You should almost always call `xform`

rather calling
than`xformpoints`

directly.

xformpoints(reg, points, ...)
# S3 method for character
xformpoints(reg, points, ...)
# S3 method for cmtkreg
xformpoints(reg, points, transformtype = c("warp",
"affine"), direction = NULL, FallBackToAffine = FALSE, ...)
# S3 method for reglist
xformpoints(reg, points, ...)
# S3 method for default
xformpoints(reg, points, ...)

## Arguments

reg |
A registration defined by a matrix, a function, a
`cmtkreg` object, a `reglist` object containing a
sequence of arbitrary registrations, or a character vector specifying
path(s) to registrations on disk (see details). |

points |
Nx3 matrix of points |

... |
Additional arguments passed to methods |

transformtype |
Which transformation to use when the CMTK file contains
both warp (default) and affine |

direction |
Whether to transform points from sample space to reference
space (called **inverse** by CMTK) or from reference to sample space
(called **forward** by CMTK). Default (when `NULL` is inverse). |

FallBackToAffine |
Whether to use the affine transformation for points
that fail to transform under a warping transformation. |

## Details

If a list of transformations is passed in, these transformations are
performed in sequence order, such that ```
xformpoints(c(a,b,c), x) ==
xformpoints(c, (xformpoints(b, xformpoints(a, x))))
```

Note that the direction of CMTK registrations can be the source of
much confusion. This is because CMTK defines the *forward* direction
as the transform required to reformat an image in *sample* (floating)
space to an image in *template* space. Since this operation involves
filling a regular grid in template space by looking up the corresponding
positions in sample space, the transformation that is required is (somewhat
counterintuitively) the one that maps template to sample. However in
neuroanatomical work, one often has points in sample space that one would
like to transform into template space. Here one needs the *inverse*
transformation.