picom is a lightweight X11 compositor that adds visual enhancements to the X Window System desktop, including configurable drop shadows behind windows, per-window transparency, and composite effects such as blur and fading. It manages window opacity so that content behind windows can become partially visible, and it renders shadows to improve the sense of depth and separation between overlapping windows.
Under the hood, picom uses a damage-event-driven render loop that redraws only the regions of the screen that have changed, minimising unnecessary full-frame renders. A double-buffered compositing pipeline renders offscreen into a back buffer while the front buffer displays the last complete frame, then swaps to avoid tearing. The compositor abstracts OpenGL context creation and buffer swapping behind interchangeable GLX and EGL backends, and applies Gaussian blur and drop shadow effects via custom GLSL fragment shaders during the compositing pass. A config-file rule engine evaluates conditional window-matching rules at runtime to apply per-window settings for transparency, shadow, and blur.
Users can fine-tune compositor performance to reduce tearing and improve responsiveness, customise the desktop aesthetic with effects like blur and fading, and define and test per-window rules that control the compositor’s behaviour. A built-in window rule testing tool helps identify which patterns match a selected window.
