From 08ada94bd270dc6851460637fb19b59617a238c3 Mon Sep 17 00:00:00 2001
From: Bradlee Speice <bradlee@speice.io>
Date: Sun, 12 Jul 2026 10:05:19 -0400
Subject: [PATCH] Implement post-transform coefficients

---
 Cargo.toml                       |  2 +-
 enkou-shaders/examples/gasket.rs | 30 +++++++++---------
 enkou-shaders/src/transform.rs   | 53 ++++++++++++++++++++++++++++++--
 enkou-shaders/src/variation.rs   |  6 ++++
 4 files changed, 73 insertions(+), 18 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 594cd29..b5fdf79 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,7 +24,7 @@ bytemuck = { version = "1.25.0", features = ["derive"] }
 glam = { version = "0.33.1", default-features = false, features = ["bytemuck", "scalar-math"] }
 image = { version = "0.25.10", default-features = false, features = ["default-formats"]}
 libm = "0.2.16"
-rand = {  version = "0.10.1", default-features = false }
+rand = { version = "0.10.1", default-features = false }
 rand_xoshiro = "0.8.1"
 rspirv = "0.13.0"
 tempfile = "3.27.0"
diff --git a/enkou-shaders/examples/gasket.rs b/enkou-shaders/examples/gasket.rs
index 55d0001..12ce467 100644
--- a/enkou-shaders/examples/gasket.rs
+++ b/enkou-shaders/examples/gasket.rs
@@ -17,21 +17,21 @@ const IMAGE_DIMENSION: UVec2 = uvec2(600, 600);
 
 pub fn main() -> Result<()> {
     let transforms = [
-        // F_0: (x / 2, y / 2)
-        Transform::new(
-            Affine2::from_coefficients(0.5, 0.0, 0.0, 0.0, 0.5, 0.0),
-            uvec2(0, 1),
-        ),
-        // F_1: ((x + 1) / 2, y / 2)
-        Transform::new(
-            Affine2::from_coefficients(0.5, 0.0, 0.5, 0.0, 0.5, 0.0),
-            uvec2(0, 1),
-        ),
-        // F_2: (x / 2, (y + 1) / 2)
-        Transform::new(
-            Affine2::from_coefficients(0.5, 0.0, 0.0, 0.0, 0.5, 0.5),
-            uvec2(0, 1),
-        ),
+        {
+            // F_0: (x / 2, y / 2)
+            let coefficients = Affine2::from_coefficients(0.5, 0.0, 0.0, 0.0, 0.5, 0.0);
+            Transform::new(coefficients, Affine2::IDENTITY, uvec2(0, 1))
+        },
+        {
+            // F_1: ((x + 1) / 2, y / 2)
+            let coefficients = Affine2::from_coefficients(0.5, 0.0, 0.5, 0.0, 0.5, 0.0);
+            Transform::new(coefficients, Affine2::IDENTITY, uvec2(0, 1))
+        },
+        {
+            // F_2: (x / 2, (y + 1) / 2)
+            let coefficients = Affine2::from_coefficients(0.5, 0.0, 0.0, 0.0, 0.5, 0.5);
+            Transform::new(coefficients, Affine2::IDENTITY, uvec2(0, 1))
+        },
     ];
 
     let weights = [1.0 / 3.0, 1.0 / 3.0, 1.0 / 3.0];
diff --git a/enkou-shaders/src/transform.rs b/enkou-shaders/src/transform.rs
index 3b8badc..0c91370 100644
--- a/enkou-shaders/src/transform.rs
+++ b/enkou-shaders/src/transform.rs
@@ -13,14 +13,16 @@ use rand::Rng;
 #[repr(C)]
 pub struct Transform {
     coefficients: Affine2,
+    coefficients_post: Affine2,
     variation_range: UVec2,
 }
 
 impl Transform {
     /// Create a new transform from an affine transformation matrix
-    pub fn new(coefficients: Affine2, variation_range: UVec2) -> Self {
+    pub fn new(coefficients: Affine2, coefficients_post: Affine2, variation_range: UVec2) -> Self {
         Transform {
             coefficients,
+            coefficients_post,
             variation_range,
         }
     }
@@ -43,6 +45,53 @@ impl Transform {
             point_output += variation.transform_point(point, rng, &self.coefficients)
         }
 
-        point_output
+        self.coefficients_post.transform_point2(point)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::rng::xoshiro256starstar_from_seed;
+    use crate::transform::Transform;
+    use crate::variation::{Variation, VariationKind};
+    use glam::{Affine2, uvec2, vec2};
+
+    #[test]
+    fn transform_scaling() {
+        let scale_coefficients = vec2(2.0, 0.5);
+        let transform = Transform::new(
+            Affine2::from_scale(scale_coefficients),
+            Affine2::IDENTITY,
+            uvec2(0, 1),
+        );
+
+        let mut rng = xoshiro256starstar_from_seed([0; 32]);
+        let variations = [Variation::IDENTITY];
+        let point = vec2(1.0, 1.0);
+
+        assert_eq!(
+            transform.transform_point(&mut rng, &variations, point),
+            scale_coefficients
+        );
+    }
+
+    #[test]
+    fn transform_scaling_post() {
+        let scale_coefficients = vec2(2.0, 0.5);
+        let transform_pdj = Transform::new(Affine2::IDENTITY, Affine2::IDENTITY, uvec2(0, 1));
+        let transform_pdj_post = Transform::new(
+            Affine2::IDENTITY,
+            Affine2::from_scale(scale_coefficients),
+            uvec2(0, 1),
+        );
+
+        let mut rng = xoshiro256starstar_from_seed([0; 32]);
+        let variations = [Variation::new(VariationKind::Pdj, 1.0, [0.0f32; 4].into())];
+        let point = vec2(1.0, 1.0);
+
+        let point_pdj = transform_pdj.transform_point(&mut rng, &variations, point);
+        let point_pdj_post = transform_pdj_post.transform_point(&mut rng, &variations, point);
+
+        assert_eq!(point_pdj * scale_coefficients, point_pdj_post);
     }
 }
diff --git a/enkou-shaders/src/variation.rs b/enkou-shaders/src/variation.rs
index 06a0df0..81c3463 100644
--- a/enkou-shaders/src/variation.rs
+++ b/enkou-shaders/src/variation.rs
@@ -20,6 +20,12 @@ use rand::{Rng, RngExt};
 #[repr(C)]
 pub struct VariationParams([f32; 4]);
 
+impl From<[f32; 4]> for VariationParams {
+    fn from(v: [f32; 4]) -> Self {
+        VariationParams(v)
+    }
+}
+
 /// Enum for all supported variation types
 ///
 /// ID numbers are chosen to match the variation identifier also used by `flam3`
-- 
2.52.0