quality deteriorates when saving

[LinPop0]

I drew art and wanted to save it in JPEG format. Everything has been saved, but the picture quality has deteriorated a lot.

[D Fosse]

Two things:

One, it's just pixels. Don't reduce pixel dimensions too much.

Two, jpeg is a lossy and destructive format. Any jpeg compression degrades the image somewhat, even at "maximum" quality. The compression is irreversible and cumulative, and every resave dgrades the image further. Don't use jpeg if you want full quality, use TIFF or PSD.

[Kukurykus]

Once someone wanted to check the loss of jpeg files quality with every save. I couldn't make it real. Of course I heard many times it's like that but I never could reproduce it. Would you be able to show some example, or just share a method for it? The only that worked was rotating the image before of each next save, but that's not the same: Jpeg Generation Loss Script / and the lacking video.

[D Fosse]

It's not a question of "how much" or whether you can see it or not in any given image. The point is that it happens, it can't not happen. It's how the jpeg algorithm works - there's no way to get exactly the same pixels out as you put in.

We've all seen the posts where users save to jpeg and can no longer get the numbers to match. No matter what they do, even at quality 12, the numbers just won't match. That's jpeg compression.

We're all perfectly fine with this. We all agree that for most practical purposes, it doesn't matter much and jpeg is just fine. The size reduction is more important. That's why we still use it.

But if it's important that nothing in the file changes, then you use TIFF or PSD, not jpeg.

[Kukurykus]

Okay, thank you. I accept of course of almost no change of saved jpg for the nth time, but my point was to find a way to make that change visible at sight after number of resavings of the same jpg, surley without any additional manipulations to produce them artificially.

[PECourtejoie]

HEllo, did you try the method in the link? between the last image and the first one?

[Kukurykus]

You mean a link from your other post? If so, not yet 😉 Is that change noticable?

[davescm]

Hi

You can compare by placing the images and using difference blend mode.

As to why it deteriorates, I have posted this before but it may be helpful here:

To understand what the control does and why data is still lost during the process regardless of its setting, it is worth understanding the steps in jpeg compression /decompression.

1. Colour transformation - the RGB image data is converted to Luminance and colour components.

2. The colour components are sampled at a lesser definition than the luminosity. This in itself loses some of the image data so is lossy.

3. The image data is split into blocks for processing using an algorithm known as the Discrete Cosine Transform which(this is not inherently lossy) but allows the next step to treat the high frequency components differently to the low frequency components.

4. The data is now quantized using algorithms that treat high frequency information differently to low but the aim throughout is to reduce the amount of data passed to the next step. This is controlled by the quality setting and the 12 setting gives more steps than lower settings. This stage is lossy and the biggest opportunity to reduce the image data in a lossy way.

5. Re-ordering and encoding using Huffman -Coding which compresses the data from the previous step but is recoverable.

Decompression reverses the steps, but of course the lossy steps cannot be reversed completely accurately so there is data lost at every compress/decompress, which is why the general advise is not to open (decompress) and re-save(compress) a jpeg file as losses are cumulative.

There is some comparative data here on the quantisation settings for different packages including Photoshop

https://www.impulseadventure.com/photo/jpeg-quantization.html

Dave

[Kukurykus]

Oh dear! When I find a time to study it? But yeah - it's much more than helpful!

[Janice5maps]

Hi Dave! I had originally asked my photographer to save the photos of my fine art in jpeg. I had absolutely no idea what I was talking about! Then I found out about them being "lossy" which I didn't even know existed. I have already uploaded them, deleted them, saved them in png. Yet they are still there. I don't know what I am doing. If I delete them from photoshop and my computer and download them again will that change anything? If I do that and save them as a tiff though the jpeg is still there, will that make a difference? Have I ruined them?

[PECourtejoie]

Hello, JPEG is a lossy format, that means that compression deteriorates the data. It is recommended to use native formats, like PSD or TIFF to store your master files, and export jpeg to different sizes needed with the lowest amount of compression possible according to the file size you need for online, or sharing purposes.

See the effects of JPEG compression: https://www.photoshopessentials.com/essentials/jpeg-compression/