Create a map of images in python numpy

Question

I have a numpy array of size image_stack 64x28x28x3 which correspond to 64 images of size 28x28x3. What I want is to construct an image of size 224x224x3 which will contain all my images that are in the initial array. How can I do so in numpy? So far I have the code for stacking the images in the same line, however I want 8 lines of 8 columns instead. My code so far:

def tile_images(image_stack):
    """Given a stacked tensor of images, reshapes them into a horizontal tiling for display.""" 
    assert len(image_stack.shape) == 4
    image_list = [image_stack[i, :, :, :] for i in range(image_stack.shape[0])]
    tiled_images = np.concatenate(image_list, axis=1)
    return tiled_images

Daniel · Accepted Answer · 2018-04-22 16:08:14Z

1

Does the following reshape, transpose, reshape trick work?

x.shape # (64, 28, 28, 3)
mosaic = x.reshape(8, 8, 28, 28, 3).transpose((0, 2, 1, 3, 4)).reshape(224, 224, 3)

The first reshape breaks your 64 into lines and columns. Transpose rearranges their order so that we can collapse them in a meaningful way.

Your function would then look like:

def tile_images(x):
    dims = x.shape
    assert len(dims) == 4
    stack_dim = int(np.sqrt(dims[0]))
    res = x.reshape(stack_dim, stack_dim, *dims[1:]).transpose((0, 2, 1, 3, 4))
    tile_size = res.shape[0] * res.shape[1]
    return res.reshape(tile_size, tile_size, -1)

edited Apr 22, 2018 at 16:08

answered Apr 22, 2018 at 15:36

Daniel

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2 Comments

Jose Ramon Over a year ago

So the mosaic needs to be the input of my function or to be inside my function too, right?

Daniel Over a year ago

No, the first example considers x to be your 4D image_stack, and assumes that you only have to do this once, so I hardcoded the numbers when computing mosaic. The second example will take as input the image_stack directly, as a 4D array, and return a 3D array.

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