One of the changes to the Plasma workspace in Plasma 5 (based on KDE Frameworks 5) is that the option of having separate Plasma widgets on each virtual desktop is now gone. This is unfortunate, because one useful side effect of being able to do that was the ability to have different desktop backgrounds (wallpaper) on different virtual desktops, so proving a useful visual cue as to the desktop currently displayed.
The most relevant KDE bug is https://bugs.kde.org/show_bug.cgi?id=341143
The position of the Plasma developers, as stated in the evaluation of that bug, is that there have been major architectural changes within Plasma and at the moment the option is not available, with no simple way to bring it back. It has been acknowledged as an omission and is currently being worked on. Please be considerate to the developers and refrain from commenting on this bug unless you really do have a useful contribution to make to the discussion.
This little system tray application attempts to being back the ability to have different wallpapers on different virtual desktops, pending the feature being reintroduced into Plasma. It allows a wallpaper image file to be configured for each virtual desktop, and will change the background when the virtual desktop changes. It does this by changing the wallpaper slideshow image available, so there is some Plasma configuration needed in order to use it. See below for instructions on how to do that.
The application is built and installed in much the same way as any other KDE application. Assuming that you have Qt 5, KDE Frameworks 5 and the Plasma desktop installed, go to a suitable build location (e.g. your home directory) and do:
git clone https://github.com/martenjj/wallpaperswitch.git
cd wallpaperswitch
mkdir build
cd build
cmake ..
make
sudo make install
Building requires CMake and also KDE Frameworks development headers and libraries. Depending on your distro, you may need to install additional packages for these. If you are running Kubuntu then see the README.kubuntu file for a list of packages required.
After the files are installed above, start the switcher either from whichever desktop launcher you use (it will appear in the "Utilities" category), or by typing the command 'wallpaperswitch' in a terminal or KRunner (summoned by typing Alt-F2). The "desktop" icon will appear in the system tray.
If this is the first time that the application has been run the the configuration dialogue will appear automatically. Otherwise, click the right mouse button over the system tray icon and select the "Configure..." option from the menu. Turn on the "Enable the desktop wallpaper switcher" check box if necessary.
Each currently configured virtual desktop will be listed there. To select the wallpaper image for a desktop, double click the entry or select it and click the "Set Wallpaper" button. From the file dialogue that appears, choose an image file. Repeat the same for each virtual desktop.
Click the "Plasma desktop settings" link at the bottom right, and make a note of the "Location" value shown there. Depending on your distro's configuration, this will be a pathname something like "/home/yourname/.local/share/wallpaperswitch". Then finally click "OK" to save the configuration.
Then the Plasma desktop settings need to be configured. Right click over the desktop background and select "Configure Desktop". Change the "Wallpaper Type" to "Slideshow" and the "Positioning" to "Scaled". Set the "Change every" setting to a long time such as 12 hours - this is not directly relevant to the wallpaper switching, which will always happen immediately, but it will avoid frequent log messages (issue #2). The "Background colour" setting is not relevant here.
If the list of folders below is not empty, then click the red "-" button to the right of each entry to remove it. Then click the "Add Folder" button and select or enter the pathname that was given for "Location" above. Then click "OK" or "Apply".
The wallpaper image that you selected for the current virtual desktop should now appear, and change when the virtual desktop changes. If nothing happens the first time, you may need to log out and log in again (this is because originally the slideshow pathname didn't exist so the slideshow plugin would not be watching it).
Switching can be enabled or disabled using the "Enable Switching" option on the system tray popup menu.
If you are able to build and install the application, but it will not run or it is not able to change the desktop background, then try starting it from a terminal window within the Plasma desktop. Observe the debugging messages for anything about missing files or any other problems.
If switching doesn't work and there are no obvious error indications in the terminal as above, then ensure that the "Location" directory as above exists and is writeable by you. It should contain a single symbolic link that changes in accordance with the virtual desktop. If this is all well, then check the Plasma slideshow settings and then log out and log in again.
Please note that, for the wallpaper switching (or any use of the slideshow wallpaper, even if not using this switcher application) needs the revision 4be156bc (see https://phabricator.kde.org/R120:4be156bc for details) to be incorporated in plasma-workspace. If you are using a self compiled KDE then check that this revision is included. If you are using KDE distro packages then check with your distro's version control tracker.
If neither of this resolves the problem then please raise an issue in GitHub (http://github.com/martenjj/wallpaperswitch). Do not raise a bug against KDE Plasma.
If you use virtual desktops and want wallpaper switching then I hope that this utility will come in useful, and also that one day it will not be necessary...
Jonathan Marten, http://github.com/martenjj