Calculating image gradient direction using OpenCV's Sobel operation

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 think I understand the theory but implementing this has been more challenging than I thought.

I would expect the gradient directions to be between 0-360 degrees, however my code shows all gradients fall between 180 - 270 degrees.

I submitted a previous version of this code which included an integer division issue. I have fixed this but it has not solved the problem of a restricted angle of direction.

I have stepped through all the code but I just can't see where I'm going wrong? Can anyone spot my mistake?

Thanks.

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);

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

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

uchar* pixelX = grad_x.data;
uchar* pixelY = 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 = -1;

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

grad_x and grad_y are Mats of type CV_16SC1, that is every pixel in them takes up two bytes.

However you declare pixelX and pixelY to pointers to 8 bit bytes. Therefore pixelX[1] is the second byte of the first gradient, rather than the second gradient.

You need

short* pixelX = grad_x.ptr<short>(0);
short* pixelY = grad_y.ptr<short>(0);

Answer 2

Problem is here

uchar* pixelX = grad_x.data;
uchar* pixelY = grad_y.data;

and here

 double directionRAD = atan2(pixelY[i], pixelX[i]);

You don`t take abs(), but use unsigned pointer. That is why you cannot get x or y negative.

Should be:

short* pixelX = (short*) grad_x.data;
short* pixelY = (short*) grad_y.data;

and

 double directionRAD = atan2((double)pixelY[i], (double)pixelX[i]);