# OpenCV PCA Compute in Python

### Question

I'm loading a set of test images via OpenCV (in Python) which are 128x128 in size, reshape them into vectors (1, 128x128) and put them all together in a matrix to calculate PCA. I'm using the new cv2 libaries...

The code:

import os
import cv2 as cv
import numpy as np

matrix_test = None
for image in os.listdir('path_to_dir'):
imgvector = imgraw.reshape(128*128)
try:
matrix_test = np.vstack((matrix_test, imgvector))
except:
matrix_test = imgvector

# PCA
mean, eigenvectors = cv.PCACompute(matrix_test, np.mean(matrix_test, axis=0))

And it allways fails on the PCA part (I tested the image loading and all, the resulting matrix is how it should be)...the error I get is:

File "main.py", line 22, in

mean, eigenvectors = cv.PCACompute(matrix_test, np.mean(matri_test, axis=0))

cv2.error: /path/to/OpenCV-2.3.1/modules/core/src/matmul.cpp:2781: error: (-215) _mean.size() == mean_sz in function operator()

1
9
5/16/2012 5:05:00 AM

I think the problem is with the size of

np.mean(matrix_test, axis=0)

Its size is (128x128,) and not (1, 128x128). Thus the code below should work

mean, eigenvectors = cv.PCACompute(matrix_test, np.mean(matrix_test, axis=0).reshape(1,-1))
8
12/19/2011 9:59:12 PM

You can also put

cv.PCACompute(matrix_test, mean = np.array([]))

and the function computes the mean.