Determining image gradient direction from sobel?

Question

I am attempting to determine the image gradient direction using the results from openCV's Sobel method.

I understand this should be a very simple task. I have copied the methods from a number of resources and answers from here but whatever I do the resultant directions are always between 0 - 57 degrees (I would expect the range to be from 0-360).

I believe all the depths are correct. I have tried calculating the direction using the 16S data as well as 8U data.

I just can't see where I'm going wrong? Can anyone spot my mistake?

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

You're using integer arithmetic so your calculations for radians and degrees are suffering badly from truncation.

Also atan2 gives a result in the range -PI to +PI, so if you want a value in degrees in the range 0..360 you'll need to add a 180 degree correction:

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

Note the use of double rather than int for directionRAD.

Pro tip: learn to use a debugger to step through you code, inspecting variables as you go - that will make fixing simple bugs like this a lot easier than waiting for responses on StackOverflow.