I am playing with the cifar-10 dataset (available here) and for now I would like to plot one of the images of a batch.

The images are represented as vectors when I get them from pickle:

From the cifar-10 documentation:
The first 1024 entries (of an image) contain the red channel values, the next 1024 the green, and the final 1024 the blue. The image is stored in row-major order, so that the first 32 entries of the array are the red channel values of the first row of the image.

What I came up with to plot an image is this:

import numpy as np
import matplotlib.pyplot as plt

# get the dataset
a = unpickle('./cifar-10/data_batch_1')
# get the first image
img = np.array(a[b'data'][0])
# transform it to a 3 x 1024 array, one row per color channel 
# and transpose it to a 1024 x 3 array, one row per rgb pixel
img = img.reshape(3, 1024).T
# reshape it so we can plot it as a 32 x 32 image with 3 color channels
img = img.reshape(32, 32, 3)

# plot

It's my first attempt at matrix manipulation so even if this is concise, I feel like it could be simpler. What do you guys think?


1 Answer 1


One alternative is to transform it to the right shapes, then use moveaxis. I don't know how much simpler this is than what you've got, I guess it avoids one reshaping operation.

img = img.reshape(3, 32, 32)
img = np.moveaxis(img, 0, -1) # move the first axis to the end

or as a one-liner:

img = np.moveaxis(img.reshape(3, 32, 32), 0, -1)

Note that moveaxis returns a view, meaning that no data is copied.


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