siftdescriptor.c

sift的matlab代码

        if( NBO <= 0 ) {
          mexErrMsgTxt("'NumOrientlBins' must be positive") ;
        }
        break ;

      case PROP_MAGNIF:
        if( !uIsRealScalar(in[arg+1]) ) {
          mexErrMsgTxt("'Magnif' should be a real scalar") ;
        }
        magnif = (float) *mxGetPr(in[arg+1]) ;
        if( magnif <= 0 ) {
          mexErrMsgTxt("'Magnif' must be positive") ;
        }
        break ;

      case PROP_UNKNOWN:
        mexErrMsgTxt("Property unknown.") ;
        break ;
      }
      arg += 2 ;
    }
  }
  
  /* -----------------------------------------------------------------
   *                                   Pre-compute gradient and angles
   * -------------------------------------------------------------- */
  /* Alloc two buffers and make sure their size is multiple of 128 for
   * better alignment (used also by the Altivec code below.)
   */
  buffer_size = (M*N*num_levels + 0x7f) & (~ 0x7f) ;
  buffer_pt = (float*) mxMalloc( sizeof(float) * 2 * buffer_size ) ;
  descr_pt  = (float*) mxCalloc( NBP*NBP*NBO*K,  sizeof(float)  ) ;

  {
    /* Offsets to move in the scale space. */
    const int yo = 1 ;
    const int xo = M ;
    const int so = M*N ;
    int x,y,s ;

#define at(x,y) (*(pt + (x)*xo + (y)*yo))

    /* Compute the gradient */
    for(s = 0 ; s < num_levels ; ++s) {
      const double* pt = G_pt + s*so ;
      for(x = 1 ; x < N-1 ; ++x) {
        for(y = 1 ; y < M-1 ; ++y) {
          float Dx = 0.5 * ( at(x+1,y) - at(x-1,y) ) ;
          float Dy = 0.5 * ( at(x,y+1) - at(x,y-1) ) ;
          buffer_pt[(x*xo+y*yo+s*so) + 0          ] = Dx ;
          buffer_pt[(x*xo+y*yo+s*so) + buffer_size] = Dy ;
        }
      }
    }
    
    /* Compute angles and modules */
    {
      float* pt = buffer_pt ;
      int j = 0 ;
      while (j < N*M*num_levels) {

#if defined( MACOSX ) && defined( __ALTIVEC__ )
        if( ((unsigned int)pt & 0x7) == 0 && j+3 < N*M*num_levels ) {
          /* If aligned to 128 bit and there are at least 4 pixels left */
          float4 a, b, c, d ;
          a.vec = vec_ld(0,(vector float*)(pt              )) ;
          b.vec = vec_ld(0,(vector float*)(pt + buffer_size)) ;
          c.vec = vatan2f(b.vec,a.vec) ;
          a.x[0] = a.x[0]*a.x[0]+b.x[0]*b.x[0] ;
          a.x[1] = a.x[1]*a.x[1]+b.x[1]*b.x[1] ;
          a.x[2] = a.x[2]*a.x[2]+b.x[2]*b.x[2] ;
          a.x[3] = a.x[3]*a.x[3]+b.x[3]*b.x[3] ;
          d.vec = vsqrtf(a.vec) ;
          vec_st(c.vec,0,(vector float*)(pt + buffer_size)) ;
          vec_st(d.vec,0,(vector float*)(pt              )) ;
          j += 4 ;
          pt += 4 ;
        } else {
#endif
          float Dx = *(pt              ) ;
          float Dy = *(pt + buffer_size) ;
          *(pt              ) = sqrtf(Dx*Dx + Dy*Dy) ;
          *(pt + buffer_size) = atan2f(Dy, Dx) ;
          j += 1 ;
          pt += 1 ;
#if defined( MACOSX ) && defined( __ALTIVEC__ )
        }
#endif

      }
    }
  }

  /* -----------------------------------------------------------------
   *                                                        Do the job
   * -------------------------------------------------------------- */ 
  if(K > 0) {    
    int p ;

    /* Offsets to move in the buffer */
    const int yo = 1 ;
    const int xo = M ;
    const int so = M*N ;

    /* Offsets to move in the descriptor. */
    /* Use Lowe's convention. */
    const int binto = 1 ;
    const int binyo = NBO * NBP ;
    const int binxo = NBO ;
    const int bino  = NBO * NBP * NBP ;

    for(p = 0 ; p < K ; ++p, descr_pt += bino) {
      /* The SIFT descriptor is a  three dimensional histogram of the position
       * and orientation of the gradient.  There are NBP bins for each spatial
       * dimesions and NBO  bins for the orientation dimesion,  for a total of
       * NBP x NBP x NBO bins.
       *
       * The support  of each  spatial bin  has an extension  of SBP  = 3sigma
       * pixels, where sigma is the scale  of the keypoint.  Thus all the bins
       * together have a  support SBP x NBP pixels wide  . Since weighting and
       * interpolation of  pixel is used, another  half bin is  needed at both
       * ends of  the extension. Therefore, we  need a square window  of SBP x
       * (NBP + 1) pixels. Finally, since the patch can be arbitrarly rotated,
       * we need to consider  a window 2W += sqrt(2) x SBP  x (NBP + 1) pixels
       * wide.
       */      
      const float x = (float) *P_pt++ ;
      const float y = (float) *P_pt++ ;
      const float s = (float) (mode == SCALESPACE) ? (*P_pt++) : 0.0 ;
      const float theta0 = (float) *P_pt++ ;

      const float st0 = sinf(theta0) ;
      const float ct0 = cosf(theta0) ;
      const int xi = (int) floor(x+0.5) ; /* Round-off */
      const int yi = (int) floor(y+0.5) ;
      const int si = (int) floor(s+0.5) - smin ;
      const float sigma = sigma0 * powf(2, s / S) ;
      const float SBP = magnif * sigma ;
      const int W = (int) floor( sqrt(2.0) * SBP * (NBP + 1) / 2.0 + 0.5) ;      
      int bin ;
      int dxi ;
      int dyi ;
      const float* pt ;
      float* dpt ;

      /* Check that keypoints are within bounds . */

      if(xi < 0   || 
         xi > N-1 || 
         yi < 0   || 
         yi > M-1 ||
         ((mode==SCALESPACE) && 
          (si < 0   ||
           si > dimensions[2]-1) ) )
        continue ;

      /* Center the scale space and the descriptor on the current keypoint. 
       * Note that dpt is pointing to the bin of center (SBP/2,SBP/2,0).
       */
      pt  = buffer_pt + xi*xo + yi*yo + si*so ;
      dpt = descr_pt + (NBP/2) * binyo + (NBP/2) * binxo ;
     
#define atd(dbinx,dbiny,dbint) (*(dpt + (dbint)*binto + (dbiny)*binyo + (dbinx)*binxo))
      
      /*
       * Process each pixel in the window and in the (1,1)-(M-1,N-1)
       * rectangle.
       */
      for(dxi = max(-W, 1-xi) ; dxi <= min(+W, N-2-xi) ; ++dxi) {
        for(dyi = max(-W, 1-yi) ; dyi <= min(+W, M-2-yi) ; ++dyi) {

          /* Compute the gradient. */
          float mod   = *(pt + dxi*xo + dyi*yo + 0           ) ;
          float angle = *(pt + dxi*xo + dyi*yo + buffer_size ) ;
#ifdef LOWE_COMPATIBLE
          float theta = fast_mod(-angle + theta0) ;
#else
          float theta = fast_mod(angle - theta0) ;
#endif
          /* Get the fractional displacement. */
          float dx = ((float)(xi+dxi)) - x;
          float dy = ((float)(yi+dyi)) - y;

          /* Get the displacement normalized w.r.t. the keypoint orientation
           * and extension. */
          float nx = ( ct0 * dx + st0 * dy) / SBP ;
          float ny = (-st0 * dx + ct0 * dy) / SBP ; 
          float nt = NBO * theta / (2*M_PI) ;

          /* Get the gaussian weight of the sample. The gaussian window
           * has a standard deviation of NBP/2. Note that dx and dy are in
           * the normalized frame, so that -NBP/2 <= dx <= NBP/2. */
          const float wsigma = NBP/2 ;
          float win =  expf(-(nx*nx + ny*ny)/(2.0 * wsigma * wsigma)) ;

          /* The sample will be distributed in 8 adijacient bins. 
           * Now we get the ``lower-left'' bin. */
          int binx = fast_floor( nx - 0.5 ) ;
          int biny = fast_floor( ny - 0.5 ) ;
          int bint = fast_floor( nt ) ;
          float rbinx = nx - (binx+0.5) ;
          float rbiny = ny - (biny+0.5) ;
          float rbint = nt - bint ;
          int dbinx ;
          int dbiny ;
          int dbint ;

          /* Distribute the current sample into the 8 adijacient bins. */
          for(dbinx = 0 ; dbinx < 2 ; ++dbinx) {
            for(dbiny = 0 ; dbiny < 2 ; ++dbiny) {
              for(dbint = 0 ; dbint < 2 ; ++dbint) {
                
                if( binx+dbinx >= -(NBP/2) &&
                    binx+dbinx <   (NBP/2) &&
                    biny+dbiny >= -(NBP/2) &&
                    biny+dbiny <   (NBP/2) ) {
                  float weight = win 
                    * mod 
                    * fabsf(1 - dbinx - rbinx)
                    * fabsf(1 - dbiny - rbiny)
                    * fabsf(1 - dbint - rbint) ;

                  atd(binx+dbinx, biny+dbiny, (bint+dbint) % NBO) += weight ;
                }
              }            
            }
          }
        }  
      }

      {
        /* Normalize the histogram to L2 unit length. */        
        normalize_histogram(descr_pt, descr_pt + NBO*NBP*NBP) ;
        
        /* Truncate at 0.2. */
        for(bin = 0; bin < NBO*NBP*NBP ; ++bin) {
          if (descr_pt[bin] > 0.2) descr_pt[bin] = 0.2;
        }
        
        /* Normalize again. */
        normalize_histogram(descr_pt, descr_pt + NBO*NBP*NBP) ;
      }
    }
  }

  /* Restore pointer to the beginning of the descriptors. */
  descr_pt -= NBO*NBP*NBP*K ;

  {
    int k ;
    double* L_pt ;
    out[OUT_L] = mxCreateDoubleMatrix(NBP*NBP*NBO, K, mxREAL) ;
    L_pt = mxGetPr(out[OUT_L]) ;
    for(k = 0 ; k < NBP*NBP*NBO*K ; ++k) {
      L_pt[k] = descr_pt[k] ;
    }
  }

  mxFree(descr_pt) ;  
  mxFree(buffer_pt) ;
}