CODEWITHSUNDEEP
pythonpython-imaging-library
user1406177
user1406177

Reputation: 1370

Image.fromarray changes size

I have data that I want to store into an image. I created an image with width 100 and height 28, my matrix has the same shape. When I use Image.fromarray(matrix) the shape changes:

from PIL import Image

img = Image.new('L', (100, 28))
tmp = Image.fromarray(matrix)
print(matrix.shape) # (100, 28)
print(tmp.size) # (28, 100)
img.paste(tmp, (0, 0, 100, 28) # ValueError: images do not match

When I use img.paste(tmp, (0, 0)) the object is pasted into the image, but the part starting with the x value 28 is missing.

Why does the dimension change?

Upvotes: 1

Views: 7485

Answers (2)

Harrison Fang
Harrison Fang

Reputation: 3

NumPy and PIL have different indexing systems. So a (100, 28) numpy array will be interpreted as an image with width 28 and height 100.

If you want a 28x100 image, then you should swap the dimensions for your image instantiation.

img = Image.new('L', (28, 100))

If you want a 100x28 image, then you should transpose the numpy array.

tmp = Image.fromarray(matrix.transpose())

More generally, if you're working with RGB, you can use transpose() to only swap the first two axes.

>>> arr = np.zeros((100, 28, 3))
>>> arr.shape
(100, 28, 3)
>>> arr.transpose(1, 0, 2).shape
(28, 100, 3)

Upvotes: 0

pommicket
pommicket

Reputation: 943

PIL and numpy have different indexing systems. matrix[a, b] gives you the point at x position b, and y position a, but img.getpixel((a, b)) gives you the point at x position a, and y position b. As a result of this, when you are converting between numpy and PIL matrices, they switch their dimensions. To fix this, you could take the transpose (matrix.transpose()) of the matrix.

Here's what's happening:

import numpy as np
from PIL import Image

img = Image.new('L', (100, 28))
img.putpixel((5, 3), 17)

matrix = np.array(img)

print matrix[5, 3] #This returns 0 

print matrix[3, 5] #This returns 17

matrix = matrix.transpose()
print matrix[5, 3] #This returns 17

print matrix[3, 5] #This returns 0

Upvotes: 3

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