somewhat stable

media/client/src/backends/subprocess.rs

@@ -1,13 +1,16 @@
 //! Subprocess backend: spawn ffmpeg CLI for capture+encode.
 //!
 //! Spawns ffmpeg with the same hardware pipeline as `stream_av.sh`:
-//!   kmsgrab → hwmap=derive_device=vaapi → scale_vaapi → h264_vaapi
+//!   kmsgrab -vblank_source vsync → hwmap=derive_device=vaapi → scale_vaapi → h264_vaapi
 //!   + PulseAudio desktop audio + mic → amix → AAC
 //!
+//! -vblank_source vsync forces a frame grab on every display vblank, regardless
+//! of page flips. Without it, kmsgrab only grabs when the compositor flips a
+//! new buffer — a static/slow screen yields 1fps.
+//!
 //! ffmpeg outputs NUT format to stdout. We demux that pipe with ffmpeg-next
 //! to get proper AVPackets (keyframe flags, timestamps) without parsing
-//! bytestreams. NUT is lighter than mpegts — no TS overhead, exact packet
-//! metadata in the container layer.
+//! bytestreams.
 
 use std::os::fd::AsRawFd;
 use std::os::unix::io::RawFd;
@@ -76,8 +79,6 @@ pub fn run(
             while !stop_watcher.load(Ordering::Relaxed) {
                 std::thread::sleep(std::time::Duration::from_millis(100));
             }
-            // Send SIGINT to ffmpeg so it flushes and closes stdout,
-            // which unblocks the packet iterator in demux_and_send.
             use nix::sys::signal::{kill, Signal};
             use nix::unistd::Pid;
             let _ = kill(Pid::from_raw(child_pid as i32), Signal::SIGINT);
@@ -148,6 +149,9 @@ fn detect_default_source(pulse_server: &str) -> Option<String> {
 fn spawn_ffmpeg(cfg: &SubprocessConfig) -> Result<Child> {
     let audio = detect_audio_sources();
 
+    // fps filter after scale_vaapi pads/duplicates frames to fill gaps when
+    // kmsgrab captures fewer frames than the target rate (e.g. compositor
+    // skips flips on static content). Keeps the output stream at a stable fps.
     let filter = format!(
         "hwmap=derive_device=vaapi,scale_vaapi=w={}:h={}:format=nv12,fps={}",
         cfg.width, cfg.height, cfg.fps,
@@ -158,6 +162,9 @@ fn spawn_ffmpeg(cfg: &SubprocessConfig) -> Result<Child> {
         "-init_hw_device".into(), format!("drm=drm:{}", cfg.device),
         "-init_hw_device".into(), "vaapi=va@drm".into(),
         // Video input (kmsgrab)
+        // -vblank_source vsync: grab on every display vblank, not just page flips.
+        // Without this, a static screen (e.g. talking-head meeting) gives 1fps
+        // because the compositor rarely flips a new buffer.
         "-thread_queue_size".into(), "512".into(),
         "-device".into(), cfg.device.clone(),
         "-f".into(), "kmsgrab".into(),