[python] How can I add new dimensions to a Numpy array?

I'm starting off with a numpy array of an image.

In[1]:img = cv2.imread('test.jpg')

The shape is what you might expect for a 640x480 RGB image.

In[2]:img.shape
Out[2]: (480, 640, 3)

However, this image that I have is a frame of a video, which is 100 frames long. Ideally, I would like to have a single array that contains all the data from this video such that img.shape returns (480, 640, 3, 100).

What is the best way to add the next frame -- that is, the next set of image data, another 480 x 640 x 3 array -- to my initial array?

There is no structure in numpy that allows you to append more data later.

Instead, numpy puts all of your data into a contiguous chunk of numbers (basically; a C array), and any resize requires allocating a new chunk of memory to hold it. Numpy's speed comes from being able to keep all the data in a numpy array in the same chunk of memory; e.g. mathematical operations can be parallelized for speed and you get less cache misses.

So you will have two kinds of solutions:

1. Pre-allocate the memory for the numpy array and fill in the values, like in JoshAdel's answer, or
2. Keep your data in a normal python list until it's actually needed to put them all together (see below)

images = []
for i in range(100):
    new_image = # pull image from somewhere
    images.append(new_image)
images = np.stack(images, axis=3)

Note that there is no need to expand the dimensions of the individual image arrays first, nor do you need to know how many images you expect ahead of time.

This worked for me:

image = image[..., None]

I followed this approach:

import numpy as np
import cv2

ls = []

for image in image_paths:
    ls.append(cv2.imread('test.jpg'))

img_np = np.array(ls) # shape (100, 480, 640, 3)
img_np = np.rollaxis(img_np, 0, 4) # shape (480, 640, 3, 100).

Consider Approach 1 with reshape method and Approach 2 with np.newaxis method that produce the same outcome:

#Lets suppose, we have:
x = [1,2,3,4,5,6,7,8,9]
print('I. x',x)

xNpArr = np.array(x)
print('II. xNpArr',xNpArr)
print('III. xNpArr', xNpArr.shape)

xNpArr_3x3 = xNpArr.reshape((3,3))
print('IV. xNpArr_3x3.shape', xNpArr_3x3.shape)
print('V. xNpArr_3x3', xNpArr_3x3)

#Approach 1 with reshape method
xNpArrRs_1x3x3x1 = xNpArr_3x3.reshape((1,3,3,1))
print('VI. xNpArrRs_1x3x3x1.shape', xNpArrRs_1x3x3x1.shape)
print('VII. xNpArrRs_1x3x3x1', xNpArrRs_1x3x3x1)

#Approach 2 with np.newaxis method
xNpArrNa_1x3x3x1 = xNpArr_3x3[np.newaxis, ..., np.newaxis]
print('VIII. xNpArrNa_1x3x3x1.shape', xNpArrNa_1x3x3x1.shape)
print('IX. xNpArrNa_1x3x3x1', xNpArrNa_1x3x3x1)

We have as outcome:

I. x [1, 2, 3, 4, 5, 6, 7, 8, 9]

II. xNpArr [1 2 3 4 5 6 7 8 9]

III. xNpArr (9,)

IV. xNpArr_3x3.shape (3, 3)

V. xNpArr_3x3 [[1 2 3]
 [4 5 6]
 [7 8 9]]

VI. xNpArrRs_1x3x3x1.shape (1, 3, 3, 1)

VII. xNpArrRs_1x3x3x1 [[[[1]
   [2]
   [3]]

  [[4]
   [5]
   [6]]

  [[7]
   [8]
   [9]]]]

VIII. xNpArrNa_1x3x3x1.shape (1, 3, 3, 1)

IX. xNpArrNa_1x3x3x1 [[[[1]
   [2]
   [3]]

  [[4]
   [5]
   [6]]

  [[7]
   [8]
   [9]]]]

Alternatively to

image = image[..., np.newaxis]

in @dbliss' answer, you can also use numpy.expand_dims like

image = np.expand_dims(image, <your desired dimension>)

For example (taken from the link above):

x = np.array([1, 2])

print(x.shape)  # prints (2,)

Then

y = np.expand_dims(x, axis=0)

yields

array([[1, 2]])

and

y.shape

gives

(1, 2)

You're asking how to add a dimension to a NumPy array, so that that dimension can then be grown to accommodate new data. A dimension can be added as follows:

image = image[..., np.newaxis]

Pythonic

X = X[:, :, None]

which is equivalent to

X = X[:, :, numpy.newaxis] and X = numpy.expand_dims(X, axis=-1)

But as you are explicitly asking about stacking images, I would recommend going for stacking the list of images np.stack([X1, X2, X3]) that you may have collected in a loop.

If you do not like the order of the dimensions you can rearrange with np.transpose()

You could just create an array of the correct size up-front and fill it:

frames = np.empty((480, 640, 3, 100))

for k in xrange(nframes):
    frames[:,:,:,k] = cv2.imread('frame_{}.jpg'.format(k))

if the frames were individual jpg file that were named in some particular way (in the example, frame_0.jpg, frame_1.jpg, etc).

Just a note, you might consider using a (nframes, 480,640,3) shaped array, instead.

You can use np.concatenate() specifying which axis to append, using np.newaxis:

import numpy as np
movie = np.concatenate((img1[:,np.newaxis], img2[:,np.newaxis]), axis=3)

If you are reading from many files:

import glob
movie = np.concatenate([cv2.imread(p)[:,np.newaxis] for p in glob.glob('*.jpg')], axis=3)