Saving a JPG at "12" image quality. What does this do and why is the file size larger?

[dnemeth01]

 When I open a JPG file and save it, I get a option to save in 1 thru 12 quality.
What does selecting 12 do besides increase the file size?

[D Fosse · Accepted solution]

The highest quality setting will inflict the least damage to the original data. But there will always be some.

https://en.wikipedia.org/wiki/JPEG 

It could be argued that saving a jpeg at quality 12 defeats the whole purpose of jpeg, which is to save bandwidth and storage space. It will be destructive in any case, so you might as well maximize payoff without losing too much quality. Around 8 is often a good compromise.

Don't resave a jpeg if it can be avoided! It will deteriorate with every new save. Use PSD or TIFF as working formats.

It should be said that content plays a part. Some types of images show the effects of the jpeg compression more easily than others, so this should be determined case by case.

[Eugene Tyson]

While I agree to a degree about this - I don't think it's hugely true.

It's been proven before that image saved with Image Quality set to 12 will have no differences.

You can try it yourself by making a change to a TIFF or PSD and save it.

Save it as  JPEG with image quality 12 

Overaly the 2 using Difference blend mode and there shouldn't be any changes.

I think resaving images as TIFF/PSD is completely fine if you're looking to retain layers etc.

Or savng as a Photoshop PDF to retain vector masks/shapes/layers etc. would be preferred (for output) from outside Photoshop 

Image Quality 12 for JPEGs is the highest quality jpeg.

I heard this many years ago - and maybe I'm wrong - but after seeing the post now I am wondering if any Photoshop Experts can confirm this:

The PS engineers actually only had originally a scale of 0-10 and expected most people to use these settings, which gave a compression level maximum of 11 steps - which I believe or remember was the old settings in Photoshop from years ago. But the PS engineers decided to add more levels 11, and 12 (making 13). And these were left in place so that the images could be used in other applications so that the other applications would manage the compression settings.

For years now I believed this to be true which is why I think saving as image quality 12 allows the compression to be handled by the output settings of where the image is being used... if that makes sense.

So - wondering if a PS expert can confirm this - and I'll happily eat humble pie and retract my claim. 

However, I believe that if you're going to save as JPEG quality 12 - then why not save as TIFF or PSD - as the compression will be handled elsewhere- unless you need JPEGs for web etc or to keep file sizes down. 

I'm not an advocate of creating additional files for no reason other than 'that's the way it always was'.

I believe in the day - with only 11 settings you had to save as TIFF or PSD to stop discarding of info.

But I have strongly believed for years that the settings up to 13 doesn't degrade the image.

Would love to know.

[D Fosse · Accepted solution]

The jpeg algorithm is destructive compression. That's how it works. It's just a matter of how much.

It may not be immediately visible at quality 12, but if you resave over itself it will degrade a little bit each time. In addition, jpeg only supports 8 bit depth.

The point is that jpeg is not a working format, it's for final delivery, and then you need to decide how much destruction is visually acceptable. And with that perspective, you can often go down quite a bit with no ill effects.

This is the same 8 bit file saved as TIFF and jpeg quality 12. This absolutely massive size reduction comes at a cost. There is no free lunch:

[Eugene Tyson]

Can you please do the comparison as I suggested overlaying both using the method I described.

I don't think there is any degradation - but could be wrong.

Thanks for the reply.

[Stephen Marsh]

"Visually lossless" (lossy) is not the same as lossless, after overlaying the jpeg12 file over the original, one should merge or flatten the two layers and use the equalise command.

[Test Screen Name]

"It's been proven before that image saved with Image Quality set to 12 will have no differences."

Then the proof was wrong, or happened upon a rare file that was not changed.

(The test you describe: can you see a greyscale value of 254,255,255 different from 255,255,255?)

JPEG is designed to change colours. It's not a fault, and there isn't an OFF switch. But that doesn't mean that you can use the human eye to see the difference. This is about maths.

[Eugene Tyson]

Fair enough. 

[jane-e]

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@dnemeth01 wrote:

 When I open a JPG file and save it, I get a option to save in 1 thru 12 quality.

When you open a jpeg and save it, don't resave it as a jpeg. Choose a format that is not lossy such as tiff or psd.

~ Jane

[Eugene Tyson]

Here's my original image I got off the internet

Here's the one I saved over 100 times as JPEG max quality 

 

And I've attached the tiff showing the differences between the two.

I maybe missing something here - but the quality isn't degrading.

[Eugene Tyson]

The image actually starts as JFIF

https://images.unsplash.com/photo-1526666923127-b2970f64b422?ixid=MnwxMjA3fDB8MHxzZWFyY2h8MXx8ZXh0cm...

[D Fosse]

But that's not really the point. I think we're discussing apples and oranges here.

We all agree that a jpeg can be visually perfectly fine, and immediately indistinguishable from the original, as long as you don't resave it. That's why we all still use jpeg.

The point is that some degradation always happens. Some bits always change, it's inevitable. But what I want is no degradation whatsoever, not the tiniest bit, visual or not. So I don't use jpeg as a working format, only as final delivery.

But when you do save as jpeg - why not go down to, say, 8 while you're at it? Most likely that too will be visually perfectly acceptable. But this time the file won't be 3 MB, it will be 500 kB! So now you're really getting payoff - from 22 MB to 500 kB. And it's still good enough to ship.

This is the same file as above, but now quality 8 vs quality 12:

[dnemeth01]

Thank you everyone for your input.

HOwever, what does saving at 12 actualy do to the JPG.  When I save to 12 the file size is jsut larger.

[D Fosse]

When you resave a jpeg at the same quality level, the file size always tends to increase a bit.

The reason for this is that the compression introduces additional artifacts, and these artifacts are seen as additional detail in the next round of compression. High frequency detail compresses less effectively and results in a larger file.

So it has nothing to do with quality level as such. It's just the destructive jpeg compression at work.

Don't resave jpegs if it can be avoided.

[Eugene Tyson]

Don't resave jpegs if it can be avoided.

By @D Fosse

This is the bit I disagree with - saving as JPEG 12 Max Quality has no degradation.

And my initial question wasn't answered either - about the levels 12/13 being added to the quality to allow other programs to handle the compression - can anyone confirm this?

[D Fosse]

Here's a psd, and a jpeg quality 12. Look closely at the histograms. The fuzzy histogram in the jpeg version is destructive jpeg compression, at highest quality. Pixels are changed:

[Eugene Tyson]

But that's saving from PSD to JPEG.

Not resaving over JPEGs.

I think we are talking about different things here.

[D Fosse]

Why would that change anything? Saving from PSD is best case scenario. Resaving over an existing jpeg is worst case scenario, resulting in even more artifacts -

Every jpeg save runs the compression algorithm from scratch. As long as you get the options dialog, it's a new round of compression.

Look, I agree it's "not much" and for many practical purposes it won't matter. The point is that it happens at all. It's not non-destructive.

[Eugene Tyson]

No it's not just larger.

JPEG has Quality levels. 

The lower the number the lower the quality - and lower the file size. 

But you're sacraficing quality for file size.

The higher the number the better to retain the information originally there.

[davescm]

Hi

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

[Eugene Tyson]

Thank you, @davescm