How to use Edge Orientation histogram for object detection? - opencv

Question

I am working on an object detection code and I chose the edge orientation histogram as a descriptor for the matching.
I am facing a problem in the back projected histogram as i don't seem to have a good matching , the back projected image is mostly white, which means that i cannot use meanshift or so for detection of the object.
Please help me regarding this matter:
here is what i've done so far:
Take an initial ROI (the target needed to be detected in the video stream).
convert ROI to grayscale
apply sobel operator for both x, y derivatives.
calculate orientation using opencv phase function (from derivative x and derivative y)
make a histogram of the generated orientation. with the following specs:
(range : 0 to 2 PI) , (single channel) , (256 bins)
normalize the histogram
the code for doing these steps is the following:
Mat ROI_grad_x, ROI_grad_y , ROI_grad , ROI_gray;
Mat ROI_abs_grad_x, ROI_abs_grad_y;
cvtColor(ROI, ROI_gray, CV_BGR2GRAY);
/// Gradient X
Sobel( ROI_gray, ROI_grad_x, CV_16S, 1, 0, 3 );
/// Gradient Y
Sobel( ROI_gray, ROI_grad_y, CV_16S, 0, 1, 3 );
convertScaleAbs( ROI_grad_x, ROI_abs_grad_x );
convertScaleAbs( ROI_grad_y, ROI_abs_grad_y );
addWeighted( ROI_abs_grad_x, 0.5, ROI_abs_grad_y, 0.5, 0, ROI_grad );
Mat ROI_orientation = Mat::zeros(ROI_abs_grad_x.rows, ROI_abs_grad_y.cols, CV_32F); //to store the gradients
Mat ROI_orientation_norm ;
ROI_grad_x.convertTo(ROI_grad_x,CV_32F);
ROI_grad_y.convertTo(ROI_grad_y,CV_32F);
phase(ROI_grad_x, ROI_grad_y, ROI_orientation , false);
Mat ROI_orientation_hist ;
float ROI_orientation_range[] = {0 , CV_PI};
const float *ROI_orientation_histRange[] = {ROI_orientation_range};
int ROI_orientation_histSize =256;
//calcHist( &ROI_orientation, 1, 0, Mat(), ROI_orientation_hist, 1, &ROI_orientation_histSize, &ROI_orientation_histRange , true, false);
calcHist( &ROI_orientation, 1, 0, Mat(), ROI_orientation_hist, 1, &ROI_orientation_histSize, ROI_orientation_histRange , true, false);
normalize( ROI_orientation_hist, ROI_orientation_hist, 0, 255, NORM_MINMAX, -1, Mat() );
then , and for each camera frame captured , I do the following steps:
convert to grayscale
apply sobel operator for both x derivative and y derivative.
compute orientation using phase opencv function (using the 2 derivatives mentioned above)
back project the histogram onto the orientation frame matrix to get the matches.
the code used for this part is the following :
Mat grad_x, grad_y , grad;
Mat abs_grad_x, abs_grad_y;
/// Gradient X
Sobel( frame_gray, grad_x, CV_16S, 1, 0, 3 );
/// Gradient Y
Sobel( frame_gray, grad_y, CV_16S, 0, 1, 3 );
convertScaleAbs( grad_x, abs_grad_x );
convertScaleAbs( grad_y, abs_grad_y );
addWeighted( abs_grad_x, 0.5, abs_grad_y, 0.5, 0, grad );
///======================
Mat orientation = Mat::zeros(abs_grad_x.rows, abs_grad_y.cols, CV_32F); //to store the gradients
Mat orientation_norm ;
grad_x.convertTo(grad_x,CV_32F);
grad_y.convertTo(grad_y,CV_32F);
phase(grad_x, grad_y, orientation , false);
Mat EOH_backProj ;
calcBackProject( &orientation, 1, 0, ROI_orientation_hist, EOH_backProj, ROI_orientation_histRange, 1, true );
So , what seems to be the problem in my approach ?!
Thanks alot.