feat(app): adaptive quality, frame rate capping, and tighter VBV

Three changes to make the stream survive constrained links:

Adaptive quality: the sender pipeline now tracks the PLI rate from
the receiver. Every 5 seconds it evaluates: >0.5 PLI/s means the
link is saturated (the receiver is dropping frames), so the CRF
increases by 2 (lower quality, fewer bits) and the encoder restarts
with a keyframe. <0.1 PLI/s means the link is stable, so the CRF
decreases by 1 (better quality) and the encoder probes upward.
Clamped to [user CRF, user CRF + 10] so quality never degrades
below what the link can handle, and never exceeds what the user
asked for. The adaptation is logged to stderr for visibility.

Frame rate capping (--fps N): throttles the capture loop to N
frames per second (0 = no cap; monitor rate). At 15fps instead of
60fps, the bandwidth requirement drops 4x at the same quality
level. Desktop content is still smooth at 15-20fps.

Tighter VBV: one frame period of buffer instead of two. A two-frame
buffer lets a keyframe spike to twice the target rate in one burst,
which overflows any constrained hop (Wi-Fi hotspot, slow switch)
and cascades into PLI storms. One frame period keeps bursts
within what the link can absorb in real time.

src/app/cli.cpp

@@ -8,15 +8,15 @@ namespace sc {
 namespace {
 
 void print_usage() {
-    std::fputs(
-        "usage: screencast --send [--target monitor|window] [--peer HOST[:PORT]] [--bitrate MAX_KBPS] [--crf 0-51]\n"
-        "       screencast --receive [--port PORT] [--signaling-port PORT] [--fullscreen] [--swdecode]\n"
-        "       screencast --discover [--timeout SECONDS]\n"
-        "       screencast waybar [--toggle]        # for waybar widgets\n"
-        "\n"
-        "--send without --peer discovers a receiver on the LAN and requires\n"
-        "that exactly one is found.\n",
-        stderr);
+    std::fputs("usage: screencast --send [--target monitor|window] [--peer HOST[:PORT]] [--bitrate MAX_KBPS] [--crf "
+               "0-51] [--fps 1-60]\n"
+               "       screencast --receive [--port PORT] [--signaling-port PORT] [--fullscreen] [--swdecode]\n"
+               "       screencast --discover [--timeout SECONDS]\n"
+               "       screencast waybar [--toggle]        # for waybar widgets\n"
+               "\n"
+               "--send without --peer discovers a receiver on the LAN and requires\n"
+               "that exactly one is found.\n",
+               stderr);
 }
 
 bool parse_int(std::string_view text, int& value) {
@@ -113,6 +113,13 @@ std::optional<Command> parse_cli(int argc, const char* const argv[]) {
                 print_usage();
                 return std::nullopt;
             }
+        } else if (argument == "--fps") {
+            std::string_view value;
+            if (!next_argument(argc, argv, index, value) || !parse_int(value, send.fps) || send.fps < 0 ||
+                send.fps > 60) {
+                print_usage();
+                return std::nullopt;
+            }
         } else if (argument == "--port") {
             std::string_view value;
             int port = 0;

src/app/main.cpp

@@ -160,7 +160,7 @@ int run_sender(const sc::SendCommand& command) {
         signaling = sc::Endpoint{hosts.front(), port};
     }
 
-    auto session_result = sc::SenderSession::start(signaling, command.bitrate_kbps, command.crf, target);
+    auto session_result = sc::SenderSession::start(signaling, command.bitrate_kbps, command.crf, command.fps, target);
     if (auto* error = std::get_if<std::string>(&session_result)) {
         std::cerr << std::format("screencast: {}\n", *error);
         return 1;

src/app/pipelines.cpp

@@ -37,7 +37,7 @@ std::string receiver_service_name() {
 
 class SenderPipeline::Impl {
   public:
-    explicit Impl(SenderPipelineConfig config) : config_(std::move(config)) {}
+    explicit Impl(SenderPipelineConfig config) : config_(std::move(config)), current_crf_(config_.encoder.crf) {}
 
     ~Impl() {
         stop();
@@ -60,15 +60,12 @@ class SenderPipeline::Impl {
         run_thread_ = std::jthread([this](std::stop_token stop_token) { run(std::move(stop_token)); });
 
         // Publish the session for status widgets and one-click restarts.
-        // The RTP peer is what a restart needs; the session id identifies it.
         write_sender_state(SenderState{
             .session_id = config_.session_id,
             .receiver = std::format("{}:{}", config_.peer_rtp_endpoint.address, config_.peer_rtp_endpoint.port),
             .bitrate_kbps = config_.encoder.bitrate_kbps,
             .pid = ::getpid(),
             .started_epoch_ms = 0});
-        // A restart targets the receiver's signaling endpoint; the RTP
-        // endpoint is re-negotiated from it.
         std::string restart_peer = std::format("{}:5005", config_.peer_rtp_endpoint.address);
         if (config_.signaling_server.has_value()) {
             restart_peer = std::format("{}:{}", config_.signaling_server->address, config_.signaling_server->port);
@@ -88,10 +85,16 @@ class SenderPipeline::Impl {
 
     void request_keyframe() {
         keyframe_requested_.store(true);
+        pli_count_.fetch_add(1, std::memory_order_relaxed);
     }
 
   private:
     void run(std::stop_token stop_token) {
+        auto last_adaptation = std::chrono::steady_clock::now();
+        auto last_pli_check = last_adaptation;
+        auto last_pli_count = 0;
+        auto last_frame_time = last_adaptation;
+
         while (!stop_token.stop_requested()) {
             if (keyframe_requested_.exchange(false) && encoder_ != nullptr) {
                 encoder_->request_keyframe();
@@ -106,6 +109,17 @@ class SenderPipeline::Impl {
                 break;
             }
 
+            // Frame rate capping: skip frames that arrive faster than the
+            // configured target. 0 = no cap (use the monitor rate).
+            if (config_.max_frame_rate > 0) {
+                const auto now = std::chrono::steady_clock::now();
+                const auto min_interval = std::chrono::microseconds(1'000'000 / config_.max_frame_rate);
+                if (now - last_frame_time < min_interval) {
+                    continue;
+                }
+                last_frame_time = now;
+            }
+
             auto encoded_result = encoder_->encode(*frame);
             if (is_codec_error(encoded_result)) {
                 std::cerr << std::format("screencast: encode failed: {}\n", codec_error(encoded_result).message);
@@ -116,18 +130,58 @@ class SenderPipeline::Impl {
                     (void)transport_->send(packet);
                 }
             }
+
+            // Adaptive quality: evaluate the link every 5 seconds by
+            // checking how many PLIs the receiver sent. Frequent PLIs
+            // mean the receiver is dropping frames — the link is
+            // saturated, so increase the CRF (lower quality, fewer
+            // bits). When the link is quiet, try lowering the CRF to
+            // probe for better quality.
+            const auto now = std::chrono::steady_clock::now();
+            if (now - last_pli_check >= std::chrono::seconds(5)) {
+                const auto current_pli = pli_count_.load(std::memory_order_relaxed);
+                const auto pli_delta = current_pli - last_pli_count;
+                const auto seconds = std::chrono::duration_cast<std::chrono::seconds>(now - last_pli_check).count();
+                const auto pli_per_second = static_cast<double>(pli_delta) / static_cast<double>(seconds);
+                last_pli_count = current_pli;
+                last_pli_check = now;
+
+                if (pli_per_second > 0.5 && current_crf_ < config_.encoder.crf + 10) {
+                    // Saturated: degrade quality (higher CRF = fewer bits)
+                    current_crf_ += 2;
+                    std::cerr << std::format(
+                        "screencast: link saturated ({} PLI/s); adapting CRF to {}\n", pli_per_second, current_crf_);
+                    if (!restart_encoder(*frame)) {
+                        break;
+                    }
+                } else if (pli_per_second < 0.1 && current_crf_ > config_.encoder.crf) {
+                    // Stable: try better quality (lower CRF = more bits)
+                    current_crf_ -= 1;
+                    std::cerr << std::format("screencast: link stable; probing CRF {}\n", current_crf_);
+                    if (!restart_encoder(*frame)) {
+                        break;
+                    }
+                }
+            }
         }
     }
 
+    bool restart_encoder(const CapturedFrame& frame) {
+        // Create a new encoder with the adjusted CRF; the next encoded
+        // frame is a keyframe, so the receiver recovers immediately.
+        encoder_ = nullptr;
+        return create_encoder(frame);
+    }
+
     bool create_encoder(const CapturedFrame& frame) {
         EncoderConfig config = config_.encoder;
         config.width = frame.width;
         config.height = frame.height;
+        config.crf = current_crf_;
 
         // Downscale to the receiver's display when the capture is larger,
         // preserving aspect ratio and rounding to even values (YUV420P
-        // requires even dimensions for the chroma planes). This avoids
-        // encoding pixels the display cannot show.
+        // requires even dimensions for the chroma planes).
         if (config_.max_encode_width > 0 && config_.max_encode_height > 0 &&
             (frame.width > config_.max_encode_width || frame.height > config_.max_encode_height)) {
             const double scale = std::min(static_cast<double>(config_.max_encode_width) / frame.width,
@@ -151,12 +205,14 @@ class SenderPipeline::Impl {
     }
 
     SenderPipelineConfig config_;
+    int current_crf_;
     std::unique_ptr<CaptureSession> capture_;
     std::unique_ptr<Encoder> encoder_;
     H264Packetizer packetizer_;
     std::unique_ptr<RtpTransport> transport_ = RtpTransportFactory::create();
     std::jthread run_thread_;
     std::atomic<bool> keyframe_requested_{false};
+    std::atomic<int> pli_count_{0};
 };
 
 void SenderPipeline::request_keyframe() {

src/app/sender_session.cpp

@@ -37,7 +37,7 @@ bool is_private_ipv4(std::string_view host) {
 } // namespace
 
 std::variant<SenderSession, std::string>
-SenderSession::start(const Endpoint& signaling_endpoint, int bitrate_kbps, int crf, CaptureTarget target) {
+SenderSession::start(const Endpoint& signaling_endpoint, int bitrate_kbps, int crf, int max_fps, CaptureTarget target) {
     auto channel_result = SignalingFactory::create_client();
     if (is_network_error(channel_result)) {
         return network_error(channel_result).message;
@@ -99,6 +99,7 @@ SenderSession::start(const Endpoint& signaling_endpoint, int bitrate_kbps, int c
     config.signaling_server = signaling_endpoint;
     config.encoder.bitrate_kbps = bitrate_kbps;
     config.encoder.crf = crf;
+    config.max_frame_rate = max_fps;
     config.session_id = session_id;
     config.max_encode_width = answer.display_width;
     config.max_encode_height = answer.display_height;

src/app/sender_session.h

@@ -42,7 +42,7 @@ class SenderSession {
     // pipeline — which includes the portal's interactive source picker.
     // Returns an error message on failure.
     static std::variant<SenderSession, std::string>
-    start(const Endpoint& signaling_endpoint, int bitrate_kbps, int crf, CaptureTarget target);
+    start(const Endpoint& signaling_endpoint, int bitrate_kbps, int crf, int max_fps, CaptureTarget target);
 
     SenderSession() = default;
     ~SenderSession();

src/codec/ffmpeg_encoder.cpp

@@ -419,7 +419,12 @@ CodecResult<std::unique_ptr<Encoder>> EncoderFactory::create(const EncoderConfig
     // peak so bursts cannot overflow the receiver's UDP buffers.
     ctx->global_quality = static_cast<int>(config.crf);
     ctx->rc_max_rate = static_cast<int64_t>(config.bitrate_kbps) * 1000;
-    ctx->rc_buffer_size = static_cast<int>(ctx->rc_max_rate * 2 / config.frame_rate_num);
+    // VBV: one frame period of budget keeps bursts tight — a two-frame
+    // buffer lets a keyframe spike beyond what a constrained link can
+    // absorb in real time, causing packet loss that cascades into PLI
+    // storms. The tighter buffer trades a small quality dip on keyframes
+    // for much better behavior on slow paths.
+    ctx->rc_buffer_size = static_cast<int>(ctx->rc_max_rate / config.frame_rate_num);
 
     // A long GOP saves the keyframe overhead for screen content (which
     // changes incrementally); PLI feedback recovers from loss within one

src/gui/gui.cpp

@@ -291,7 +291,7 @@ class SenderWindow : public Gtk::ApplicationWindow {
         status(std::format("connecting to {}… (choose a source in the portal dialog)", receiver.name));
 
         worker_ = std::jthread([this, signaling, bitrate, crf](std::stop_token) {
-            auto result = sc::SenderSession::start(signaling, bitrate, crf, sc::CaptureTargetWholeScreen{});
+            auto result = sc::SenderSession::start(signaling, bitrate, crf, 0, sc::CaptureTargetWholeScreen{});
             if (auto* error = std::get_if<std::string>(&result)) {
                 Glib::signal_idle().connect_once([this, message = *error] {
                     start_button_->set_label("Start");