From ed181a809ea448ffe87606fe297ad20bd08c8264 Mon Sep 17 00:00:00 2001
From: Brandon Thomas <echelon@gmail.com>
Date: Tue, 13 Oct 2026 06:32:45 -0400
Subject: [PATCH] Render: pictures with opposite corrections no longer share
 decoded pixels

The decoded-picture cache keyed adjustments with an xor-multiply mix in
which brightness b, contrast c and -b, -c collide (the sign bits cancel),
so of two copies of a picture, one +40% bright and contrast and one -40%,
both showed whichever was decoded first. The key is now a real hash.

Wild synthetic set: brightness/contrast grid 27.86 -> 15.93.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
---
 crates/render/src/images.rs | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/crates/render/src/images.rs b/crates/render/src/images.rs
index 4d4f96d..238821c 100644
--- a/crates/render/src/images.rs
+++ b/crates/render/src/images.rs
@@ -66,11 +66,11 @@ fn with<R>(f: impl FnOnce(&mut Cache) -> R) -> R {
 }
 
 fn adjust_key(a: &PictureAdjust) -> u64 {
-    let mut h: u64 = 1469598103934665603;
-    let mut mix = |v: u64| {
-        h ^= v;
-        h = h.wrapping_mul(1099511628211);
-    };
+    use std::hash::Hasher;
+    // A real hash: an FNV-style xor-multiply mix let (b, c) and (-b, -c) collide (the sign bits
+    // cancel), so a picture brightened and contrasted showed the darkened copy's pixels.
+    let mut hs = std::hash::DefaultHasher::new();
+    let mut mix = |v: u64| hs.write_u64(v);
     mix(a.brightness.to_bits());
     mix(a.contrast.to_bits());
     mix(a.saturation.unwrap_or(1.0).to_bits());
@@ -84,7 +84,7 @@ fn adjust_key(a: &PictureAdjust) -> u64 {
             mix(((c.r as u64) << 24) | ((c.g as u64) << 16) | ((c.b as u64) << 8) | c.a as u64 | 1 << 41);
         }
     }
-    h
+    hs.finish()
 }
 
 /// The duotone's dark and light colours. The painter resolves them to plain RGB first (see
@@ -368,6 +368,17 @@ mod tests {
         assert_ne!(adjust_key(&adj), adjust_key(&PictureAdjust::default()));
     }
 
+    #[test]
+    fn opposite_brightness_and_contrast_are_cached_apart() {
+        let a = PictureAdjust { brightness: 0.4, contrast: 0.4, ..Default::default() };
+        let b = PictureAdjust { brightness: -0.4, contrast: -0.4, ..Default::default() };
+        assert_ne!(adjust_key(&a), adjust_key(&b));
+        let c = PictureAdjust { brightness: 0.4, contrast: -0.4, ..Default::default() };
+        let d = PictureAdjust { brightness: -0.4, contrast: 0.4, ..Default::default() };
+        assert_ne!(adjust_key(&c), adjust_key(&d));
+        assert_eq!(adjust_key(&a), adjust_key(&a.clone()));
+    }
+
     #[test]
     fn a_gif_of_many_tiny_frames_keeps_only_the_frame_cap() {
         use image::{Delay, Frame, Rgba, RgbaImage};