[Python] OpenCV: Saving image files - Difference between cv.imwrite and f.write()?

I'm importing images from a security camera for image processing. After importing the image, I want to save the image for archiving (processing will happen on the fly).

I see two available methods for doing so:

• Python's built in file writer (which I will refer to as the "file method")
• OpenCV's imwrite (which I will refer to as the "OpenCV method"), which requires me to decode the image using NumPy

During testing using the code below, I found the following results:

• OpenCV method file (292 KB) is considerably larger than the file method's file (153 KB)
• Values such as dpi (96), bit depth (24) and pixel resolution (1920x1080) are identical between the files
• The file method's file contains more metadata, such as creation time, color representation and camera info (see screenshot ), likely due to NumPy/OpenCV not passing this data on.

Thus, my question is: Which method is better? I want to retain as much information as possible. Why is there such a disparity between the images?

import requests
import numpy as np
import cv2 as cv

response = requests.get(cam_url, auth=login)
image = cv.imdecode(np.frombuffer(response.content, dtype=np.uint8), -1)

with open('file_method.jpg', 'wb') as f:
f.write(response.content)

cv.imwrite('opencv_method.jpg', image)


N.B.:

• I cannot link the snapshots themselves for privacy reasons. But trust me, there's no discernable difference between the two.
• I am aware that using PNG files is preferred over JPEG, but that's beside the point here).

System info:

• Python 3.12.7
• OpenCV 4.10.0
• NumPy 2.1.3
• WSL2 (Ubuntu 22.04.5 LTS (GNU/Linux 5.15.167.4-microsoft-standard-WSL2 x86_64)

EDIT:

Conclusion (summary of the kind comments below):

• The first method saves the raw file directly, the second encodes it (NumPy) then decodes it (OpenCV) and then saves it.
• The encoding / decoding process strips the image of its EXIF metadata.
• The difference in file size is likely due to greater compatibility of the re-encoded version, which in my case is redundant.
• The first method results in a smaller file with EXIF data included, albeit with the constraint that the output image format needs to be the same as the input. The second method is "safer", allowing the user to choose the output format.

EXIF Info (from ExifTool):

File Method

ExifTool Version Number : 12.40
File Name : file_method.jpg
Directory : .
File Size : 153 KiB
File Modification Date/Time : 2025:01:20 11:26:40+01:00
File Access Date/Time : 2025:01:20 11:44:37+01:00
File Inode Change Date/Time : 2025:01:20 11:44:31+01:00
File Permissions : -rw-r--r--
File Type : JPEG
File Type Extension : jpg
MIME Type : image/jpeg
JFIF Version : 1.02
Resolution Unit : None
X Resolution : 1
Y Resolution : 1
Exif Byte Order : Little-endian (Intel, II)
Make : AXIS
Camera Model Name : P1455-LE
Software : 10.4.5
Modify Date : 2025:01:20 11:16:51
Exposure Time : 1/82
F Number : 429496729.5
ISO : 65535
Sensitivity Type : Unknown
Time Zone Offset : 1
Exif Version : 0231
Date/Time Original : 2025:01:20 11:16:51
Create Date : 2025:01:20 11:16:51
Sub Sec Time Original : 987
Sub Sec Time Digitized : 987
Flashpix Version : 0100
Color Space : sRGB
Exposure Mode : Auto
Serial Number : B8A44F0C0631
Comment : ..g�"�bg�"�b.
Image Width : 1920
Image Height : 1080
Encoding Process : Baseline DCT, Huffman coding
Bits Per Sample : 8
Color Components : 3
Y Cb Cr Sub Sampling : YCbCr4:2:0 (2 2)
Aperture : 429496729.5
Image Size : 1920x1080
Megapixels : 2.1
Shutter Speed : 1/82
Create Date : 2025:01:20 11:16:51.987
Date/Time Original : 2025:01:20 11:16:51.987
Light Value : 54.4


OpenCV Method

ExifTool Version Number : 12.40
File Name : opencv_file.jpg
Directory : .
File Size : 292 KiB
File Modification Date/Time : 2025:01:20 11:13:20+01:00
File Access Date/Time : 2025:01:20 11:26:58+01:00
File Inode Change Date/Time : 2025:01:20 11:13:20+01:00
File Permissions : -rw-r--r--
File Type : JPEG
File Type Extension : jpg
MIME Type : image/jpeg
JFIF Version : 1.01
Resolution Unit : None
X Resolution : 1
Y Resolution : 1
Image Width : 1920
Image Height : 1080
Encoding Process : Baseline DCT, Huffman coding
Bits Per Sample : 8
Color Components : 3
Y Cb Cr Sub Sampling : YCbCr4:2:0 (2 2)
Image Size : 1920x1080
Megapixels : 2.1

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