Low quality wallpaper, really?

[j7n]

Windows NT6 keeps on giving. This one's kinda trivial. But considering the increased size of the system and focus on sleek visuals, I don't understand how they could have messed this up.

I have some wallpapers in bitmap format in a directory, prepared for a few screen resolutions. They are ready to be displayed, and work just fine in Windows XP. If I set the wallpaper using the Desktop Properties or the right-click menu on an image file, the data gets encoded to a poor quality JPEG file with color subsampling. It is 1 "actively used" file and should not be compressed.

%appdata%\Roaming\Microsoft\Windows\Themes\Application Data\TranscodedWallpaper.jpg

To set the wallpaper properly I need to enter the path to the image file into the registry.

HKCU\Control Panel\Desktop\Wallpaper

Then execute this command from the Run box to refresh it (a bat file does not work) {REF}:

rundll32.exe user32.dll, UpdatePerUserSystemParameters

Seriously?

[UCyborg]

Yup, it imports them as JPEG with 85% quality. I didn't find whether there was any reason for this. Windows 8 added ability to set background per-monitor and PNG format support and AFAIK this one doesn't get further compressed or converted to JPEG. Windows 10 added JPEGImportQuality setting in registry (must be manually created), so at least compression can be disabled this way.

One is better off with a 3rd party tool. I personally let Actual Window Manager manage desktop wallpapers, which also addresses lack of native ability to set background per-monitor.

[j7n]

Wow, Actual Window Manager still claims Windows 2000 compatibility. Not that I plan to use Windows 10 for many reasons, but I'm curious if you set JPEGImportQuality to 100, does color subsampling remain active (blurry red shapes)?

[vinifera]

i dont have this issue

i even compared with hashtab, the copied one matches original

pic in question is:

type: JPG
dimension: 4000x3000
size: 1.6 MB

or am i missing something here ?

Edited September 8, 2021 by vinifera

[UCyborg]

On 9/8/2021 at 12:30 AM, j7n said:

Not that I plan to use Windows 10 for many reasons, but I'm curious if you set JPEGImportQuality to 100, does color subsampling remain active (blurry red shapes)?

Unfortunately, yes.

10 hours ago, vinifera said:

or am i missing something here ?

I don't know, AFAIK it's an over decade old quirk (from MS point of view it's probably by design) and computers have only gotten more powerful since.

https://social.technet.microsoft.com/Forums/en-US/0c02f2cb-39cd-4f2d-bd4e-154fd2165b69/my-wallpaper-gets-badly-compressed?forum=w7itproui

[j7n]

So I solved this problem on Windows 2008 R2 by simply pointing the registry entry to my chosen wallpaper and it works like on normal Windows.

HKEY_CURRENT_USER\Control Panel\Desktop ! Wallpaper

But it gets worse on Windows 2022 (Windows 10). The wallpaper is repeatedly transcoded into two files stored here:

C:\Users\Administrator\AppData\Roaming\Microsoft\Windows\Themes\TranscodedWallpaper

You'll find a storm of complaints on the web about it. If you give an image in BMP format, it will be converted into two JPEG files with chroma subsampling. I was not able to lock these files down with permissions. No matter what settings, Administrator can still delete them even if the owner is Network Service or whatever and there is no Write permission (opening for writing is not possible, but deletion is). I could not assign them to TrustedInstaller.

The way to partially bypass this is to give an image in PNG format. It still gets transcoded using CPU but without loss of quality. This is not wise because PNG gives poor compression on photos and only uses CPU. It seems that the desktop is now implicitly "active desktop".

The strangeness of modern Windows is so multifactorial and goes beyond the Start menu.

[vinifera]

now you got me curious again
so i did comparison to different "chosen wallpaper" and transcoded one

resolution: 6674 x 3923
size: 18.3 MB

and they both are the same, even CRC matches
but all my "chosen" wp's are JPG already

maybe the OS does transcode only if you chose non JPG pics ?

also there is reg trick for HKEY_CURRENT_USER\Control Panel\Desktop
with DWORD32 of JPEGImportQuality with Decimal value of 100
but this one as i see is for w10/11 not 7