Why numpy.asarray return an array full of boolean

Question

If I want to have an array filled whith 0 or 1 depending of the pixels value in a image, I write this :

image = "example.jpg"
imageOpen = Image.open(image)
bwImage = imageOpen.convert("1", dither=Image.NONE)
bw_np = numpy.asarray(bwImage)
print(type(bw_np[0, 0]))

Result :

<class 'numpy.bool_'>

Because of the .convert bilevel mode "1", the array must be full of 1 and 0. https://pillow.readthedocs.io/en/3.1.x/reference/Image.html#PIL.Image.Image.convert

When I try something simpler :

bw_np = numpy.asarray([0, 1])
print(type(bw_np[0]))

Result :

<class 'numpy.int32'>

But instead of the second example, the first is full of true and false. So Why ?

Dorian Turba · Accepted Answer · 2018-06-25 13:37:19Z

2

In a nutshell : In python True is 1 and False is 0. This should correct this weird behavior :

bw_np = numpy.asarray(bwImage, dtype=int)

Long answer : Maybe imageOpen.convert("1", dither=Image.NONE) prefer bool instead of int32 for a better memory management :

import sys
import numpy

print("Size of numpy.bool_() :", sys.getsizeof(numpy.bool_()))
print("Size of numpy.int32() :", sys.getsizeof(numpy.int32()))

Result :

Size of numpy.bool_() : 13
Size of numpy.int32() : 16

edited Jun 25, 2018 at 13:37

answered Jun 12, 2018 at 16:50

Dorian Turba

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