## Chapter 3: Replacing Lost Features

KLTTrackFeatures() sometimes loses features (either because they actually became occluded or go out of bounds, or because the computation fails for one reason or another). If it is desired to always maintain a certain number of features, e.g. 100, then the lost features can be replaced by calling KLTReplaceLostFeatures(). This function calls the same underlying computation as KLTSelectGoodFeatures() to find all the features in the image and rank them accordingly. Then, if k features have been lost, the k best features are used to replace them.

Below is sample code using this function.

### Example 2

    /**********************************************************************
    Finds the 100 best features in an image, tracks these
    features to the next image, and replaces the lost features with new
    features in the second image.  Saves the feature
    locations (before and after tracking) to text files and to PPM files.
    **********************************************************************/

    #include "pnmio.h"
    #include "klt.h"

    void main()
    {
         unsigned char *img1, *img2;
         KLT_TrackingContext tc;
         KLT_FeatureList fl;
         int nFeatures = 100;
         int ncols, nrows;

         tc = KLTCreateTrackingContext();
         fl = KLTCreateFeatureList(nFeatures);

         img1 = pgmReadFile("img0.pgm", NULL, &ncols, &nrows);
         img2 = pgmReadFile("img1.pgm", NULL, &ncols, &nrows);

         KLTSelectGoodFeatures(tc, img1, ncols, nrows, fl);

         KLTWriteFeatureListToPPM(fl, img1, ncols, nrows, "feat1.ppm");
         KLTWriteFeatureList(fl, "feat1.txt", "%3d");

         KLTTrackFeatures(tc, img1, img2, ncols, nrows, fl);
         KLTReplaceLostFeatures(tc, img2, ncols, nrows, fl);

         KLTWriteFeatureListToPPM(fl, img2, ncols, nrows, "feat2.ppm");
         KLTWriteFeatureList(fl, "feat2.txt", "%3d");
    }