THIS PROJECT HAS MOVED -> https://github.com/n4n0GH/hello
This is a fork of https://sourceforge.net/projects/shapecorners/
When you disable window borders in System Settings --> Application Style --> Window Decorations --> Border size: No Borders you get sharp bottom window corners.
This effect rounds the corners of your windows.
Debian based (Ubuntu, Kubuntu, KDE Neon):
sudo apt install git cmake g++ gettext extra-cmake-modules qttools5-dev libqt5x11extras5-dev libkf5configwidgets-dev libkf5crash-dev libkf5globalaccel-dev libkf5kio-dev libkf5notifications-dev kinit-dev kwin-dev
Fedora based
sudo dnf install git cmake gcc-c++ extra-cmake-modules qt5-qttools-devel qt5-qttools-static qt5-qtx11extras-devel kf5-kconfigwidgets-devel kf5-kcrash-devel kf5-kguiaddons-devel kf5-kglobalaccel-devel kf5-kio-devel kf5-ki18n-devel kf5-knotifications-devel kf5-kinit-devel kwin-devel qt5-qtbase-devel libepoxy-devel
git clone https://github.com/n4n0GH/KDE-Rounded-Corners
cd KDE-Rounded-Corners; mkdir qt5build; cd qt5build; cmake ../ -DCMAKE_INSTALL_PREFIX=/usr -DQT5BUILD=ON && make && sudo make install && (kwin_x11 --replace &)
It should be now activated.
For better results turn off the border size in:
System Settings --> Application Style --> Window Decoration