Move curve calculation to a dedicated struct

So we can reuse this in the analyzer.

plugins/spectral_compressor/src/compressor_bank.rs

@@ -20,6 +20,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 
 use crate::analyzer::AnalyzerData;
+use crate::curve::{Curve, CurveParams};
 use crate::SpectralCompressorParams;
 
 // These are the parameter name prefixes used for the downwards and upwards compression parameters.
@@ -983,32 +984,34 @@ impl CompressorBank {
     /// are updated in accordance to the atomic flags set on this struct.
     fn update_if_needed(&mut self, params: &SpectralCompressorParams) {
         // The threshold curve is a polynomial in log-log (decibels-octaves) space
-        let intercept = params.threshold.threshold_db.value();
+        let curve_params = CurveParams {
-        // The cheeky 3 additional dB/octave attenuation is to match pink noise with the default
+            intercept: params.threshold.threshold_db.value(),
-        // settings. When using sidechaining we explicitly don't want this because the curve should
+            center_frequency: params.threshold.center_frequency.value(),
-        // be a flat offset to the sidechain input at the default settings.
+            // The cheeky 3 additional dB/octave attenuation is to match pink noise with the
-        let slope = match params.threshold.mode.value() {
+            // default settings. When using sidechaining we explicitly don't want this because
-            ThresholdMode::Internal => params.threshold.curve_slope.value() - 3.0,
+            // the curve should be a flat offset to the sidechain input at the default settings.
-            ThresholdMode::SidechainMatch | ThresholdMode::SidechainCompress => {
+            slope: match params.threshold.mode.value() {
-                params.threshold.curve_slope.value()
+                ThresholdMode::Internal => params.threshold.curve_slope.value() - 3.0,
-            }
+                ThresholdMode::SidechainMatch | ThresholdMode::SidechainCompress => {
+                    params.threshold.curve_slope.value()
+                }
+            },
+            curve: params.threshold.curve_curve.value(),
         };
-        let curve = params.threshold.curve_curve.value();
+        let curve = Curve::new(&curve_params);
-        let log2_center_freq = params.threshold.center_frequency.value().log2();
 
         if self
             .should_update_downwards_thresholds
             .compare_exchange(true, false, Ordering::SeqCst, Ordering::SeqCst)
             .is_ok()
         {
-            let intercept = intercept + params.compressors.downwards.threshold_offset_db.value();
+            let downwards_intercept = params.compressors.downwards.threshold_offset_db.value();
             for (log2_freq, threshold_db) in self
                 .log2_freqs
                 .iter()
                 .zip(self.downwards_thresholds_db.iter_mut())
             {
-                let offset = log2_freq - log2_center_freq;
+                *threshold_db = curve.evaluate_log2(*log2_freq) + downwards_intercept;
-                *threshold_db = intercept + (slope * offset) + (curve * offset * offset);
             }
         }
 
@@ -1017,14 +1020,13 @@ impl CompressorBank {
             .compare_exchange(true, false, Ordering::SeqCst, Ordering::SeqCst)
             .is_ok()
         {
-            let intercept = intercept + params.compressors.upwards.threshold_offset_db.value();
+            let upwards_intercept = params.compressors.upwards.threshold_offset_db.value();
             for (log2_freq, threshold_db) in self
                 .log2_freqs
                 .iter()
                 .zip(self.upwards_thresholds_db.iter_mut())
             {
-                let offset = log2_freq - log2_center_freq;
+                *threshold_db = curve.evaluate_log2(*log2_freq) + upwards_intercept;
-                *threshold_db = intercept + (slope * offset) + (curve * offset * offset);
             }
         }
 

plugins/spectral_compressor/src/curve.rs

@@ -0,0 +1,61 @@
+//! Abstractions for the parameterized threshold curve.
+//!
+//! This was previously computed directly inside of the `CompressorBank` but this makes it easier to
+//! reuse it when drawing the GUI.
+
+/// Parameters for a curve, similar to the fields found in `ThresholdParams` but using plain floats
+/// instead of parameters.
+#[derive(Debug, Clone, Copy)]
+pub struct CurveParams {
+    /// The compressor threshold at the center frequency. When sidechaining is enabled, the input
+    /// signal is gained by the inverse of this value. This replaces the input gain in the original
+    /// Spectral Compressor. In the polynomial below, this is the intercept.
+    pub intercept: f32,
+    /// The center frqeuency for the target curve when sidechaining is not enabled. The curve is a
+    /// polynomial `threshold_db + curve_slope*x + curve_curve*(x^2)` that evaluates to a decibel
+    /// value, where `x = log2(center_frequency) - log2(bin_frequency)`. In other words, this is
+    /// evaluated in the log/log domain for decibels and octaves.
+    pub center_frequency: f32,
+    /// The slope for the curve, in the log/log domain. See the polynomial above.
+    pub slope: f32,
+    /// The, uh, 'curve' for the curve, in the logarithmic domain. This is the third coefficient in
+    /// the quadratic polynomial and controls the parabolic behavior. Positive values turn the curve
+    /// into a v-shaped curve, while negative values attenuate everything outside of the center
+    /// frequency. See the polynomial above.
+    pub curve: f32,
+}
+
+/// Evaluates the quadratic threshold curve. This used to be calculated directly inside of the
+/// compressor bank since it's so simple, but the editor also needs to compute this so it makes
+/// sense to deduplicate it a bit.
+///
+/// The curve is evaluated in log-log space (so with octaves being the independent variable and gain
+/// in decibels being the output of the equation).
+pub struct Curve<'a> {
+    params: &'a CurveParams,
+    /// The 2-logarithm of [`CurveParams::cemter_frequency`].
+    log2_center_frequency: f32,
+}
+
+impl<'a> Curve<'a> {
+    pub fn new(params: &'a CurveParams) -> Self {
+        Self {
+            params,
+            log2_center_frequency: params.center_frequency.log2(),
+        }
+    }
+
+    /// Evaluate the curve for the 2-logarithm of the frequency value. This can be used as an
+    /// optimization to avoid computing these logarithms all the time.
+    #[inline]
+    pub fn evaluate_log2(&self, log2_freq: f32) -> f32 {
+        let offset = log2_freq - self.log2_center_frequency;
+        self.params.intercept + (self.params.slope * offset) + (self.params.curve * offset * offset)
+    }
+
+    /// Evaluate the curve for a value in Hertz.
+    #[inline]
+    pub fn evaluate_plain(&self, freq: f32) -> f32 {
+        self.evaluate_log2(freq.log2())
+    }
+}

plugins/spectral_compressor/src/lib.rs

@@ -28,6 +28,7 @@ use triple_buffer::TripleBuffer;
 
 mod analyzer;
 mod compressor_bank;
+mod curve;
 mod dry_wet_mixer;
 mod editor;