feat: live operating-point fill + -6 dB refs on the gain curve

Add a per-channel input_level meter (post pre-gain, peak-with-decay) and use it
to shade the gain curve: a translucent fill under the curve from the floor up to
the current input, its right edge riding the curve (width = input, height =
output), plus a dot at the operating point. Add -6 dBFS reference lines on both
axes with a tick label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/editor/gain_curve.rs

@@ -1,11 +1,14 @@
-//! Static gain-curve display (visualisation only): output level vs input level for the channel
-//! currently selected in the plot. Plots the full channel transfer for the wet path:
-//! `out = (in + pre_gain) + gain_reduction(in + pre_gain) + makeup`. Mix is not folded in (the
-//! curve shows the wet/100% path, matching the GR-meter convention); output is clamped at 0 dBFS.
+//! Static gain-curve display: output level vs input level for the channel currently selected in
+//! the plot. Plots the wet transfer `out = (in + pre_gain) + gain_reduction(...) + makeup` (mix not
+//! folded in; output clamped at 0 dBFS), plus a live **operating-point fill** under the curve up to
+//! the channel's current input level — its right edge rides the curve (width = input, height = out).
 
+use nih_plug::prelude::util;
 use nih_plug_egui::egui::{self, pos2, vec2, Align2, Color32, CornerRadius, FontId, Sense, Stroke};
+use std::sync::atomic::Ordering;
 
 use crate::dsp::compressor::Compressor;
+use crate::meters::Meters;
 use crate::params::Codename206Params;
 
 /// Side length of the square plot.
@@ -13,7 +16,7 @@ const CURVE_SIZE: f32 = 150.0;
 /// dB extent of both axes (bottom/left = FLOOR_DB, top/right = 0 dBFS).
 const FLOOR_DB: f32 = -60.0;
 
-pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usize) {
+pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usize, meters: &Meters) {
     let labels = ["LOW", "MID", "HIGH", "ALL"];
     let ch = selected.min(3);
     let cp = match ch {
@@ -43,6 +46,13 @@ pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usiz
     let x_for = |db: f32| left + (db - FLOOR_DB) / -FLOOR_DB * w;
     let y_for = |db: f32| bottom - (db - FLOOR_DB) / -FLOOR_DB * h;
 
+    // -6 dBFS reference lines on both axes.
+    let g6 = Color32::from_gray(38);
+    let x6 = x_for(-6.0);
+    let y6 = y_for(-6.0);
+    p.line_segment([pos2(x6, top), pos2(x6, bottom)], Stroke::new(1.0, g6));
+    p.line_segment([pos2(left, y6), pos2(right, y6)], Stroke::new(1.0, g6));
+
     // Unity reference (out = in), bottom-left to top-right.
     p.line_segment([pos2(left, bottom), pos2(right, top)], Stroke::new(1.0, Color32::from_gray(45)));
     // Threshold marker on the input axis — shifted left by pre-gain (the comp sees in + pre).
@@ -59,9 +69,38 @@ pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usiz
         let out_db = (driven + gr + makeup).clamp(FLOOR_DB, 0.0);
         pts.push(pos2(x_for(in_db), y_for(out_db)));
     }
+    // Operating-point fill: shade under the curve from the floor up to the current input level.
+    let driven_now = util::gain_to_db(meters.input_level[ch].load(Ordering::Relaxed));
+    let ext_in = (driven_now - pre).clamp(FLOOR_DB, 0.0); // external input -> curve x
+    let x_now = x_for(ext_in);
+    let fill_col = Color32::from_rgba_unmultiplied(120, 200, 160, 45);
+    for seg in pts.windows(2) {
+        let a = seg[0];
+        let mut b = seg[1];
+        if a.x >= x_now {
+            break;
+        }
+        if b.x > x_now {
+            let f = ((x_now - a.x) / (b.x - a.x)).clamp(0.0, 1.0); // clip the last quad at x_now
+            b = pos2(x_now, a.y + (b.y - a.y) * f);
+        }
+        p.add(egui::Shape::convex_polygon(
+            vec![pos2(a.x, bottom), a, b, pos2(b.x, bottom)],
+            fill_col,
+            Stroke::NONE,
+        ));
+    }
+
     p.add(egui::Shape::line(pts, Stroke::new(1.6, Color32::from_rgb(120, 200, 160))));
 
-    // Corner dB ticks.
+    // Operating-point dot, on the curve at the current input.
+    let driven = ext_in + pre;
+    let gr = Compressor::gain_computer(driven, threshold, ratio, knee, low_slope, low_curve);
+    let out_op = (driven + gr + makeup).clamp(FLOOR_DB, 0.0);
+    p.circle_filled(pos2(x_now, y_for(out_op)), 3.0, Color32::from_rgb(235, 240, 235));
+
+    // Corner dB ticks + the -6 dB reference.
     p.text(pos2(left + 1.0, top + 1.0), Align2::LEFT_TOP, "0", FontId::proportional(9.0), Color32::from_gray(90));
     p.text(pos2(left + 1.0, bottom - 1.0), Align2::LEFT_BOTTOM, "-60", FontId::proportional(9.0), Color32::from_gray(90));
+    p.text(pos2(x6 + 2.0, bottom - 1.0), Align2::LEFT_BOTTOM, "-6", FontId::proportional(9.0), Color32::from_gray(80));
 }

src/editor/mod.rs

@@ -84,7 +84,7 @@ pub(crate) fn create(params: Arc<Codename206Params>, meters: Arc<Meters>) -> Opt
                         // Gain curve (left, square) beside the scrolling plot (right, fills the rest).
                         let selected = state.plot.selected;
                         ui.horizontal_top(|ui| {
-                            ui.vertical(|ui| gain_curve::draw(ui, &params, selected));
+                            ui.vertical(|ui| gain_curve::draw(ui, &params, selected, &meters));
                             ui.vertical(|ui| plot::draw(ui, &meters, &mut state.plot));
                         });
                         ui.separator();