First of all I would like to thank the quickness and the great number of
contributions in answering my question about shark morphometrics:
although in some concret aspects there are contraditory- or perphaps
complementary- advices, in general it has clarified me lots of doubts.
I would like to make a comment with respect to the artifact of Partial
Warp 1 and 3 that I talk of in my previos mail. Using the visualization
plot of the partial warps, choosing to see the changes as vectors and
using the resistant fit algorithm I've "discovered" that the change was
due to few landmarks (in case of the PW1 the bending of the tail, not a
bending of the whole body), which is more consistent with the reality.
Maybe this is useful to somebody...
Finally, I would like to make one question (or-better said- I would like
you to criticise what I think) about the overall ,general (and-I
suppose-oversimplified) procedure that the program performs to find the
relative warps from the landmarks of the digital images:
-As far as I know, the first step is to find the consensus configuration
from the superimposition of a set of configurations, which implies the
use of the Generalized procrustes analysis (GPA) (also called the
Generalized least squares (GLS)), the resistant fit or the use of other
algorithm.
-Once this is obtained, it is computed the "energy" needed to warp or
deform landmarks of each individual to the consensus configuration. From
this it is obtained the bending energy matrix.
-The partial warps are the eigenvectors of the bending energy matrix.
-The relative warps are the result of a PCA performed from the weight
matrix (the matrix of the partial warp scores and the uniform
component).
Do I have guessed something right?
Javier
Mariana
Barcelona
(Spain)
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