Capture pipelines¶

Kartoza Screencaster has three screen-capture paths. Which one runs is decided at runtime by Platform::compositor() and Platform::supportsWlrCapture().

wlroots (Hyprland, Sway, Wayfire, River, Niri, …)¶

wl-screenrec --output <name> --filename <path>

wl-screenrec reads frames over the wlr-screencopy protocol. Single process, hardware-accelerated encode where supported, near-zero CPU on desktop GPUs. If it exits early (e.g. a VAAPI init failure) the recorder falls back once to wf-recorder with software libx264.

Both tools require wlr-screencopy-unstable-v1, so this path is strictly for wlroots-family compositors. COSMIC is not one of them despite its lineage — it implements its own capture protocol, and wf-recorder fails there with compositor doesn't support wlr-screencopy-unstable-v1. COSMIC uses the portal path below.

GNOME / KDE / COSMIC Wayland — xdg-desktop-portal + PipeWire¶

gst-launch-1.0 -e \
    pipewiresrc path=<node> fd=<portal-fd> do-timestamp=true \
    ! videoconvert \
    ! openh264enc bitrate=8000000 complexity=medium rate-control=bitrate \
    ! h264parse \
    ! mp4mux fragment-duration=1000 \
    ! filesink location=<path>

Needs a portal backend for the desktop: xdg-desktop-portal-gnome, xdg-desktop-portal-kde or xdg-desktop-portal-cosmic. Because the portal picks a single source, only the primary monitor is captured — additional monitor insets are dropped on this path.

The portal flow runs CreateSession → SelectSources → Start → OpenPipeWireRemote asynchronously. The PipeWire node id and the private connection FD come back from Start's response and the OpenPipeWireRemote call respectively. The FD is dup'd, CLOEXEC cleared, and setChildProcessModifier dup2's it onto a fixed slot (23) in the forked gst-launch child before exec() — that's how pipewiresrc fd=23 reaches the stream.

X11¶

ffmpeg -y -f x11grab \
    -framerate 30 \
    -video_size <WxH> -i <display>+<x>,<y> \
    -c:v libx264 -preset ultrafast -crf 18 -pix_fmt yuv420p \
    <path>

Classic x11grab. Monitor geometry comes from xrandr (or xdpyinfo as a fallback).

Stop semantics¶

All three pipelines react to SIGINT for a clean shutdown — the gst-launch -e flag is what makes the EOS propagate so mp4mux finalises the moov atom. The Recorder::stopProcess helper sends SIGINT first, escalates to terminate() after 5 seconds, then kill() after another 3.