From e2954ba5b02bad786ee443a0ffe363ad19470a98 Mon Sep 17 00:00:00 2001
From: Nathan Vegdahl <cessen@cessen.com>
Date: Wed, 8 Jun 2016 23:00:09 -0700
Subject: [PATCH] Added more code for handling XYZ color and spectral samples.

---
 src/color/mod.rs | 143 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 142 insertions(+), 1 deletion(-)

diff --git a/src/color/mod.rs b/src/color/mod.rs
index 8c139dc..ea78703 100644
--- a/src/color/mod.rs
+++ b/src/color/mod.rs
@@ -2,6 +2,147 @@ mod spectra_xyz;
 
 use self::spectra_xyz::{spectrum_xyz_to_p, EQUAL_ENERGY_REFLECTANCE};
 
-pub fn xyz_to_spectrum(xyz: (f32, f32, f32), wavelength: f32) -> f32 {
+// Minimum and maximum wavelength of light we care about, in nanometers
+const WL_MIN: f32 = 380.0;
+const WL_MAX: f32 = 700.0;
+const WL_RANGE: f32 = WL_MAX - WL_MIN;
+const WL_RANGE_Q: f32 = WL_RANGE / 4.0;
+
+pub trait Color {
+    fn sample_spectrum(&self, wavelength: f32) -> f32;
+}
+
+
+#[derive(Copy, Clone, Debug)]
+pub struct SpectralSample {
+    e: [f32; 4],
+    hero_wavelength: f32,
+}
+
+impl SpectralSample {
+    pub fn new(wavelength: f32) -> SpectralSample {
+        debug_assert!(wavelength >= WL_MIN && wavelength <= WL_MAX);
+        SpectralSample {
+            e: [0.0; 4],
+            hero_wavelength: wavelength,
+        }
+    }
+
+    pub fn add_color<T: Color>(&mut self, color: &T) {
+        self.e[0] += color.sample_spectrum(self.wl_n(0));
+        self.e[1] += color.sample_spectrum(self.wl_n(1));
+        self.e[2] += color.sample_spectrum(self.wl_n(2));
+        self.e[3] += color.sample_spectrum(self.wl_n(3));
+    }
+
+    pub fn mul_color<T: Color>(&mut self, color: &T) {
+        self.e[0] *= color.sample_spectrum(self.wl_n(0));
+        self.e[1] *= color.sample_spectrum(self.wl_n(1));
+        self.e[2] *= color.sample_spectrum(self.wl_n(2));
+        self.e[3] *= color.sample_spectrum(self.wl_n(3));
+    }
+
+    fn wl_n(&self, n: usize) -> f32 {
+        let mut wl = self.hero_wavelength + (WL_RANGE_Q * n as f32);
+        if wl > WL_MAX {
+            wl - WL_RANGE
+        } else {
+            wl
+        }
+    }
+}
+
+
+#[derive(Copy, Clone, Debug)]
+pub struct XYZ {
+    pub x: f32,
+    pub y: f32,
+    pub z: f32,
+}
+
+impl XYZ {
+    fn new(x: f32, y: f32, z: f32) -> XYZ {
+        XYZ { x: x, y: y, z: z }
+    }
+
+    fn from_wavelength(wavelength: f32, intensity: f32) -> XYZ {
+        XYZ {
+            x: x_1931(wavelength) * intensity,
+            y: y_1931(wavelength) * intensity,
+            z: z_1931(wavelength) * intensity,
+        }
+    }
+}
+
+impl Color for XYZ {
+    fn sample_spectrum(&self, wavelength: f32) -> f32 {
+        xyz_to_spectrum((self.x, self.y, self.z), wavelength)
+    }
+}
+
+
+/// Samples an CIE 1931 XYZ color at a particular wavelength, according to
+/// the method in the paper "Physically Meaningful Rendering using Tristimulus
+/// Colours" by Meng et al.
+fn xyz_to_spectrum(xyz: (f32, f32, f32), wavelength: f32) -> f32 {
     spectrum_xyz_to_p(wavelength, xyz) * (1.0 / EQUAL_ENERGY_REFLECTANCE)
 }
+
+
+/// Close analytic approximations of the CIE 1931 XYZ color curves.
+/// From the paper "Simple Analytic Approximations to the CIE XYZ Color Matching
+/// Functions" by Wyman et al.
+fn x_1931(wavelength: f32) -> f32 {
+    let t1 = (wavelength - 442.0) *
+             (if wavelength < 442.0 {
+        0.0624
+    } else {
+        0.0374
+    });
+    let t2 = (wavelength - 599.8) *
+             (if wavelength < 599.8 {
+        0.0264
+    } else {
+        0.0323
+    });
+    let t3 = (wavelength - 501.1) *
+             (if wavelength < 501.1 {
+        0.0490
+    } else {
+        0.0382
+    });
+    (0.362 * (-0.5 * t1 * t1).exp()) + (1.056 * (-0.5 * t2 * t2).exp()) -
+    (0.065 * (-0.5 * t3 * t3).exp())
+}
+
+fn y_1931(wavelength: f32) -> f32 {
+    let t1 = (wavelength - 568.8) *
+             (if wavelength < 568.8 {
+        0.0213
+    } else {
+        0.0247
+    });
+    let t2 = (wavelength - 530.9) *
+             (if wavelength < 530.9 {
+        0.0613
+    } else {
+        0.0322
+    });
+    (0.821 * (-0.5 * t1 * t1).exp()) + (0.286 * (-0.5 * t2 * t2).exp())
+}
+
+fn z_1931(wavelength: f32) -> f32 {
+    let t1 = (wavelength - 437.0) *
+             (if wavelength < 437.0 {
+        0.0845
+    } else {
+        0.0278
+    });
+    let t2 = (wavelength - 459.0) *
+             (if wavelength < 459.0 {
+        0.0385
+    } else {
+        0.0725
+    });
+    (1.217 * (-0.5 * t1 * t1).exp()) + (0.681 * (-0.5 * t2 * t2).exp())
+}