从索贝尔确定图像梯度方向？

Question

我试图使用openCV的Sobel方法的结果确定图像梯度方向.

我明白这应该是一个非常简单的任务.我从这里复制了许多资源和答案的方法,但无论我做什么,结果方向总是在0到57度之间(我希望范围是0-360).

我相信所有的深度都是正确的.我尝试使用16S数据和8U数据计算方向.

我只是看不出我哪里出错了？谁能发现我的错误？

void getGradients(IplImage* original, cv::Mat* gradArray)
{
    cv::Mat original_Mat(original, true);

    // Convert it to gray
    cv::cvtColor( original_Mat, original_Mat, CV_RGB2GRAY );
    //cv::blur(original_Mat, original_Mat, cv::Size(7,7));

    /// Generate grad_x and grad_y
    cv::Mat grad_x = cv::Mat::zeros(original->height, original->width, CV_16S); 
    cv::Mat grad_y = cv::Mat::zeros(original->height, original->width, CV_16S);

    cv::Mat abs_grad_x = cv::Mat::zeros(original->height, original->width, CV_8U);
    cv::Mat abs_grad_y = cv::Mat::zeros(original->height, original->width, CV_8U);;

    /// Gradient X
    cv::Sobel(original_Mat, grad_x, CV_16S, 1, 0, 3);
    cv::convertScaleAbs( grad_x, abs_grad_x );

    /// Gradient Y
    cv::Sobel(original_Mat, grad_y, CV_16S, 0, 1, 3);
    cv::convertScaleAbs( grad_y, abs_grad_y );

    uchar* pixelX = abs_grad_x.data;
    uchar* pixelY = abs_grad_y.data;
    uchar* grad1 = gradArray[0].data;
    uchar* grad2 = gradArray[1].data;
    uchar* grad3 = gradArray[2].data;
    uchar* grad4 = gradArray[3].data;
    uchar* grad5 = gradArray[4].data;
    uchar* grad6 = gradArray[5].data;
    uchar* grad7 = gradArray[6].data;
    uchar* grad8 = gradArray[7].data;
    int count = 0;
    int min = 999999;
    int max = 0;

    for(int i = 0; i < grad_x.rows * grad_x.cols; i++) 
    {
            int directionRAD = atan2(pixelY[i], pixelX[i]);
            int directionDEG = directionRAD / PI * 180;

            if(directionDEG < min){min = directionDEG;}
            if(directionDEG > max){max = directionDEG;}

            if(directionDEG >= 0 && directionDEG <= 45)         { grad1[i] = 255; count++;}         
            if(directionDEG >= 45 && directionDEG <= 90)        { grad2[i] = 255; count++;}         
            if(directionDEG >= 90 && directionDEG <= 135)       { grad3[i] = 255; count++;}         
            if(directionDEG >= 135 && directionDEG <= 190)      { grad4[i] = 255; count++;}         
            if(directionDEG >= 190 && directionDEG <= 225)      { grad5[i] = 255; count++;}         
            if(directionDEG >= 225 && directionDEG <= 270)      { grad6[i] = 255; count++;}     
            if(directionDEG >= 270 && directionDEG <= 315)      { grad7[i] = 255; count++;}
            if(directionDEG >= 315 && directionDEG <= 360)      { grad8[i] = 255; count++;}

            if(directionDEG < 0 || directionDEG > 360)
            {
                cout<<"Weird gradient direction given in method: getGradients.";
            }               
    }
}

Answer 1

您正在使用整数运算,因此您对弧度和度数的计算会因截断而受到严重影响.

也atan2给范围内的结果-PI来+PI,所以如果你想在范围度的值0..360你需要添加一个180度的修正:

        double directionRAD = atan2(pixelY[i], pixelX[i]);
        int directionDEG = (int)(180.0 + directionRAD / M_PI * 180.0);

注意使用的double,而不是int为directionRAD.

专业提示:学习使用调试器来逐步调试代码,随时检查变量 - 这将使得修复这样的简单错误比等待StackOverflow上的响应容易得多.

Answer 2

您可以使用Sobel算子获得x导数dx和y导数dy.然后,您可以使用公式计算渐变的大小和方向.G=sqrt(dx^2+dy^2), theta=arctan(dy/dx).你可以发现这只是将descartes坐标系(x,y)转换为极坐标(rho,theta)!

还有就是你做的绝对值一些错误代码dx和dy,这使得方向总是在笛卡尔的第一象限坐标系.您使用的函数convertScaleAbs将结果转换为8位,这会导致截断错误.

我有一个演示来部分根据您的代码计算幅度.

    const string imgname = "F:/OpenCV/square.jpg";
    Mat img = imread(imgname, CV_LOAD_IMAGE_COLOR);

    // 1. convert it to gray value
    Mat gray;
    cvtColor(img, gray, CV_BGR2GRAY);
    // 2. blur the image
    blur(gray, gray, Size(7, 7));
    // 3. sobel
    Mat grad_x, grad_y;
    Scharr(gray, grad_x, CV_32FC1, 1, 0);
    Scharr(gray, grad_y, CV_32FC1, 0, 1);
    // 4. calculate gradient magnitude and direction
    Mat magnitude, direction;
    bool useDegree = true;    // use degree or rad
    // the range of the direction is [0,2pi) or [0, 360)
    cartToPolar(grad_x, grad_y, magnitude, direction, useDegree);

    // test, the histogram of the directions
    vector<int> cnt(8, 0);   // 0-45, 45-90, ..., 315-360

    for(auto iter = direction.begin<float>(); iter != direction.end<float>(); ++iter)
    {
        int idx = static_cast<int>(*iter) / 45;
        ++cnt[idx];
    }

    Mat scaled;
    convertScaleAbs(magnitude, scaled);
    imshow("magnitude", scaled);
    for(auto v : cnt)
        cout << v << " ";