feat(audio): add simplified HRTF with ITD and ILD

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/voltex_audio/src/hrtf.rs

@@ -0,0 +1,126 @@
+use std::f32::consts::PI;
+
+/// Head radius in meters (average human).
+const HEAD_RADIUS: f32 = 0.0875;
+/// Speed of sound in m/s.
+const SPEED_OF_SOUND: f32 = 343.0;
+
+/// HRTF filter result for a sound at a given azimuth angle.
+#[derive(Debug, Clone, Copy)]
+pub struct HrtfResult {
+    pub left_delay_samples: f32,   // ITD: delay for left ear in samples
+    pub right_delay_samples: f32,  // ITD: delay for right ear in samples
+    pub left_gain: f32,            // ILD: gain for left ear (0.0-1.0)
+    pub right_gain: f32,           // ILD: gain for right ear (0.0-1.0)
+}
+
+/// Calculate HRTF parameters from azimuth angle.
+/// azimuth: angle in radians, 0 = front, PI/2 = right, -PI/2 = left, PI = behind.
+pub fn calculate_hrtf(azimuth: f32, sample_rate: u32) -> HrtfResult {
+    // ITD: Woodworth formula
+    // time_diff = (HEAD_RADIUS / SPEED_OF_SOUND) * (azimuth + sin(azimuth))
+    let itd = (HEAD_RADIUS / SPEED_OF_SOUND) * (azimuth.abs() + azimuth.abs().sin());
+    let delay_samples = itd * sample_rate as f32;
+
+    // ILD: simplified frequency-independent model
+    // Sound is louder on the side facing the source
+    let shadow = 0.5 * (1.0 + azimuth.cos()); // 1.0 at front, 0.5 at side, 0.0 at back
+
+    let (left_delay, right_delay, left_gain, right_gain);
+    if azimuth >= 0.0 {
+        // Sound from right side
+        left_delay = delay_samples;
+        right_delay = 0.0;
+        left_gain = (0.3 + 0.7 * shadow).min(1.0); // shadowed side
+        right_gain = 1.0;
+    } else {
+        // Sound from left side
+        left_delay = 0.0;
+        right_delay = delay_samples;
+        left_gain = 1.0;
+        right_gain = (0.3 + 0.7 * shadow).min(1.0);
+    }
+
+    HrtfResult {
+        left_delay_samples: left_delay,
+        right_delay_samples: right_delay,
+        left_gain,
+        right_gain,
+    }
+}
+
+/// Calculate azimuth angle from listener position/forward to sound position.
+pub fn azimuth_from_positions(
+    listener_pos: [f32; 3],
+    listener_forward: [f32; 3],
+    listener_right: [f32; 3],
+    sound_pos: [f32; 3],
+) -> f32 {
+    let dx = sound_pos[0] - listener_pos[0];
+    let dy = sound_pos[1] - listener_pos[1];
+    let dz = sound_pos[2] - listener_pos[2];
+    let len = (dx * dx + dy * dy + dz * dz).sqrt();
+    if len < 1e-6 {
+        return 0.0;
+    }
+    let dir = [dx / len, dy / len, dz / len];
+
+    // Dot with right vector gives sin(azimuth)
+    let right_dot =
+        dir[0] * listener_right[0] + dir[1] * listener_right[1] + dir[2] * listener_right[2];
+    // Dot with forward vector gives cos(azimuth)
+    let fwd_dot = dir[0] * listener_forward[0]
+        + dir[1] * listener_forward[1]
+        + dir[2] * listener_forward[2];
+
+    right_dot.atan2(fwd_dot)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_hrtf_front() {
+        let r = calculate_hrtf(0.0, 44100);
+        assert!((r.left_delay_samples - 0.0).abs() < 1.0);
+        assert!((r.right_delay_samples - 0.0).abs() < 1.0);
+        assert!((r.left_gain - r.right_gain).abs() < 0.01); // symmetric
+    }
+
+    #[test]
+    fn test_hrtf_right() {
+        let r = calculate_hrtf(PI / 2.0, 44100);
+        assert!(r.left_delay_samples > r.right_delay_samples);
+        assert!(r.left_gain < r.right_gain);
+    }
+
+    #[test]
+    fn test_hrtf_left() {
+        let r = calculate_hrtf(-PI / 2.0, 44100);
+        assert!(r.right_delay_samples > r.left_delay_samples);
+        assert!(r.right_gain < r.left_gain);
+    }
+
+    #[test]
+    fn test_azimuth_front() {
+        let az = azimuth_from_positions(
+            [0.0; 3],
+            [0.0, 0.0, -1.0],
+            [1.0, 0.0, 0.0],
+            [0.0, 0.0, -5.0],
+        );
+        assert!(az.abs() < 0.1); // roughly front
+    }
+
+    #[test]
+    fn test_azimuth_right() {
+        let az = azimuth_from_positions(
+            [0.0; 3],
+            [0.0, 0.0, -1.0],
+            [1.0, 0.0, 0.0],
+            [5.0, 0.0, 0.0],
+        );
+        assert!((az - PI / 2.0).abs() < 0.1);
+    }
+}

crates/voltex_audio/src/lib.rs

@@ -5,6 +5,7 @@ pub mod mixing;
 pub mod wasapi;
 pub mod audio_system;
 pub mod spatial;
+pub mod hrtf;
 pub mod mix_group;
 pub mod audio_source;