I have a question when I read the source code in opencv/modules/objdetect/src/hog.cpp(opencv/hog.cpp at master · opencv/opencv · GitHub ).
In the menber function of
cv::HOGDescriptor Struct which is
void HOGDescriptor::computeGradient(const Mat& img, Mat& grad, Mat& qangle,Size paddingTL, Size paddingBR) const
,after calculating the gradient Dx,Dy,we need to convert them to magnitude and angle,Just as the following codes in line 566-584 do.
In my opinion, dbuf[x+width*3]*angleScale is the position of the corresponding angle. Why does it subtract 0.5f？I think the position will change if it subtract 0.5f.
for( ; x < width; x++ )
{
float mag = dbuf[x+width*2], angle = dbuf[x+width*3]*angleScale **- 0.5f**;
int hidx = cvFloor(angle);
angle -= hidx;
gradPtr[x*2] = mag*(1.f - angle);
gradPtr[x*2+1] = mag*angle;
if( hidx < 0 )
hidx += nbins;
else if( hidx >= nbins )
hidx -= nbins;
CV_Assert( (unsigned)hidx < (unsigned)nbins );
qanglePtr[x*2] = (uchar)hidx;
hidx++;
hidx &= hidx < nbins ? -1 : 0;
qanglePtr[x*2+1] = (uchar)hidx;
}