OpenCV SURF comparing descriptors

Question

Folowing snippet is from OpenCV find_obj.cpp which is demo for using SURF,

double
compareSURFDescriptors( const float* d1, const float* d2, double best, int length )
{
    double total_cost = 0;
    assert( length % 4 == 0 );
    int i;
    for( i = 0; i  best )
            break;
    }
    return total_cost;
}

As far as I can tell it checking the euclidian distance, what I do not understand is why is it doing it in groups of 4? Why not calculate the whole thing at once?

Answer 1

Usually things like this are done for making SSE optimizations possible. SSE registers are 128 bits long and can contain 4 floats, so you can do the 4 subtractions using one instruction, parallelly.

Another upside: you have to check the loop counter only after every fourth difference. That makes the code faster even if the compiler doesn't use the opportunity to generate SSE code. For example, VS2008 didn't, not even with -O2:

    
      double t0 = d1[i] - d2[i];
00D91666  fld         dword ptr [edx-0Ch] 
00D91669  fsub        dword ptr [ecx-4] 
        double t1 = d1[i+1] - d2[i+1];
00D9166C  fld         dword ptr [ebx+ecx] 
00D9166F  fsub        dword ptr [ecx] 
        double t2 = d1[i+2] - d2[i+2];
00D91671  fld         dword ptr [edx-4] 
00D91674  fsub        dword ptr [ecx+4] 
        double t3 = d1[i+3] - d2[i+3];
00D91677  fld         dword ptr [edx] 
00D91679  fsub        dword ptr [ecx+8] 
        total_cost += t0*t0 + t1*t1 + t2*t2 + t3*t3;
00D9167C  fld         st(2) 
00D9167E  fmulp       st(3),st 
00D91680  fld         st(3) 
00D91682  fmulp       st(4),st 
00D91684  fxch        st(2) 
00D91686  faddp       st(3),st 
00D91688  fmul        st(0),st 
00D9168A  faddp       st(2),st 
00D9168C  fmul        st(0),st 
00D9168E  faddp       st(1),st 
00D91690  faddp       st(2),st 

Answer 2

I think it is because for each subregion we get 4 numbers. Totally 4x4x4 subregions making 64 length vector. So its basically getting the difference between 2 sub regions.