Question

i’m very new to opencv…
i need to predict in between value from the two linear data array in opencv.Wat i’m going to do for that…For example we use interp1 function in matlab.

tab =
    1950    150.697
    1960    179.323
    1970    203.212
    1980    226.505
    1990    249.633
then the population in 1975, obtained by table lookup within the matrix tab, is

p = interp1(tab(:,1),tab(:,2),1975)
p =
    214.8585

How can we do this in opencv…please help me..Thanks in advance.

Answer 1

You could try to use the regression functions build into OpenCV. But for the simple linear interpolation you seem to be doing it might be easier to just write it yourself.

double interpolate(int x1, double y1, int x2, double y2, int targetX)
{
  int diffX = x2 - x1;
  double diffY = y2 - y1;
  int diffTarget = targetX - x1;

  return y1 + (diffTarget * diffY) / diffX;
}

This function linearly interpolates the target value given to two given datapoints.

If you want to use it like the matlab function, supplying all datapoints at once, you need a function which picks the two nearest neighbors. Something like this:

double interpolate(Mat X, Mat Y, int targetX)
{
  Mat dist = abs(X-targetX);
  double minVal, maxVal;
  Point minLoc1, minLoc2, maxLoc;

  // find the nearest neighbour
  Mat mask = Mat::ones(X.rows, X.cols, CV_8UC1);
  minMaxLoc(dist,&minVal, &maxVal, &minLoc1, &maxLoc, mask);

  // mask out the nearest neighbour and search for the second nearest neighbour
  mask.at<uchar>(minLoc1) = 0;
  minMaxLoc(dist,&minVal, &maxVal, &minLoc2, &maxLoc, mask);

  // use the two nearest neighbours to interpolate the target value
  double res = interpolate(X.at<int>(minLoc1), Y.at<double>(minLoc1), X.at<int>(minLoc2), Y.at<double>(minLoc2), targetX);
  return res;
}

And here is a small example showing how to use it:

int main()
{
  printf("res = %f\n", interpolate(1970, 203.212, 1980, 226.505, 1975));

  Mat X = (Mat_<int>(5, 1) <<
  1950, 1960, 1970, 1980, 1990);
  Mat Y = (Mat_<double>(5, 1) <<
  150.697, 179.323, 203.212, 226.505, 249.633);
  printf("res = %f\n", interpolate(X, Y, 1975));

  return 0;
}

I did not test this extensively. So you might need to fix some bugs.