VTK/Image Interpolators

Image interpolation is done internally by several VTK classes, but there is no easy way for users to interpolate an image directly. Neither is there any straightforward way of adding new interpolation methods to VTK, since each VTK class that uses image interpolation has its own internal code for this purpose. The goal of this project is to add a vtkImageInterpolator class to VTK that provides a clean API for image interpolation.

Background

Within the imaging pipeline, interpolation is performed by vtkImageReslice and its subclass vtkImageResample. These classes are use interpolation to resample a whole image, they are not useful when a user wants to interpolate just a single point. Also, the code for vtkImageReslice is quite complex, so adding new interpolation modes to this class is beyond the abilities of most VTK programmers, and any programmer who did so would either have to submit their changes to Kitware, or maintain their own version of vtkImageReslice.

The obvious way to resolve this issue is to move the interpolation code out of vtkImageReslice and into its own interpolator class (or classes), and add a SetInterpolator() method to vtkImageReslice. This is, in fact, something that I have wanted to do for around 10 years, but found very difficult to do because the vtkImageReslice interpolation code was very tightly integrated with the class for performance reasons. It was a requirement of this project that factoring out the interpolation code would not result in any significant loss of performance.

I had also planned to unify the interpolation code between the imaging pipeline and the geometry pipeline, so that these interpolators could be used with e.g. vtkProbeFilter, but decided to delay the unification until a future project. The main difference between the VTK image pipeline and the VTK geometry pipeline is that the former utilizes only scalars and structured coordinates for interpolation, while the latter utilizes all the arrays in the data set's PointData along with the point and connectivity information. It will be possible to provide a unified interface at some point in the future, by providing interpolation methods that operate on all of the PointData instead of just on the scalars.

The interpolator code is currently under review: [1]

Hierarchy

• vtkAbstractImageInterpolator - provides abstract interface
  • vtkImageInterpolator - basic linear, cubic, nearest-neighbor interpolation
  • vtkImageSincInterpolator - sinc interpolation
  • vtkImageBSplineInterpolator - b-spline interpolation (pending)