From e12ff12dbf5474ff63bfbcb45a3f447d4b154701 Mon Sep 17 00:00:00 2001
From: Walther <veeti.haapsamo@gmail.com>
Date: Sat, 23 Sep 2023 15:20:49 +0300
Subject: [PATCH] feature: add spectral rendering helper methods

---
 clovers/src/lib.rs      |  1 +
 clovers/src/spectral.rs | 72 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 clovers/src/spectral.rs

diff --git a/clovers/src/lib.rs b/clovers/src/lib.rs
index f2b53787..e69bac22 100644
--- a/clovers/src/lib.rs
+++ b/clovers/src/lib.rs
@@ -89,6 +89,7 @@ pub mod pdf;
 pub mod random;
 pub mod ray;
 pub mod scenes;
+pub mod spectral;
 pub mod textures;
 
 /// Rendering options struct
diff --git a/clovers/src/spectral.rs b/clovers/src/spectral.rs
new file mode 100644
index 00000000..80c833c9
--- /dev/null
+++ b/clovers/src/spectral.rs
@@ -0,0 +1,72 @@
+//! Spectral rendering functionalities for the renderer
+
+use core::{array::from_fn, ops::Range};
+use rand::rngs::SmallRng;
+use rand_distr::uniform::SampleRange;
+
+use crate::{color::Color, Float};
+
+/// Wavelength in nanometers
+pub type Wavelength = usize;
+
+const MIN_WAVELENGTH: Wavelength = 380;
+const MAX_WAVELENGTH: Wavelength = 780;
+const SPECTRUM: Range<Wavelength> = MIN_WAVELENGTH..MAX_WAVELENGTH;
+const SPECTRUM_SIZE: usize = MAX_WAVELENGTH - MIN_WAVELENGTH;
+const WAVE_SAMPLE_COUNT: usize = 4;
+
+/// Return a random wavelength, sampled uniformly from the visible spectrum.
+pub fn random_wavelength(rng: &mut SmallRng) -> Wavelength {
+    SPECTRUM.sample_single(rng)
+}
+
+/// Given a hero wavelength, create additional equidistant wavelengths in the visible spectrum. Returns an array of wavelengths, with the original hero wavelength as the first one.
+#[must_use]
+pub fn rotate_wavelength(hero: Wavelength) -> [Wavelength; WAVE_SAMPLE_COUNT] {
+    from_fn(|j| {
+        (hero - MIN_WAVELENGTH + ((1 + j) / WAVE_SAMPLE_COUNT) * SPECTRUM_SIZE)
+            % (SPECTRUM_SIZE + MIN_WAVELENGTH)
+    })
+}
+
+/// Helper function adapted from <https://en.wikipedia.org/wiki/CIE_1931_color_space#Analytical_approximation>
+fn gaussian(x: Float, alpha: Float, mu: Float, sigma1: Float, sigma2: Float) -> Float {
+    let t = (x - mu) / (if x < mu { sigma1 } else { sigma2 });
+    alpha * (-(t * t) / 2.0).exp()
+}
+
+/// Color component transfer function.
+/// Note: Produces `sRGB` digital values with a range 0 to 1, which must then be multiplied by 2^(bit depth) – 1 and quantized.
+/// <https://color.org/chardata/rgb/sRGB.pdf>
+#[must_use]
+pub fn color_component_transfer(c: Float) -> Float {
+    if c.abs() < 0.003_130_8 {
+        12.92 * c
+    } else {
+        1.055 * c.powf(1.0 / 2.4) - 0.055
+    }
+}
+
+/// Helper function adapted from <https://en.wikipedia.org/wiki/CIE_1931_color_space#Analytical_approximation>
+impl From<Wavelength> for Color {
+    // TODO: precision loss
+    #[allow(clippy::cast_precision_loss)]
+    fn from(lambda: Wavelength) -> Self {
+        // With the wavelength λ measured in nanometers, we then approximate the 1931 color matching functions:
+        let l: Float = lambda as Float;
+        let x = 0.0 // for readability of next lines
+            + gaussian(l, 1.056, 599.8, 37.9, 31.0)
+            + gaussian(l, 0.362, 442.0, 16.0, 26.7)
+            + gaussian(l, -0.065, 501.1, 20.4, 26.2);
+        let y = gaussian(l, 0.821, 568.8, 46.9, 40.5) + gaussian(l, 0.286, 530.9, 16.3, 31.1);
+        let z = gaussian(l, 1.217, 437.0, 11.8, 36.0) + gaussian(l, 0.681, 459.0, 26.0, 13.8);
+        // Convert from XYZ to sRGB
+        // https://color.org/chardata/rgb/sRGB.pdf
+        // TODO: more correct color management!
+        let r = 3.240_625_5 * x - 1.537_208 * y - 0.498_628_6 * z;
+        let g = -0.968_930_7 * x + 1.875_756_1 * y + 0.041_517_5 * z;
+        let b = 0.055_710_1 * x - 0.204_021_1 * y + 1.056_995_9 * z;
+
+        Color { r, g, b }
+    }
+}