Python Decode base64 to picture not working

Question

I have got the following Base64 representation of an image I get send to my redis server from an application: https://1drv.ms/t/s!AkTtiXv5QMtWliMbeziGpd0t1EjW?e=JDHEkt

Here's an excerpt of the data for those who don't want to download the whole 13MB data file:

b'\\/9j\\/4b6URXhpZgAASUkqAAgAAAAMAAABBAABAAAAoA8AAAEBBAABAAAAuAsAAA8BAgAIAAAAngAA\\nABABAgAJAAAApgAAABIBAwABAAAAAQAAABoBBQABAAAA0gAAABsBBQABAAAA2gAAACgBAwABAAAA\\nAgAAADEBAgAOAAAAsAAAADIBAgAUAAAAvgAAABMCAwABAAAAAQAAAGmHBAABAAAA4gAAAIQCAABz\\nYW1

I tried to repaire the b64 with the following :

import base64

with open('Outputfirst.txt', 'r') as file:
    imgstring = file.read().replace('\n', '')

#get rid of wrong characters
imgstring = imgstring.replace("b'",'')
imgstring = imgstring.replace('\\','')
imgstring = imgstring.replace('"','')
imgstring = imgstring.replace('\'','')
imgstring = imgstring.replace(',','')
#take care of padding
if(len(imgstring)%4 ==1):
    imgstring = imgstring +"==="
if(len(imgstring)%4 ==2):
    imgstring = imgstring +"=="
if(len(imgstring)%4 ==3):
    imgstring = imgstring +"="

imgstring = bytes(imgstring,'utf8')

with open(filename, 'wb') as f:
    f.write(imgstring)
    
imgdata = base64.b64decode(imgstring)
filename = 'some_image3.jpg' 
with open(filename, 'wb') as f:
    f.write(imgdata)

But somehow I dont get the image back properly.

When I use this tool though https://base64.guru/tools/repair and take its output as input for my script, I get the image I want.

Would you be able to include the base64-encoded image, or a link to it, so that we can give it a try? — Adam Oellermann
– Adam Oellermann, Commented Mar 4, 2021 at 19:21
Thanks for your response.Can you get it from the onedrive link above ? 1drv.ms/t/s!AkTtiXv5QMtWliMbeziGpd0t1EjW?e=JDHEkt — Klaus Krawunkel
– Klaus Krawunkel, Commented Mar 4, 2021 at 19:35
What makes you think that all the things you've flagged as "bad characters" are really bad? Many of them look like artifacts of printing a string from Python, for example the leading b'. There's a 1/65536 chance of any 2-character sequence in a random bunch of bytes to match that. — Mark Ransom
– Mark Ransom, Commented Mar 4, 2021 at 20:04
I just tested it with and without and it seemed not to work with an initial b'. But this might be wrong because of my initial misstake. — Klaus Krawunkel
– Klaus Krawunkel, Commented Mar 4, 2021 at 20:58

jps · Accepted Answer · 2021-03-04 21:27:35Z

1

It seems \\n doesn't get filtered out.

The whole filtering and padding can be done like so:

with open('Outputfirst.txt', 'r') as file:
    imgstring = file.read().replace('\\n', '').replace('\\','').replace("b'",'')
    imgstring = imgstring + '=' * (4 - len(imgstring) % 4)

Padding with 3 '=' is invalid:

if(len(imgstring)%4 ==1):
    imgstring = imgstring +"==="

edited Mar 4, 2021 at 21:27

answered Mar 4, 2021 at 19:42

jps

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1 Comment

Klaus Krawunkel Over a year ago

Thank you so much. I missed one \. at the first filterig, because I thought replace('\n') would do the job . This took me hours. Hope this helps someody in the future.

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