Landmark Warp Frequently Asked Questions
How do I choose a basis function?
Some of the characteristics that differ between basis functions are number of parameters, and model used to derive them. A basis function with no parameters will be easy to use without much testing, but has no flexibility. Usually the value of the parameters will depend on the data set or how elastic the warp should be. Some functions have more local effects than others. A function that goes to zero rapidly will have less of a global effect on the warp. Finally, the some of the basis function were derived from physical models, and the user may wish to choose the basis function with this in mind. For example, thin plate splines model metal sheets, and elastic body splines model an elastic material in equilibrium (with options for the types of forces used).
Are there conditions such that a warp cannot be computed?
Yes, if the points are not in general position, an exception will be thrown. For example, to create a 3D warp, the points cannot be coplanar or collinear. Additionally, there cannot be duplicate points in the set of source points.
Why are contours resampled?
Two UCF contours could very well have different numbers of points representing them, due to the way they were created. Also, for most UCFs, fewer points will do a sufficient job in less time. Finally, to make correspondences, the homologous contours must have the same number of points.
What does the spline order have to do with the warping?
Very little. The spline order is the order of the polynomials used when resampling the contours. The only thing that this affects is the accuracy of the resampling.
