp, cvRect(tempX, tempY, src->width, src->height));
	cvCopy(src, temp, NULL);
	cvResetImageROI(temp);

	if (clockwise)
		flag = 1;

	float m[6];
	int w = temp->width;
	int h = temp->height;
	m[0] = (float) cos(flag * angle * CV_PI / 180.);
	m[1] = (float) sin(flag * angle * CV_PI / 180.);
	m[3] = -m[1];
	m[4] = m[0];
	// 将旋转中心移至图像中间
	m[2] = w * 0.5f;
	m[5] = h * 0.5f;
	//
	CvMat M = cvMat(2, 3, CV_32F, m);
	cvGetQuadrangleSubPix(temp, dst, &M);
	cvReleaseImage(&temp);
	return dst;
}

int main(int argc, char **argv)
{
	IplImage *src = 0;
	IplImage *dst = 0;
	// 旋转角度
	int angle = 90;

	src = cvLoadImage("Images\\cones\\imL.png",CV_LOAD_IMAGE_COLOR);
	cvNamedWindow("src", 1);
	cvShowImage("src", src);

	dst = rotateImage(src, angle, false);
	cvNamedWindow("dst", 2);
	cvShowImage("dst", dst);
	cvWaitKey(0);

	cvReleaseImage(&src);
	cvReleaseImage(&dst);
	return 0;
}