A variety of techniques have been used.
Deferring calculations until needed
Each face had a normal vector and area stored in the nx,ny,nz,orig_area members of the FACE struct.
mris_fix_topology has a hot loop
mrisComputeOptimalRetessellation and mrisComputeRandomRetessellation have a similar structure
_: _: a computeDefectContext is constructed here
_:_: then it loops over a set of patches, or iterates on one patch. For each patch it calls
_:_:_: mrisDefectPatchFitness, which calls
_:_:_: _: mrisComputeDefectLogLikelihood, which calls
_:_:_: _: _: mrisComputeDefectMRILogUnlikelihood, which
_:_:_: _: _:_: does an expensive computation all the face normals for ALL the faces
_:_:_: _: _:_: does two other expensive steps, which only use a few of the face normals