What is the correct way to change image channel ordering between channels first and channels last?

Question

I can not for the life of me figure out how to switch the image ordering. images are read in (x,x,3) format, theano requires it to be in (3,x,x) format. I tried changing the order with

numpy.array([img[:,:,i] for i in range(3)])

which I guess gets the job done, but it is both ugly and I can't figure out how to reverse it to get the original image back.

Answer 1

There are various methods to do that:

Reorder data

You can simply use numpy.rollaxis to move the axis 3 to position 1 

For example:

np.rollaxis(imagesArray, 3, 1)  

When using keras, you might want to change its configuration. But Theano library doesn't need anything if Keras is being used.

Keras can be configured with channels first or channels last, besides allowing you to define it in every individual layer, so you don't have to change your data.

To configure keras

You should find the keras.json file, then change it. The file is usually installed in

C:\Users\yourusername\.keras 

or 

~/.keras 

depending on your OS.

You should change "image_data_format": "channels_last" to "channels_first" or vice-versa

When working with "channels_last" is less troublesome because of a great number of other functions that work only on the last axis.