Add some tonecurve stuff that I'm about to remove in favour of the splines package

Author: capnfabs

blitz/src/render_settings.rs

@@ -1,3 +1,41 @@
+use itertools::Itertools;
+use std::iter::once;
+
+pub struct ToneCurve {
+    x_vals: Vec<f32>,
+    tangents: Vec<f32>,
+}
+
+impl ToneCurve {
+    pub fn new(points: &Vec<f32>) -> Self {
+        // SPLINE TIME
+        // https://en.wikipedia.org/wiki/Cubic_Hermite_spline#Catmull%E2%80%93Rom_spline
+        let x_inc = 1.0 / points.len() as f32;
+        // for 4 points, this should be 12.5, 37.5, 62.5, 87.5, i.e. split into 4 even bands
+        let x_vals = (0..points.len()).map(|x| (x as f32 + 0.5) * x_inc);
+        let xs = once(0.0).chain(x_vals).chain(once(1.0)).collect_vec();
+        let ys = once(0.0)
+            .chain(points.iter().copied())
+            .chain(once(1.0))
+            .collect_vec();
+        assert_eq!(xs.len(), ys.len());
+
+        let tangents = (1..(xs.len() - 1))
+            .map(|k| {
+                // as per wikipedia:
+                // m_k =
+                (ys[k + 1] - ys[k - 1]) / (xs[k + 1] - xs[k - 1])
+            })
+            .collect_vec();
+        ToneCurve {
+            x_vals: xs,
+            tangents,
+        }
+    }
+
+    pub fn apply(val: f32) -> f32 {}
+}
+
 #[derive(Debug, Clone)]
 pub struct RenderSettings {
     pub tone_curve: Vec<f32>,