I'm aware of the age of this question and anyway I will provide my answer for future reference, as the one posted by @clk didn't resolve the issue for me on kde Plasma 5.2, which task switcher (ALT+TAB) was showing blurry icons for some applications, e.g. Gimp and Emacs.
In that regard I double checked the
StartupWMClass key/value within the
<application>.desktop file, both:
WM_CLASS for the app was already set to the right one, that is, the string given in output of the
xprop command applied to the specific window of the app with blurry icon.
I also checked to have good resolution icons within
/usr/share/icons/hicolor/<resolution>/apps/ and that was not the case as hi-res icons where already there.
Simple solution tested with KDE Plasma
- open the the application with blurry icon
- click on the title bar of that window
- choose more actions->application settings. This will open a dialog with some specific setting for the application window.
- bottom left of the dialog, click add property, scroll down and choose name of the desktop file, from the dropdown choose force and within the text field, add in the name for the specific application (
- click apply
The task switcher should now use the right high-resolution icon for the application.