GIMP JPEG Compression Subsampling

On Monday night, I was preparing 4 photos for a works photography competition. The images were ready and all that was left to do was to compress the photos down to under the 2mb maximum limit and get them emailed to myself at work ready to sumbit from my works email address in the morning.

The software I use to do the compression, and any other editting other than processing RAW files, is GIMP. I’ve never tried any other editting package and find the GIMP seems to be able to meet all of my needs at the moment.

In order to compress files in GIMP, it’s as simple as opening a JPEG selecting “Save As” and then giving the file a “.jpg” file name. GIMP will then prompt you to export the image file giving control over the jpeg compression settings. The two settings I change are:

  1. Qualtiy – Percentage of image quality to maintain
  2. Subsampling – How the compression process handles the colour spectrum of the image

When compressing the the images for the competition, I did what I normally do which is to set the subsampling value to “Smallest file” and dropped the quality value until the file size dropped below 2mb.

Once I arrived at work on Tuesday, I opened the images and to my disappointment found that the images all looked darker, softer and the colour detail seemed to be much less. I didn’t have time to wait till I got home and check the files on my home pc and had to submit them but I couldn’t help thinking I had made a big mistake using the “Smallest File” option and should have stuck with “Best Quality”.

Anyway, once I got home, I checked the files I had submitted on my home pc and found the detail was as it had been when I first emailed them to myself on Monday night and I’m putting it down to monitor settings at work. After checking the files, I thought it would be worth doing a direct comparision of the differences between subsampling. Below are cropped images using the two different sampling methods from 500kb versions of one of the submitted file:

Smallest File Crop

Smallest File Crop

Best Quality Crop

Best Quality Crop

From the above examples, zoomed in to 150%, I think it shows that there is virtually no difference in the actual sampling methods. I was quite suprised, but reasured by this.

For interest, the full size image is below:

Highlands Cow

Highlands Cow

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