Chapter 7: Customizing the Tracker

Because computer vision is an experimental science, feature tracking cannot be accomplished without introducing parameters that must be empirically determined. These parameters are hopefully stable enough that they can be considered constant and therefore ignored; nevertheless, KLT provides access to nearly all the parameters for those users who wish to change them. As mentioned in Chapters 1 and 2, these parameters are contained in the tracking context, whose members we now list, along with a brief description of each:

int mindist;              /* minimum distance between selected features */
int window_width;         /* dimensions of feature window */
int window_height;                       "
KLT_BOOL sequentialMode;  /* whether to save most recent image */
KLT_BOOL smoothBeforeSelecting; /* whether to smooth image before selecting features */
KLT_BOOL writeInternalImages;   /* whether to write internal images for later viewing */
int min_eigenvalue;       /* smallest eigenvalue allowed for selecting */
float min_determinant;    /* min determinant for declaring tracking failure */
float min_displacement;   /* amount of pixel shift for stopping tracking iterations */
int max_iterations;       /* max iterations before declaring tracking failure */
float max_residue;        /* max residue before declaring tracking failure */
float grad_sigma;         /* sigma of gaussian for computing gradient */
float smooth_sigma_fact;  /* sigma factor of gaussian for smoothing image */
float pyramid_sigma_fact; /* sigma factor of gaussian for computing image pyramid */
int nSkippedPixels;       /* used to speed up feature selection */
int borderx;              /* border in which features will not be selected, and */
int bordery;              /*    tracked features will be declared out-of-bounds */
int nPyramidLevels;       /* number of pyramid levels */
int subsampling;          /* amount of subsampling between pyramid levels */
(There are three additional members which point to the most recent image when the sequential mode is set, but these should never be accessed directly by the user. A complete description of the tracking context parameters can be found in the reference manual.)

All of these members are initialized to default values when the tracking context is created. Afterwards, they can be read and changed at leisure. The only exception is that sequentialMode must be set to FALSE via the function KLTStopSequentialMode().

A convenience function named KLTChangeTCPyramid() is provided for changing nPyramidLevels and subsampling. This function accepts the maximum search range as input, and computes and changes the two members accordingly. Therefore, the user can control these rather low-level parameters from a more high-level, intuitive objective. If desired, however, these members can be changed directly.

Another convenience function, KLTUpdateTCBorder(), automatically recomputes the borderx and bordery fields, which are dependent upon smooth_sigma_fact, pyramid_sigma_fact, window_width, window_height, and subsampling. Unless the user desires direct control over the border, this function should be called whenever one of these parameters is changed.

The example below illustrates how to change some of the parameters. It should be self-explanatory.

Example 5

Demonstrates manually tweaking the tracking context parameters.

#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();
     tc->mindist = 20;
     tc->window_width  = 9;
     tc->window_height = 9;
     KLTChangeTCPyramid(tc, 15);
     fl = KLTCreateFeatureList(nFeatures);

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

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

     KLTWriteFeatureListToPPM(fl, img1, ncols, nrows, "feat1b.ppm");

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

     KLTWriteFeatureListToPPM(fl, img2, ncols, nrows, "feat2b.ppm");