OpenCV鼠标滑轮事件

鼠标的滑轮事件实现图像的缩放很方便，具体在回调函数中如下写：

其中scale可以在外部定义为全局变量，通过响应CV_EVENT_MOUSEWHEEL滑轮事件获取Scale的具体值。

获取Scale值需要关注两个问题，滑轮滑动的方向和滑动量的大小。滑动方向通过getMouseWheelDelta（flags）获取，当返回值＞0时，表示向前滑动；当返回值＜0时，表示向后滑动。滑动量根据滑动方向自行设置相应的滑动步长即可。

void onMouse(int event, int x, int y, int flags,  void* )
{
    double value;
    float step=0.02;
    switch (event)
    {
    case CV_EVENT_MOUSEWHEEL:
        value = getMouseWheelDelta(flags);
        if (value>)
            scale +=step;
        else if(value<)
            scale -=step;
        break;
    default:
        break;
    }
}

下面是简单编写的滑动滑轮实现图像的缩放操作代码：

 #include <iostream>
 #include <string>
 #include <opencv2/opencv.hpp>

 using namespace std;
 using namespace cv;

 float scale=1.0;

 void zoomInAndOut(const float scale, const Mat srcImg, Mat &dstImg)
 {
     Mat M=Mat::eye(,,CV_32FC1);
     int imgHeight=srcImg.rows;
     int imgWidth=srcImg.cols;

     uchar* pSrcData = (uchar*)srcImg.data;
     uchar* pDstData = (uchar*)dstImg.data;

     Point2f center(imgWidth / 2.0, imgHeight / 2.0);
     //计算仿射矩阵
     M.at<float>(, ) = scale;
     M.at<float>(, ) = ( - scale)*center.x;
     M.at<float>(, ) = scale;
     M.at<float>(, ) = ( - scale)*center.y;

     float a11 = M.at<float>(, );
     float a12 = M.at<float>(, );
     float a13 = M.at<float>(, );
     float a21 = M.at<float>(, );
     float a22 = M.at<float>(, );
     float a23 = M.at<float>(, );
     float a31 = M.at<float>(, );
     float a32 = M.at<float>(, );
     float a33 = M.at<float>(, );

     float bx = a11*a22 - a21*a12;
     float by = a12*a21 - a11*a22;
     if ( abs(bx) > 1e- && abs(by) > 1e-)
     {
         bx = 1.0 / bx;
         by = 1.0 / by;
         float cx = a13*a22 - a23*a12;
         float cy = a13*a21 - a23*a11;

         for (int j =; j < imgHeight; j++)
         {
             for (int i = ; i < imgWidth; i++)
             {
                 float u = (a22*i - a12*j - cx) *bx;
                 float v = (a21*i - a11*j - cy) *by;

                 int u0 = floor(u);
                 int v0 = floor(v);
                 int u1 = floor(u0 + );
                 int v1 = floor(v0 + );
                 if (u0 >=  && v0 >=  && u1 < imgWidth && v1 < imgHeight)
                 {
                     float dx = u - u0;
                     float dy = v - v0;
                     float weight1 = ( - dx)*( - dy);
                     float weight2 = dx*( - dy);
                     float weight3 = ( - dx)*dy;
                     float weight4 = dx*dy;

                     for (int k=; k<srcImg.channels(); k++)
                     {
                         pDstData[j*imgWidth *  + i *  + k] = weight1*pSrcData[v0*imgWidth *  + u0 *  + k] +
                         weight2*pSrcData[v0*imgWidth *  + u1 *  + k] +
                         weight3*pSrcData[v1*imgWidth *  + u0 *  + k] +
                         weight4*pSrcData[v1*imgWidth *  + u1 *  + k];
                     }
                 }
                 else
                 {
                    for (int k=; k<srcImg.channels(); k++)
                     {
                         pDstData[j*imgWidth *  + i *  + k] = ;
                     }
                 }
             }
         }
     }
 }

 void onMouse(int event, int x, int y, int flags,  void* )
 {
     double value;
     float step=0.02;
     switch (event)
     {
     case CV_EVENT_MOUSEWHEEL:
         value = getMouseWheelDelta(flags);
         if (value>)
             scale +=step;
         else if(value<)
             scale -=step;
         break;
     default:
         break;
     }
 }

 void main()
 {
     string imgPath="data/source_images/";
     Mat srcImg = imread(imgPath+"moon.jpg");
     pyrDown(srcImg, srcImg);
     pyrDown(srcImg, srcImg);

     Mat dstImg = srcImg.clone();
     dstImg.setTo();

     string windowName="showImg";
     namedWindow(windowName);
     imshow(windowName, srcImg);
     waitKey();

     setMouseCallback(windowName, onMouse, NULL);
     float scaleMin=0.5;
     float scaleMax=;
     while (true)
     {
         scale = (scale<scaleMin)? (scaleMin): scale;
         scale = (scale>scaleMax)? (scaleMax): scale;
         zoomInAndOut(scale, srcImg, dstImg);
         imshow(windowName, dstImg);
         waitKey();
     }
 }