PDF JPEG Compression Question

[ChristiduToit]

Quick question regarding exporting PDF's and their settings. 
 

In Affinity apps, when exporting any PDF format, why is the "Allow JPEG Compression" box automatically ticked and lowered to 85 (Quality) under the advanced settings/"More" tab?
What exactly does this do - does this value affect all raster layers/graphics when exporting the PDF?
For instance, if I work in Affinity Photo, and I have a bunch of pixel layers (let's say, a detailed digital painting), when exporting, will the quality of those layers and the final output all drop to 85%?
Unchecking the "Allow JPEG Compression" box dramatically increases the PDF file size, and I've been unchecking it for client work to be safe, but I still don't exactly know what it does or if it's even necessary.

Will this affect the final print quality of raster graphics in the PDF?
If I were to set up and send a raster t-shirt design to a client, should this box be unchecked to avoid loss of print quality?
If so, should this not be unchecked by default?

Any insight on this would be much appreciated!
I fear that I may have sent a lot of print-ready PDF files to clients over the last year that were, accidentally, at 85% of their original quality. 😨
If this is really what it does, then it's a bit scary that it's hidden under advanced settings when exporting, as I've unknowingly completely overlooked it for a long time.

[lacerto]

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[GarryP]

To add to what Lagarto said above, the “Allow JPEG Compression” option, when active, tells the software that it can – if it thinks it is necessary – compress any or all raster images in the PDF. Whether any particular image in the PDF will be compressed, when this option is active, is down to how the software is coded and the user has no say in that decision is made.
As for “loss of quality” that’s pretty much down to the “eye of the beholder”. “Quality”, in this case, is something very subjective. Basically (and it’s way more complicated in practice than this), 85% Quality means that you are allowing the software to remove 15% of the detail in the image to be able to make the image smaller, if it thinks it can do so without losing major detail. However, we have no control over which detail is lost, but that might not matter at all depending on the image.
I’ve attached three images, one as a lossless PNG, another as a JPEG at 100% quality and another as a JPEG at 85% quality. Without looking at the filenames, can you tell which is which? I’m guessing that most people can’t.
Another thing to take into consideration is what will happen to the image. For instance, when printing onto a T-Shirt, the medium that’s being printed on will probably remove a good amount of the “quality” of the image purely by down to the fact that it’s not an entirely flat surface. T-Shirt material is relatively mountainous (with massive dips) when compared to a piece of paper and so detail will be lost during the printing process anyway – probably more detail lost than during the JPEG compression.
My recommendation would be to look at an 85% quality version of the image and see if you can tell where detail has been lost. If you can’t see any glaring problems within the first couple of seconds then there’s probably a good chance that no-one else will either.
In other words, don’t worry about it until/if it actually becomes a problem.
If you do worry then you can always set your Export Preset to be 100% and never have to worry about it again.

[ChristiduToit]

@Lagarto @GarryP
Thanks for the insight!
I'm not too worried about digital, because in that realm 'what you see if what you get' for the most part - i.e. if it looks fine on screen, then it's probably good to go.
Social media sites, for the most part, compress images so much already anyway.

In print, however, any loss of quality can quickly become apparent - often times you'll be able to see unwanted artefacts and subtle pixelation a lot more easily than you would on screen. 
The "Allow JPEG Compression" setting is fairly foreign to me, as I've never had to pay any attention to that with other apps, such as Photoshop.
With that in mind, I'm also not sure why it would automatically be ticked for presets like PDF/X-4 which is specifically intended for professional print quality, and definitely not for something like a web-based on interactive PDF.

Let's say, for arguments sake, an unaware professional photographer is exporting work for a gallery show (archival giclee prints), and decides to use PDF/X-4 instead of TIFF (most printing companies, fine art or otherwise, prefer PDFs nowadays). Said photographer may have switched from Adobe to Affinity, may not be aware that "Allow JPEG Compression" even exists, and would end up with sub par print quality and may be completely oblivious as to why that is. 

I would hate to spend days working on a professional industry project (which a client is paying me to do), and then to send them a file with lower quality without even realising.

It's a very vague setting, and if it really does impact any and all raster work being exported in PDF format, surely it should be made a bit more obvious to the user when exporting?

I'd love for one of the devs to shine some more light on this just so that we're all aware what exactly this setting does under the hood, and how it affects the final outcome and quality of the exported file. 
Basically, what does it do, and should I be losing sleep over this setting? 😅

In the meantime I'll definitely create a preset with the box unchecked, but I'd still love to know what the significance of the setting is so that I can avoid any potential future pitfalls.

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[GarryP]

I think 85% is the ‘normal’ compression percentage for most image processing software.
I don’t use Photoshop but I think GIMP uses 85% (or thereabouts) by default and I remember seeing similar values elsewhere too.
However, as Lagarto said above, the ‘best’ value really depends on the specific image and there’s no value that will be ‘best’ for all images, other than 100%. If you are using compression, and image quality is important, then you need to check each image individually.
If you are sending things to be printed and you have no problem with file sizes then just switch compression off permanently and you never have to worry about it again.

I think it’s deliberately vague as explaining what it does would take up way too much space in the UI. It’s a bit like saying “I’d like this sponge cake to be 15% less yummy.” How would you explain what “yummy” meant? It might not mean the same to others as it does to you. Is creaminess more “yummy” than juiciness? How much is 10% less creamy? Is juiciness more “yummy” than sponginess? How much is 15% less spongy? Is sponginess more “yummy” than… etc. etc.

Here’s a link to an explanation of JPEG compression: https://en.wikipedia.org/wiki/JPEG

It's very complicated. I think this is one of those cases where you just have to let the software do its thing and not fret over how it does it.

[ChristiduToit]

@GarryP
At design college we were always told that, under normal circumstances, PDF/X (or TIFF) would always yield the best printing results - I feel betrayed, haha!

I must admit, I still feel that it's a bit vague in that I'm exporting a PDF, not a JPEG, so perhaps a label like "Image Compression" or "Pixel/Raster Compression" would be a bit more clear and self-explanatory - at least then I know what to expect. What's even weirder is that when you export a JPEG, there's no "JPEG Compression" slider, but rather a "Quality" slider. 

I understand that each image is affected differently though, and a lot of what I do (especially for t-shirts) is solid flat colours for burning screens, so the compression, if any, would be fairly minor.

Aaaanyway, I think I'll probably play it safe and stick to 100% "yummy" for now, especially for client work, but I appreciate all the effort you've put into looking into this.

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[Dave Harris]

2 hours ago, Lagarto said:

Normally PDF compression is a one-time procedure and does not accumulate but now that people can open PDF files (and seem to frequently do so, especially with Affinity apps), there is a danger that PDF files are treated as a kind of standard documents, and then the compression effect would also accumulate and quality of exported images get increasingly worse.  

Affinity will store the compressed data from the PDF in that case, and will only recompress if something is done to the image.

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[GarryP]

I just meant that 85% (or thereabouts), rather than 100%, seemed to be the default quality percentage given out-of-the-box, as far as I can remember, in most software I have used when the user is asked about compression of images. I can’t remember ever seeing a default value of 100% being given – if the user has not previously specified it – but my memory is bad.

[Dave Harris]

2 hours ago, Lagarto said:

Good to know. Is this something that non-Affinity apps know, as well, or proprietary data saved off-the-file by Affinity apps / documents?

It's stored with the rest of the image data in the Affinity document. Our file format isn't documented and shouldn't be a concern of other apps. It should just mean that importing a PDF and then exporting it repeatedly won't degrade the image.

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