resize.cc

#include <math.h>
#include <assert.h>
#include <string.h>
#include "mex.h"

/*
 * Fast image subsampling.
 * This is used to construct the feature pyramid.
 */

// struct used for caching interpolation values
struct alphainfo {
  int si, di;
  double alpha;
};

// copy src into dst using pre-computed interpolation values
void alphacopy(double *src, double *dst, struct alphainfo *ofs, int n) {
  struct alphainfo *end = ofs + n;
  while (ofs != end) {
    dst[ofs->di] += ofs->alpha * src[ofs->si];
    ofs++;
  }
}

// resize along each column
// result is transposed, so we can apply it twice for a complete resize
void resize1dtran(double *src, int sheight, double *dst, int dheight, 
		  int width, int chan) {
  double scale = (double)dheight/(double)sheight;
  double invscale = (double)sheight/(double)dheight;
  
  // we cache the interpolation values since they can be 
  // shared among different columns
  int len = (int)ceil(dheight*invscale) + 2*dheight;
  alphainfo ofs[len];
  int k = 0;
  for (int dy = 0; dy < dheight; dy++) {
    double fsy1 = dy * invscale;
    double fsy2 = fsy1 + invscale;
    int sy1 = (int)ceil(fsy1);
    int sy2 = (int)floor(fsy2);       

    if (sy1 - fsy1 > 1e-3) {
      assert(k < len);
      assert(sy-1 >= 0);
      ofs[k].di = dy*width;
      ofs[k].si = sy1-1;
      ofs[k++].alpha = (sy1 - fsy1) * scale;
    }

    for (int sy = sy1; sy < sy2; sy++) {
      assert(k < len);
      assert(sy < sheight);
      ofs[k].di = dy*width;
      ofs[k].si = sy;
      ofs[k++].alpha = scale;
    }

    if (fsy2 - sy2 > 1e-3) {
      assert(k < len);
      assert(sy2 < sheight);
      ofs[k].di = dy*width;
      ofs[k].si = sy2;
      ofs[k++].alpha = (fsy2 - sy2) * scale;
    }
  }

  // resize each column of each color channel
  bzero(dst, chan*width*dheight*sizeof(double));
  for (int c = 0; c < chan; c++) {
    for (int x = 0; x < width; x++) {
      double *s = src + c*width*sheight + x*sheight;
      double *d = dst + c*width*dheight + x;
      alphacopy(s, d, ofs, k);
    }
  }
}

// main function
// takes a double color image and a scaling factor
// returns resized image
mxArray *resize(const mxArray *mxsrc, const mxArray *mxscale) {
  double *src = (double *)mxGetPr(mxsrc);
  const int *sdims = mxGetDimensions(mxsrc);
  if (mxGetNumberOfDimensions(mxsrc) != 3 || 
      mxGetClassID(mxsrc) != mxDOUBLE_CLASS)
    mexErrMsgTxt("Invalid input");  

  double scale = mxGetScalar(mxscale);
  if (scale > 1)
    mexErrMsgTxt("Invalid scaling factor");   

  int ddims[3];
  ddims[0] = (int)round(sdims[0]*scale);
  ddims[1] = (int)round(sdims[1]*scale);
  ddims[2] = sdims[2];
  mxArray *mxdst = mxCreateNumericArray(3, ddims, mxDOUBLE_CLASS, mxREAL);
  double *dst = (double *)mxGetPr(mxdst);

  double *tmp = (double *)mxCalloc(ddims[0]*sdims[1]*sdims[2], sizeof(double));
  resize1dtran(src, sdims[0], tmp, ddims[0], sdims[1], sdims[2]);
  resize1dtran(tmp, sdims[1], dst, ddims[1], ddims[0], sdims[2]);
  mxFree(tmp);

  return mxdst;
}

// matlab entry point
// dst = resize(src, scale)
// image should be color with double values
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { 
  if (nrhs != 2)
    mexErrMsgTxt("Wrong number of inputs"); 
  if (nlhs != 1)
    mexErrMsgTxt("Wrong number of outputs");
  plhs[0] = resize(prhs[0], prhs[1]);
}