Repairing corrupted JPEG images from character replacement - jpeg

Question

Recently I got some corrupted JPEG images after a mistakingly input command:
~$> sed -i 's/;/_/g' *
After that, in the working directory and the subdirectories, Every byte '0x3b' in JPEG images became '0x5f'. Viewer apps displays the images corrupted, such as below:
corrupted image sample
I could not identify which byte should be recovered, and when I tried to validate the warning/error flags from the images with toolkits such as EXIFtool, they just returns OK as the corrupted JPEG is not literally BROKEN not to be opened by a viewer.
Images should be repaired, since there is no duplicated image backup for them, but I don't know how to start. Just replacing 0x5f with 0x3b again is not effective, since the number of cases would be too big (2^n I guess where there are n candidate 0x5f) to take the trial-and-error replacing way. I've just started parsing huffman table in a JPEG image header and hoping to identify the conflict point between huffman coded statement and binary, but not sure.
How can I recover the images in this situation? I appreciate your help.

Answer 1

There appear to be 57 incidences of 0x5f in your corrupted image. If you can't find a better way, you could maybe "eyeball" the effects of replacing the incorrect bytes in the image fairly quickly like this:
open the image in binary mode and read it all with JPEG = open('PdQpR.jpg','rb').read()
use offsets = [m.start() for m in re.finditer(b'_', JPEG)] to find the byte offsets of the 57 occurrences
display the image with cv2.imdecode() and cv2.imshow() and then enter a loop accepting keypresses with cv2.waitkey()
p = move to previous one of 57 occurrences
n = move to next one of 57 occurrences
SPACE = toggle between 0x5f and 0x3b
s = save current state
q = quit
I had a quick attempt at this but haven't had much success using it yet:
#!/usr/bin/env python3
import cv2
import re
import numpy as np
# Load the image
filename = 'PdQpR.jpg'
JPEG = open(filename,'rb').read()
JPEG = bytearray(JPEG)
# Find the byte offsets of all the underscores
offsets = [m.start() for m in re.finditer(b'_', JPEG)]
N = len(offsets)
index = 0
while True:
# Show user which entry we are at
print(f'{index}/{N}: n=next, p=previous, space=toggle, q=quit')
# Decode and display the JPEG
im = cv2.imdecode(np.frombuffer(JPEG, dtype=np.uint8), cv2.IMREAD_COLOR)
cv2.imshow(filename, im)
key = cv2.waitKey(0)
# n = next offset
if key == ord('n'):
index = (index + 1) % N
next
# p = previous offset
if key == ord('p'):
index = index -1
if index < 0:
index = N - 1
next
# q = Quit
if key == ord('q'):
break
# space = toggle between underscore and semicolon
if key == ord(' '):
if JPEG[offsets[index]] == ord('_'):
print(f'{index}/{N}: Toggling to ;')
JPEG[offsets[index]] = ord(';')
else:
print(f'{index}/{N}: Toggling to _')
JPEG[offsets[index]] = ord('_')
next
Note: Toggling some bytes between '_' and ';' results in illegal images and error messages from cv2.imdecode() and/or cv2.imshow(). Ideally you would wrap these inside a try/except and back out the last change if they occur. I didn't do that, yet.
Note: I didn't implement save function, it is just something like open('corrected.jpg', 'wb').write(JPEG)