CODEWITHSUNDEEP
pythonopencvimage-processingcompressionimage-compression
Oaknut
Oaknut

Reputation: 11

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:

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

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.:

System info:

EDIT:

Conclusion (summary of the kind comments below):

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

Upvotes: 0

Views: 50

Answers (1)

Mark Setchell
Mark Setchell

Reputation: 207315

If, as stated, your goal is to retain as much quality as possible, you should use the write() method because it retains exactly the quality and metadata you got from the original feed.

Using cv2.imwrite() will definitely lose metadata and any attached colour profiles and also risks either losing quality, or adding unnecessary bloat but it cannot improve the quality.

The reason the two are different is most likely due to a different JPEG quality setting. You can explore the parameters of the original vs OpenCV-saved image with:

exiftool YOURIMAGE

Or, in a more targeted way, with:

exiftool -filesize -JPEGQualityEstimate -n YOURIMAGE

Output will look similar to this:

File Size                       : 292000
JPEG Quality Estimate           : 75

Upvotes: 1

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