Image from array and image using point

Question

I am using Image.point and Image.fromarray to do exactly the same operation on an image, increase the value of all pixels together by the same value. The thing is that i get to absolutelly different images.

using point

def getValue(val):
    return math.floor(255*float(val)/100)

def func(i):
    return int(i+getValue(50))

out = img.point(func)

using array and numpy

arr = np.array(np.asarray(img).astype('float'))
value = math.floor(255*float(50)/100)
arr[...,0] += value
arr[...,1] += value
arr[...,2] += value

out = Image.fromarray(arr.astype('uint8'), 'RGB')

I am using the same image (a jpg).

the initial image

the image with point using point

the image with arrays using arrays

How can they be so much different?

You have values greater than 255 in your array, which you then convert to uint8 ... what do you want those values to become in the image? — askewchan
– askewchan, Commented Nov 1, 2013 at 14:56
Actually your image looks right, but you've ruined palette (see previous comment). This link is relevant: stackoverflow.com/questions/2181292/… — nickzam
– nickzam, Commented Nov 1, 2013 at 14:58

askewchan · Accepted Answer · 2013-11-01 18:45:56Z

4

You have values greater than 255 in your array, which you then convert to uint8 ... what do you want those values to become in the image? If you want them to be 255, clip them first:

out_arr_clip = Image.fromarray(arr.clip(0,255).astype('uint8'), 'RGB')

By the way, there's no need to add to each color band separately:

arr = np.asarray(img, dtype=float)   # also simplified
value = math.floor(255*float(50)/100)
arr += value                           # the same as doing this in three separate lines

If your value is different for each band, you can still do this because of broadcasting:

percentages = np.array([25., 50., 75.])
values = np.floor(255*percentages/100)
arr += values   # the first will be added to the first channel, etc.

edited Nov 1, 2013 at 18:45

answered Nov 1, 2013 at 14:58

askewchan

46.7k18 gold badges125 silver badges135 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

askewchan Over a year ago

@Apostolos Yeah, clip is definitely easier, but kudos for solving it and posting your own solution :)

Apostolos Over a year ago

Thanks for helping me around...I am getting new to this image processing and its the first time i used numpy. Though arr contains alpha channel too and i don't want to alter it. So i have to do it to each band separately, i think i have to that is :)

askewchan Over a year ago

@Apostolos Ah, I see; if you want to edit only channels 0,1,2, then you can still do it in one line! arr[...,:3] += value

Apostolos · Accepted Answer · 2013-11-01 14:59:35Z

1

Fixxed it :)

Didn't take under consideration getting out of bounds. So i did

for i in range(3):
    conditions = [arr[...,i] > 255, arr[...,i] < 0]
    choices = [255, 0]
    arr[...,i] = np.select(conditions, choices, default=arr[...,i]

Worked like a charm....:)

answered Nov 1, 2013 at 14:59

Apostolos

8,18117 gold badges89 silver badges158 bronze badges

Collectives™ on Stack Overflow

Image from array and image using point

2 Answers 2

3 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related