Sliding semilandmark registration

This tutorial describes details of an alternative registration method for cortical bone mapping. It assumes that the reader is already familiar with the general wxRegSurf workflow. If you have not done so already, you should first read the femur tutorial.

When homologous point and curve landmarks can be identified on the surfaces, you might prefer to use the sliding semilandmark method for registering the surfaces. The sliding semilandmark algorithm is commonly used for geometric morphometrics, and guarantees alignment of the homologous features, while minimizing the bending energy of the thin plate spline that warps the LR landmarks onto the HR landmarks.
The first step is to manually identify landmarks on each of the surfaces. wxRegSurf understands two types of manually localized landmarks: points and curves. It is best to locate the landmarks using some other software package that allows visualization of the underlying data from which the surfaces were segmented. For example, you might use Stradwin to place landmarks with reference to the underlying CT data. Stradwin then exports the surface landmarks in the corresponding .ply files. In extremis, wxRegSurf does offer a rudimentary landmark editor. Clicking on a surface while holding down the shift key defines a point landmark. For open curves, press o, then shift-click on points along the curve, then press o again. For closed curves, press c instead of o. The example on the left shows several points (large spheres) and open curves (dashes) defined on an individual surface.
Homologous landmarks, with matching names, must also be defined on the canonical (LR) surface. In addition, a set of surface semilandmarks (small spheres, left) is required to ensure that the LR surface stays close to the HR surface everywhere. The surface semilandmarks should be a suitable subset of the LR vertices. There is currently no facility for generating them in wxRegSurf or Stradwin, though the femur and L1 vertebra canonical meshes, supplied with wxRegSurf, do include suitable sets of semilandmarks. Surface semilandmarks do not need to be defined on each individual, only on the canonical (LR) mesh.

Landmarks are stored in .ply|.wrl files as comments. The following .ply example should be self-explanatory:

comment LANDMARK "Radial extent of radial epicondyle" 0.13939 5.67478 57.00443
comment OPEN_LANDCURVE "Radial palmar condyle" 7
comment 4.28018 14.76402 56.43016
comment 3.12137 14.12788 55.24648
comment 2.30985 13.25270 54.62091
comment 1.42572 12.51070 54.90279
comment 0.76691 11.86253 55.92390
comment 0.70025 11.06597 57.07935
comment 0.79940 11.06698 58.44813
comment SEMILANDMARK 13.75 10.8253 4.70761 
The syntax is the same for .wrl files, except that comment lines start with <\tt># instead of comment. Note that the stored landmark coordinates do not have to lie precisely on the surface, since wxRegSurf projects them onto the surface when the .ply|.wrl file is read in. Note also that landmark locations may be specified using a single, integer vertex index, instead of the (x,y,z) coordinates.
Here are the individual and canonical surfaces (third metacarpals), aligned using wxRegSurf's Registration->Similarity option. Thus far, the landmarks have not been used in any way. The surfaces have just been brought into reasonably close proximity. But this is a good starting point for the next stage. Selecting Registration->Similarity+TPS, and pressing Start, projects the surface semilandmarks from the LR surface onto the HR surface, and the curve landmarks from the LR curves onto the corresponding HR curves. Landmarks are then slid on the HR surface, so as to minimize the bending energy of the thin plate spline that warps the LR landmarks onto the HR landmarks. Surface semilandmarks can slide in the surface tangent planes, while curve landmarks can slide along tangents to the curves. Point landmarks do not slide. After sliding, surface semilandmarks are projected back onto the HR surface, and curve landmarks are projected back onto the corresponding HR curve, ready for the next iteration.
After twenty iterations, the sliding semilandmarks have converged to stable positions. The homologous points and curves are perfectly aligned. Note that curve landmarks are allowed to slide off the end of open curves. If this behaviour is not appropriate, you should specify the end points of the curves as point landmarks.

A suitable workflow for sliding semilandmark registration and mapping, expressed as a batch file, is provided below.


# LR surface                    HR surface      LR data  HR data                   reflect  smth LR   smth HR  range  fussy    n iter  reg type  save reg  save LR data  save map  review       rims
#                                                                                                                     -1=auto          -=as->sm            -1 = no                 0 = no       0 = no
#                                                                                                                                                          0 = yes                 1-6 = angle  1 = yes
#                                                                                                                                                          >0 = smth               - = LR only  2 = to rims

# similarity registration
canonical_mc3_6756_lmarked.ply  individual.ply  n/a      n/a                       0        0         0        999    0        200     1         1         -1            0         0            1

# similarity+TPS registration
canonical_mc3_6756_lmarked.ply  individual.ply  n/a      n/a                       0        0         0        999    0        20      10        1         -1            0         0            1

# review
canonical_mc3_6756_lmarked.ply  individual.ply  n/a      n/a                       0        0         0        999    0        200     10        0         -1            0         1            1

# map
canonical_mc3_6756_lmarked.ply  individual.ply  n/a      individual_thickness.bin  0         0        2        999    0        0       10        0          4            0         0            1