From 0d2e80145cb4fbc8ae2ddcb7dc2898116660755d Mon Sep 17 00:00:00 2001
From: Bradlee Speice <bradlee@speice.io>
Date: Sat, 1 Aug 2026 15:52:44 -0400
Subject: [PATCH] Add color blending

---
 enkou-shaders/src/image.rs | 79 ++++++++++++++++++++++++++++++++++++++
 enkou-shaders/src/lib.rs   |  1 +
 2 files changed, 80 insertions(+)
 create mode 100644 enkou-shaders/src/image.rs

diff --git a/enkou-shaders/src/image.rs b/enkou-shaders/src/image.rs
new file mode 100644
index 0000000..62b49f9
--- /dev/null
+++ b/enkou-shaders/src/image.rs
@@ -0,0 +1,79 @@
+//! Image
+use glam::Vec4;
+use libm::floorf;
+
+/// Blending modes for mapping IFS color values (which are on a scale `[0, 1]`)
+/// to RGBA colors.
+#[derive(Copy, Clone, Default)]
+#[repr(u32)]
+pub enum BlendMode {
+    /// Map IFS color values to a linear blend of the nearest two palette colors
+    #[default]
+    Linear = 0,
+
+    /// Map IFS color values to the nearest single palette color
+    Step = 1,
+}
+
+impl BlendMode {
+    /// Map an IFS color value to RGBA color from the provided palette.
+    pub fn ifs_to_rgb(&self, color: f32, palette: &[Vec4]) -> Vec4 {
+        let colors_m_one = palette.len() - 1;
+        let period = 1.0 / colors_m_one as f32;
+        let index_lower = floorf(color / period) as usize;
+        let index_upper = (index_lower + 1).clamp(0, colors_m_one);
+        let rem = color % period / period;
+
+        match self {
+            BlendMode::Linear => palette[index_lower].lerp(palette[index_upper], rem),
+            BlendMode::Step => palette[index_lower],
+        }
+    }
+}
+
+// UNSAFE: Sound because enum has guaranteed layout (u32) and defined zero-value
+unsafe impl bytemuck::Zeroable for BlendMode {}
+// UNSAFE: Sound because enum has guaranteed layout (u32) and defined zero-value
+unsafe impl bytemuck::Pod for BlendMode {}
+
+#[cfg(test)]
+mod test {
+    use glam::Vec4;
+    use crate::image::BlendMode;
+
+    #[test]
+    fn blend_linear() {
+        let ifs_to_rgb = |color, palette| BlendMode::Linear.ifs_to_rgb(color, palette);
+
+        let palette = &[Vec4::splat(0.0), Vec4::splat(1.0)];
+        assert_eq!(ifs_to_rgb(0.0, palette), Vec4::splat(0.0));
+        assert_eq!(ifs_to_rgb(0.5, palette), Vec4::splat(0.5));
+        assert_eq!(ifs_to_rgb(1.0, palette), Vec4::splat(1.0));
+
+        let palette = &[Vec4::splat(1.0), Vec4::splat(2.0), Vec4::splat(3.0)];
+        assert_eq!(ifs_to_rgb(0.0, palette), Vec4::splat(1.0));
+        assert_eq!(ifs_to_rgb(0.5, palette), Vec4::splat(2.0));
+        assert_eq!(ifs_to_rgb(1.0, palette), Vec4::splat(3.0));
+
+        let palette = &[Vec4::splat(1.0), Vec4::splat(2.0), Vec4::splat(3.0), Vec4::splat(4.0)];
+        assert_eq!(ifs_to_rgb(0.0, palette), Vec4::splat(1.0));
+        assert_eq!(ifs_to_rgb(0.5, palette), Vec4::splat(2.5));
+        assert_eq!(ifs_to_rgb(1.0, palette), Vec4::splat(4.0));
+    }
+
+    #[test]
+    fn blend_step() {
+        let ifs_to_rgb = |color, palette| BlendMode::Step.ifs_to_rgb(color, palette);
+
+        let palette = &[Vec4::splat(0.0), Vec4::splat(1.0)];
+        assert_eq!(ifs_to_rgb(0.5, palette), Vec4::splat(0.0));
+
+        let palette = &[Vec4::splat(1.0), Vec4::splat(2.0), Vec4::splat(3.0)];
+        assert_eq!(ifs_to_rgb(0.0, palette), palette[0]);
+        assert_eq!(ifs_to_rgb(0.25, palette), palette[0]);
+        assert_eq!(ifs_to_rgb(0.4, palette), palette[0]);
+        assert_eq!(ifs_to_rgb(0.5, palette), palette[1]);
+        assert_eq!(ifs_to_rgb(0.7, palette), palette[1]);
+        assert_eq!(ifs_to_rgb(1.0, palette), palette[2]);
+    }
+}
diff --git a/enkou-shaders/src/lib.rs b/enkou-shaders/src/lib.rs
index 0e485ae..38d0cba 100644
--- a/enkou-shaders/src/lib.rs
+++ b/enkou-shaders/src/lib.rs
@@ -6,6 +6,7 @@ pub mod camera;
 pub mod chaos_game;
 pub mod transform;
 pub mod variation;
+mod image;
 
 use glam::Affine2;