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[6233638]

I really care about album art - especially since I got an iPad 3 with its 2048x1536 resolution display. It is now more important than ever to grab the highest resolution, highest quality files possible for it.

Similar to initially ripping your CD library as MP3s because "it sounds good enough" or to save space, and then having to go through the whole process again to rip to lossless (I'm sure many of us have done that) finding good album art is not something I want to go through again at some point in the future.


A major issue - that I have just discovered as I was writing this post - is that Media Center is only writing JPEG files into file tags. (Specifically Apple Lossless in my case) While more formats are supported, Media Center is converting them to JPG and doing a bad job of it at that. Source PNG, Resulting JPG.

Previously I had always been storing a copy of my cover art with the files just in case anything ever happened to them, but I stopped this practice and removed the existing files, because Media Center wouldn't handle file renaming properly unless the folders only contained music files. (and I did not have the patience to set up a "maintenance view" as suggested)

Fortunately most of my cover art was already JPEGs, and Media Center doesn't appear to re-compress those, but it's not something I have actually tested.
The problem is that I don't know which albums were actually using PNG art - I can think of a few, but not all of them, and I don't know that I will be able to source the originals again.


And now what I had intended discussing:

While Media Center's "Get from internet..." option is sometimes useful, more often than not, I will get similar or better results from a Google search.
There are a few big issues I have with "Get from internet..." as it currently stands.

Firstly, it seems to primarily look for files under 1000px. It is very rare for there to be anything above 1000x1000 shown in the search results, even if much better quality artwork exists.

For example, here's a search for Norah Jones' Come Away with Me:


You can see that there are five results, all of which are under 1000px, and most of them are half that resolution.

The Google search I use typically finds much better results.

Code: [Select]

https:////www.google.com//search?as_st=y&tbm=isch&as_q=Hexify([Album Artist (auto)] [Album])&safe=images&tbs=isz:lt,islt:2mp,iar:s,imgo:1&biw=1920&bih=984
Now there are two good results from that search, but even if they had shown up in Media Center's results, I have no way of evaluating the quality of them.
(p.s. is there a way to have Media Center open multiple links at once? I would prefer search results for "Large Images", "Larger than 2MP", and "Larger than 4MP" to open with a single click)

If I maximize the window, I get a somewhat larger preview of the covers, but nothing that is going to show me the difference in quality between all the 500px images it found.
While you might look at file size as an indicator of quality, a high quality scan will often have a smaller file size than one which is covered in moiré or other artefacts.

If you hover over a thumbnail for a few seconds (I would prefer this to be instant) you get a 480px preview, but that generally means that images are all going to be scaled down, and it is not a good representation of quality.

What I would find much more useful would be a loupe, rather than a scaled-down preview.


In many cases, the difference in quality should be obvious. For example, the highest resolution result for Queen - News of the World on Google is 1417x1417 - but it is horribly compressed.
After that, there are a number of files at 1000x1000 but they vary in quality hugely - some of them are even from the same source, but have been re-compressed, or sometimes edited and re-compressed: Example 1, Example 2, Example 3

If those had been the results that I got in Media Center's search, I wouldn't know which one to choose. (though that 1417px file and Example 1 are so bad they should be obvious even when scaled down to 480x480)

Only that last result is acceptable, because all the others are showing horrible compression artefacts in the red text. You might think it is washed out (though I seem to recall my LP of it looking like that) but that is an easy fix - which leads me on to my third issue:


Cover art submissions seem to be ignored.

Any time I am dissatisfied with the results from Media Center - whether it's the quality of the results or the resolution - I will seek out the best quality cover art I can find. If I can't find anything suitable at all, I will even go as far as to scan in the cover myself and touch it up, or edit one of the better scans out there. (for example using FFT to remove raster patterns) Now I don't have the best scanner - it's not color managed, and it's a sheet-fed document scanner rather than a flatbed, but it does give better results than are available online for some more obscure albums, or even some that aren't so obscure, but haven't had a recent re-release/remaster. (Queen Rocks, for example)

I submit these covers to the database, but I have not had a single one show up in my search results when I have tried it.

It would also be nice if there was some indicator to show whether a cover has been user-submitted - which I would expect to be high quality - or if it is from a general cover search.


And finally, thumbnails in the library seem to be of very poor quality. As you scale them up it seems to improve, but at the size I normally leave the thumbnails (128x128) they can look really bad:

[Matt]

Cover Art > Get From Internet is a JRiver web service (part of YADB).

We cap the size of cover art it is willing to store.

Currently it considers 500x500 the sweet spot and doesn't store art larger than about 1000x1000.

[JimH]

If you use MC to get art, it uses a pretty sophisticated system.  It was built five or six years ago, and it has proven to work very well.

When you do one album at a time, it will let you choose which art to use.  It will never replace your art unless you tell it to.

When you do multiple files at a time, it will use an automatic mode and choose the best art it finds.

Best is a relative term.  The highest pixel count may not be the best.  Morey patterns, bad scans, bad crops, etc.

Best is determined over time by a "voting" system.  The art that is chosen by users most often gradually rises to the top.  The art that is never chosen eventually gets discarded.

But you are free to do whatever you think is best.

[JimH]

Currently it considers 500x500 the sweet spot and doesn't store art larger than about 1000x1000.

We might reconsider that now.

[6233638]

Cover Art > Get From Internet is a JRiver web service (part of YADB).

We cap the size of cover art it is willing to store.

Currently it considers 500x500 the sweet spot and doesn't store art larger than about 1000x1000.

With the increasing number of high DPI devices (1080p displays on phones, even) doesn't that seem awfully low resolution? Especially when there is some very high resolution art available for more recent albums, that isn't even scanned. (example)

That's not to say that super high resolution art is useful today - but as I said at the start of my post, it's equivalent to having the "lossless" version of your music stored, rather than a compressed track. At some point you're going to regret only having 500x500 art (I do today) and that 11MP art is going to look great on your 4K or 8K display.

Can anything be done about Media Center converting PNG art to JPG? For the files where I still have the original, I have re-saved them as 100% quality JPG in the meantime. It increases the size, but at least it looks OK.

What about the quality of thumbnails? I'm sure that's done for speed, but perhaps it could be a user preference?

Best is a relative term.  The highest pixel count may not be the best.  Morey patterns, bad scans, bad crops, etc.

Best is determined over time by a "voting" system.  The art that is chosen by users most often gradually rises to the top.  The art that is never chosen eventually gets discarded.

I agree that resolution is not the only determining factor (and I posted some examples of that) and that some kind of "voting" system would be best - even if that simply goes by the art most people choose from the list. But as it stands right now, there's no way to actually evaluate the quality of the art within Media Center itself.

[Matt]

YADB's only limit is at 1000x1000 (or close).  Voting decides the rest.

But you'll see a lot of good 500x500 artwork (I think Amazon made this common), so that size often has the most votes.

As for thumbnail quality, the thumbnail engine will switch between small / medium / large / full resolution images as you change sizes.  You may find boundary conditions between the size changes that look slightly less sharp than right after the size change.  By definition, thumbnailing requires some compromises between speed, performance, and size (or else you'd just always use the full resolution image).

[JimH]

...  as it stands right now, there's no way to actually evaluate the quality of the art within Media Center itself.

Your screenshot of Nora Jones was exactly that.  You can look at the choices and make one of your own.

[6233638]

Your screenshot of Nora Jones was exactly that.  You can look at the choices and make one of your own.

So which of them is best? Other than some contrast differences, they all look pretty similar in the previews.

If we go with the largest dimensions, you get this. It's clearly a scan, full of compression artefacts, and it has a light border that you won't notice until it's displayed over a dark background.
Based off the 480px preview that Media Center gives you, it looks fine though - it actually seems to be the best choice. (good contrast & color)
At least with some kind of loupe that gives you a 100% (or greater) view of the file, you can see if there are obvious compression or scanner artefacts.

If you use filesize, you get this file - and it looks like two of the 500px images are based off this source. (based off the contrast)
The contrast is jacked up and the left sixth of the image is just black - no detail. Color is wrong too - the blue background has a purplish hue.

And then you have the Google result: A very high quality file that clearly came from the source art that was used on the album cover, and not based on a scan or otherwise manipulated. (contrast etc.)
And it's only 68KB more than the 600px cover.


I understand that it's very difficult to have an automated system that gets good results - I am actually impressed by how few unrelated results have shown up with the "Get from internet..." command.
But there's an increasing number of cover art going online that is sourced from the original art rather than being scanned in - but you often won't find it if you are only looking at images 1000px or smaller. (it's usually 1500px+)


I guess I will just stick with what I am doing and manually search for good art, I was just wondering why it wasn't finding the newer high resolution art available for some of my albums, and why my submissions weren't appearing. (there are some I submitted when I first imported a lot of my library, and I have since then imported another release of the same album)

[Matt]

So which of them is best? Other than some contrast differences, they all look pretty similar in the previews.

If you mouse-over an image, the tooltip shows a full resolution version of each image.

[6233638]

If you mouse-over an image, the tooltip shows a full resolution version of each image.

I'm only seeing a 480x480 preview, which is almost useful for comparing 500px art:


(mousing over the 1650px art)

I'm not trying to make a big issue out of this - the bigger problem for me is actually that PNG art is being recompressed as JPEG.

It just seemed that Media Center was missing high resolution art, and lacked a way to compare at 100% from the selection that is there.

[Fred1]

It is not exactly on topic here, but nevertheless, i have a little tip here:

iTunes delivers very good album art mostly at 600x600 pixels.
But it doesn't do it without work.

You have to remove the album art it automatically assigns when you import into iTune's library.
Once the album art is cleared out, you have to re-load CD covers.
This time you get the 600x600 art in very good quality.

I do this procedure with the help of some of Dough's AppleScripts.
It is a bit time consuming, however.

[vagskal]

You could check out Album Art Downloader http://sourceforge.net/projects/album-art/

You can invoke a search from within MC. It can download a folder.jpg to the folder where the album you searched for resides, for example.

[Matt]

I'm only seeing a 480x480 preview, which is almost useful for comparing 500px art:

We have a general tooltip size cap at 50% of the height of the monitor, since a tooltip that covers too much can be confusing and/or frustrating.

Maybe this should be an exception to that rule?  Thoughts welcome.

[Matt]

As for PNG vs JPG, the problem is that not all file formats support PNG artwork and not all programs and devices read them.  It's possible we could add an option for PNG (I would use it myself) but I'm a little afraid of causing new problems elsewhere.

If we do convert to JPG, we save using a quality of 90 which should look very good.  If you're getting bad JPEG results, a step-by-step would be appreciated because it would be a bug.

[Neco]

I'm not a fan of Jpeg either.


MC uses a thumbnail cache right?    So,  could we not maintain the original file itself, in whatever format (PNG, Targa, JPG, w/e) but when we build the cache use Jpegs because those are gonna be small anyway.

For those who want, I guess you could institute a tickbox that does not convert to JPG for the cache.    We could have an option to not convert to JPG  when embedding album art?     Additionally letting us control the quality slider (so we can set 100%) for JPG compression would be a nice  option for all users too.

Just ideas of course..

Also +1 for  full resolution preview.

[vagskal]

If we do convert to JPG, we save using a quality of 90 which should look very good.

1. Is this 90 quality applied also if I paste a jpg image (not the url) from clipboard and, if so, is this resampling (or whatever it is called for images) applied to both the jpg saved in the music file and the resulting folder.jpg (or jpg in the cover art folder)?

No complaint. Just curious.

2. I have, to be helpful, selected the Submit cover art changes to online database option. I have recently updated a lot of singles cover art (applied the cover art of the single to individual files in VA and artist compilation albums). Was that a bad thing for your database and should I untick the option when applying singles cover art? (If you do not accept anything above 1,000x1,000 to the database you will not have gotten many album cover art files from me and I might as well leave that option unticked all the time.)

Feature request: Would it be possible to have a "Save cover art as image file" toggle option (like the "Update Tags When File Info Changes" option) available as a toolbar button? When saving singles cover art to VA and artist compilation albums I want to disable this option so that the singles cover art does not overwrite the cover art image file (a folder jpg in my case) representing the entire album. Now I have to switch the save folder.jpg option to the cover art folder, which I do not really use, when pasting singles cover art to individual music files, but I tend to forget that...

[Matt]

1. Is this 90 quality applied also if I paste a jpg image (not the url) from clipboard and, if so, is this resampling (or whatever it is called for images) applied to both the jpg saved in the music file and the resulting folder.jpg (or jpg in the cover art folder)?

Yes.

As you touch on, pasting a URL is better since it avoids the save (it just downloads the file as-is).  If the downloaded image is not JPEG, then it might get saved as JPEG.

[6233638]

We have a general tooltip size cap at 50% of the height of the monitor, since a tooltip that covers too much can be confusing and/or frustrating.

Maybe this should be an exception to that rule?  Thoughts welcome.

Well as I suggested, a loupe that shows the image at 100% (or greater) where you are mousing over would be much more helpful than a preview of the image - that way you can actually check the file quality.

These two files look similar at a glance: One, Two - even if you are viewing them at 100%.
But when you magnify them - or view them on a high DPI display like the iPad - the difference between them becomes obvious.

But I'm not sure how useful that would actually be to be honest. It would let you see if a cover is highly compressed or not, but it would still be difficult to actually compare different versions of the same art.

Coming back to it today, I think what I really want would be a completely revamped UI for selecting cover art that allowed you to mark covers as good/bad, and allow for A/B comparisons. This is what I am doing now - but outside of Media Center - and it seems like an unreasonable thing to request if no-one else is bothered by the current Album Art implementation.

As for PNG vs JPG, the problem is that not all file formats support PNG artwork and not all programs and devices read them.  It's possible we could add an option for PNG (I would use it myself) but I'm a little afraid of causing new problems elsewhere.

If we do convert to JPG, we save using a quality of 90 which should look very good.  If you're getting bad JPEG results, a step-by-step would be appreciated because it would be a bug.

Personally, I have not come across a player or device that has not been able to read PNG artwork - though Media Center itself might be the exception - I seem to recall a few albums when I did my first library import that would not display art no matter what I did, until I replaced them with a JPEG file. (this was on the first day of using Media Center - I had forgotten about that)

I would rather not see the art on a device that didn't support PNG artwork, than compress it to JPEG.


What I would suggest though, is that if you are going to insist on converting art to JPG, you warn the user if he tries to use a non-JPEG file, and save it at the maximum quality possible.
You should also store an original copy of the PNG artwork somewhere rather than discarding it. (and not in the general %appdata%\J River\Media Center 18\Cover Art\ folder where it may be cleaned out)

And if you are supposed to be saving a 90% quality JPG, I think something must be going wrong in your conversion process.
Source PNG
90% quality in Photoshop "Save for Web & Devices" - compression is still visible.
Media Center's conversion - the image is blurred and general quality is horrible.

Even as a 100% quality JPG, which is a 55KB file rather than a 5KB PNG, there is visible compression added to the image.


Am I right in thinking that if you add a JPG file as cover art though, Media Center does not recompress it?
That does appear to be the case - but the file sizes reported by dBpoweramp's tag viewer don't match up 100% with Explorer's size for the original file.

[Matt]

And if you are supposed to be saving a 90% quality JPG, I think something must be going wrong in your conversion process.
Source PNG
90% quality in Photoshop "Save for Web & Devices" - compression is still visible.
Media Center's conversion - the image is blurred and general quality is horrible.

When I paste the URL of that PNG to an APE file, let it save it to the tag, then do Cover Art > Copy To Clipboard, the result looks great and nothing like your screenshot.

Is it possible you're taking a screenshot of our thumbnail or something instead of the data in the tag?

[6233638]

When I paste the URL of that PNG to an APE file, let it save it to the tag, then do Cover Art > Copy To Clipboard, the result looks great and nothing like your screenshot.

Is it possible you're taking a screenshot of our thumbnail or something instead of the data in the tag?

I added it as a file (rather than an url) and then used "Save Cover Art to External Location Specified in Options" and pulled it out of %appdata%\J River\Media Center 18\Cover Art\Albums
But it looks the same if I use "Cover Art > View (first file)"

And it's worse when I added it by pasting the URL as a test just now - I get this as a result. (re-saved it as a PNG to guarantee it wouldn't be compressed on upload)

[Matt]

I added it as a file (rather than an url) and then used "Save Cover Art to External Location Specified in Options" and pulled it out of %appdata%\J River\Media Center 18\Cover Art\Albums
But it looks the same if I use "Cover Art > View (first file)"

And it's worse when I added it by pasting the URL as a test just now - I get this as a result. (re-saved it as a PNG to guarantee it wouldn't be compressed on upload)

What type of file are you testing?

Could you try an APE file, just so we're doing the same test?

[Matt]

Looking more, I think it's the chroma subsampling making that image look soft as JPEG.

Our JPEG engine currently uses 4:2:0 chrominance subsampling for all saves.  It's most notable on hard lines like the sample image.

Maybe this should be 4:4:4 for all saves, or switch based on the quality.

[vagskal]

Yes.

As you touch on, pasting a URL is better since it avoids the save (it just downloads the file as-is).  If the downloaded image is not JPEG, then it might get saved as JPEG.

OK, thanks. I might have to reconsider and copy the image URL in Google Chrome instead (and figure out if I can do the same in Album Art Downloader).

[6233638]

Looking more, I think it's the chroma subsampling making that image look soft as JPEG.

Our JPEG engine currently uses 4:2:0 chrominance subsampling for all saves.  It's most notable on hard lines like the sample image.

Maybe this should be 4:4:4 for all saves, or switch based on the quality.

That'll be what it is. Subsampled chroma looks terrible in JPEGs. It would also explain why some thumbnails look particularly bad.

Media Center, Resized externally:


I would still prefer it if we could use PNGs as art without any conversions though. Any file that is actually well suited to PNG compression (large flat areas of color etc.) will show artefacts even as a 100% quality 4:4:4 JPEG.

[Matt]

Next build:
Changed: When saving a JPEG with a high quality (90 or higher quality, which is used for cover art saving), no chrominance subsampling will be used.

I would still prefer it if we could use PNGs as art without any conversions though.

I think I agree, but I'm still thinking about how to do it.  The change would have repercussions with YADB (the speed of the cover art tool for users and our bandwidth usage), as well some some possible compatibility things.

Would you be willing to help with testing if we go down this road?

[6233638]

Next build:
Changed: When saving a JPEG with a high quality (90 or higher quality, which is used for cover art saving), no chrominance subsampling will be used.

Excellent, thanks. I don't suppose that also impacts thumbnail quality?

I think I agree, but I'm still thinking about how to do it.  The change would have repercussions with YADB (the speed of the cover art tool for users and our bandwidth usage), as well some some possible compatibility things.

Perhaps Media Center could compare the filesize of a 100 quality JPEG (rather than 90) against the PNG art? That way you will get the space savings of PNG if it is actually smaller, and use JPEG if it doesn't actually compress well - for uploading to/downloading from YADB I mean. Locally, if I add a PNG, I want it to stay a PNG - even a maximum quality JPEG still has some compression.

The covers for game soundtracks and chiptune music are typically good examples for this.

17kB as a PNG inflates to 63kB as a JPG:


And 7kB as a PNG inflates to 279kB as a JPG!

This one is also a good example of an image that looks incredibly bad as a thumbnail with the current compression levels/chroma sampling.

EDIT: At 200%, here is iTunes on the left and Media Center on the right:


I wonder if they are actually just using PNGs for thumbnails. (or at least very high quality JPG)

Would you be willing to help with testing if we go down this road?

I'm happy to do what I can to help test things.

[Frobozz]

Next build:
Changed: When saving a JPEG with a high quality (90 or higher quality, which is used for cover art saving), no chrominance subsampling will be used.

Yea!  Thank you. 
The chroma subsampling makes a mess of red hues, especially solid reds.
Next build I'll rebuild all my thumbnails.   

For anyone curious here's a little bit about chroma subsampling and how it affects jpeg compression.
When I edit my own cover images I always save without chroma subsampling.  Tools like GIMP, Photoshop, and IrfanView allow you set options do disable the chroma subsampling when saving as JPG.  Tools like MS Paint do not and will always create JPGs with chroma subsampling enabled, even if you set 100% quality.

I think I agree, but I'm still thinking about how to do it.  The change would have repercussions with YADB (the speed of the cover art tool for users and our bandwidth usage), as well some some possible compatibility things.

Would you be willing to help with testing if we go down this road?

How many portable devices support PNG for embedded cover art display?
Everyone supports JPG, I don't know how many support PNG.  Adding PNG could cause problems for people who sync to a portable that doesn't support PNG.

[6233638]

How many portable devices support PNG for embedded cover art display?
Everyone supports JPG, I don't know how many support PNG.  Adding PNG could cause problems for people who sync to a portable that doesn't support PNG.

I only have Apple devices for portable audio, and they all seem to support PNG just fine. I would have thought that any recent device would be the same.

When syncing to portable devices via Media Center, don't most people convert their audio to AAC/MP3? In that case, you could surely convert the embedded art at the same time. (reduce the resolution, and save as JPG)

[Frobozz]

I only have Apple devices for portable audio, and they all seem to support PNG just fine. I would have thought that any recent device would be the same.

When syncing to portable devices via Media Center, don't most people convert their audio to AAC/MP3? In that case, you could surely convert the embedded art at the same time. (reduce the resolution, and save as JPG)

I don't know how many portable players support PNG cover art.  I'd suspect that recent models do.  But older ones?  People with older devices still deserve to see cover art and shouldn't need to jump through more hoops to do so.

High quality JPG compression is essentially lossless as far as what the eye can see.  Even when enlarged I can't see the difference between a high quality JPG and the original BMP.  High quality being 100% quality or very high 90's and no chroma subsampling.  I see no reason to go to PNG for image quality reasons.  Just up the JPG compression quality and the images will be visually indistinguishable.

If you are seeing differences it is possible those differences are due to embedded color profiles.  If embedded color profiles are being handled differently for PNG and JPG then you could see differences in colors and black levels and shadows between a PNG and JPG.  I save an embedded color profile when I save as JPG.  For some reason I lose the color profile when saving as PNG (PNG is supposed to support color profiles, I don't know what's going on).

Cover art images that compress better as PNG are rare special cases.  Those game soundtrack and chiptune covers are an example.  Covers that are mostly a solid color.  Most cover art is not like that and JPG will compress better.

[6233638]

I don't know how many portable players support PNG cover art.  I'd suspect that recent models do.  But older ones?  People with older devices still deserve to see cover art and shouldn't need to jump through more hoops to do so.

Well I would be surprised if people were adding PNG art if their devices don't support it. But that is the sort of thing that would probably come up at some point. ("Why is the cover art for ___ missing on my device?")

High quality JPG compression is essentially lossless as far as what the eye can see.  Even when enlarged I can't see the difference between a high quality JPG and the original BMP.

In most cases, it is perceptually lossless, I agree. But particularly with complicated images - even at 100% with no chroma subsampling - the compression can become visible. It's more obvious on high DPI devices than most current displays.

If you are seeing differences it is possible those differences are due to embedded color profiles.  If embedded color profiles are being handled differently for PNG and JPG then you could see differences in colors and black levels and shadows between a PNG and JPG.  I save an embedded color profile when I save as JPG.  For some reason I lose the color profile when saving as PNG (PNG is supposed to support color profiles, I don't know what's going on).

It's a compression difference, not a color profile issue - I'm well aware of those problems.

Using the same image as an example, here's what happens when you remove all the colors of the original 7kB PNG from the 279kB JPG:


Now this is not an image where compression is actually visible when you save it as a 100% JPEG with no chroma subsampling, but it was an easy way to illustrate that a JPEG is still lossy at 100%, while inflating the file size 40x.

Cover art images that compress better as PNG are rare special cases.  Those game soundtrack and chiptune covers are an example.  Covers that are mostly a solid color.  Most cover art is not like that and JPG will compress better.

I agree that it's rare that PNG will be better - but I have maybe 5-10% of my library with art that compresses a lot better as a PNG.

Thinking about it some more though, perhaps converting PNG and other lossless file types to JPEG is the best solution after all - but only if they are stored as a 100% JPEG with no chroma subsampling, rather than 90%.
In most cases, it's not going to inflate the file size to ridiculous levels. While 7kB vs 279kB is a big difference, 279kB itself is not outrageous, and it works out as a 22MB difference when you add it up for the whole album, so it's probably less than 200MB across my entire library.

I still maintain that if you are using a non-JPEG source for album art however, that the source file should retained, even if it is not currently being used.
Fundamentally, it seems like a bad idea to throw away a lossless file in favor of a lossy one for the sake of convenience. (would you be happy to convert all your FLAC/ALAC files to MP3?)

[6233638]

So I had some problems with my system after trying out Drive Bender.
I backed up my library, reinstalled Windows and Media Center, restored my license, and then restored my library.

Now after fixing the file locations for my music, most cover art was restored - but not for DFF files (Multichannel DSD, or DST compressed Stereo DSD) presumably because they can't store metadata.
Luckily, I had the foresight to back up %APPDATA%\J River\Media Center 18\Cover Art, so I was able to restore most of it via Cover Art > Quick Find in File / Cover Art Directory (one album is still missing art) but I feel like that shouldn't have happened from a manually initiated backup of my library.

And strangely, there were two ALAC albums where the first track didn't have any cover art, but the rest did. Easily fixed, but very strange.

EDIT: Actually, I didn't lose any album art - for some reason that last SACD which wasn't displaying any, did have it - I just had to rebuild the thumbnail for it. (I tried getting art from the internet and it showed up in there, saying it was in my library)