[scikit-learn] Should we standardize data before PCA?

[Shiheng Duan]

Hello all,

I wonder is it necessary or correct to do z score transformation before
PCA? I didn't see any preprocessing for face image in the example of Faces
recognition example using eigenfaces and SVMs, link:
http://scikit-learn.org/stable/auto_examples/applications/plot_face_recognition.html#sphx-glr-auto-examples-applications-plot-face-recognition-py

I am doing on a similar dataset and got a weird result if I standardized
data before PCA. The components figure will have a strong gradient and it
doesn't make any sense. Any ideas about the reason?

Thanks.
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