From bf466ab1d71afafa4ab428f83167de7d14f45a78 Mon Sep 17 00:00:00 2001
From: James Hurst <james@jameshurst.ca>
Date: Sat, 18 Jan 2025 23:21:34 -0500
Subject: [PATCH] feat: enforce minimum contrast ratio for reverse video cursor

---
 color-types/src/lib.rs                   | 52 +++++++++++++++++++-----
 wezterm-gui/src/termwindow/render/mod.rs | 26 ++++++++----
 2 files changed, 60 insertions(+), 18 deletions(-)

diff --git a/color-types/src/lib.rs b/color-types/src/lib.rs
index bd12a1ae3cd..efa228e0f86 100644
--- a/color-types/src/lib.rs
+++ b/color-types/src/lib.rs
@@ -538,16 +538,8 @@ impl SrgbaTuple {
         *deltae::DeltaE::new(a, b, deltae::DEMethod::DE2000).value()
     }
 
-    pub fn contrast_ratio(&self, other: &Self) -> f64 {
-        let (_, _, l_a, _) = self.to_hsla();
-        let (_, _, l_b, _) = other.to_hsla();
-        let a = l_a + 0.05;
-        let b = l_b + 0.05;
-        if a > b {
-            a / b
-        } else {
-            b / a
-        }
+    pub fn contrast_ratio(&self, other: &Self) -> f32 {
+        self.to_linear().contrast_ratio(&other.to_linear())
     }
 }
 
@@ -891,6 +883,22 @@ impl LinearRgba {
             self.3,
         )
     }
+
+    pub fn relative_luminance(&self) -> f32 {
+        0.2126 * self.0 + 0.7152 * self.1 + 0.0722 * self.2
+    }
+
+    pub fn contrast_ratio(&self, other: &Self) -> f32 {
+        let l_a = self.relative_luminance();
+        let l_b = other.relative_luminance();
+        let a = l_a + 0.05;
+        let b = l_b + 0.05;
+        if a > b {
+            a / b
+        } else {
+            b / a
+        }
+    }
 }
 
 #[cfg(test)]
@@ -960,4 +968,28 @@ mod tests {
         let grey = SrgbaTuple::from_str("rgb:f0f0/f0f0/f0f0").unwrap();
         assert_eq!(grey.to_rgb_string(), "#f0f0f0");
     }
+
+    #[test]
+    fn linear_rgb_contrast_ratio() {
+        let a = LinearRgba::with_srgba(255, 0, 0, 1);
+        let b = LinearRgba::with_srgba(0, 255, 0, 1);
+        let contrast_ratio = a.contrast_ratio(&b);
+        assert!(
+            (2.91 - contrast_ratio).abs() < 0.01,
+            "contrast({}) == 2.91",
+            contrast_ratio
+        );
+    }
+
+    #[test]
+    fn srgba_contrast_ratio() {
+        let a = SrgbaTuple::from_str("hsl:0   100  50").unwrap();
+        let b = SrgbaTuple::from_str("hsl:120 100  50").unwrap();
+        let contrast_ratio = a.contrast_ratio(&b);
+        assert!(
+            (2.91 - contrast_ratio).abs() < 0.01,
+            "contrast({}) == 2.91",
+            contrast_ratio
+        );
+    }
 }
diff --git a/wezterm-gui/src/termwindow/render/mod.rs b/wezterm-gui/src/termwindow/render/mod.rs
index 7e164459b04..c819a8515ec 100644
--- a/wezterm-gui/src/termwindow/render/mod.rs
+++ b/wezterm-gui/src/termwindow/render/mod.rs
@@ -484,6 +484,8 @@ impl crate::TermWindow {
     }
 
     pub fn compute_cell_fg_bg(&self, params: ComputeCellFgBgParams) -> ComputeCellFgBgResult {
+        const CURSOR_MIN_CONTRAST: f32 = 2.5;
+
         if params.cursor.is_some() {
             if let Some(bg_color_mix) = self.get_intensity_if_bell_target_ringing(
                 params.pane.expect("cursor only set if pane present"),
@@ -524,12 +526,14 @@ impl crate::TermWindow {
                 self.dead_key_status != DeadKeyStatus::None || self.leader_is_active();
 
             if dead_key_or_leader && params.is_active_pane {
-                let (fg_color, bg_color) =
-                    if self.config.force_reverse_video_cursor && params.cursor_is_default_color {
-                        (params.bg_color, params.fg_color)
-                    } else {
-                        (params.cursor_fg, params.cursor_bg)
-                    };
+                let (fg_color, bg_color) = if self.config.force_reverse_video_cursor
+                    && params.cursor_is_default_color
+                    && params.fg_color.contrast_ratio(&params.bg_color) >= CURSOR_MIN_CONTRAST
+                {
+                    (params.bg_color, params.fg_color)
+                } else {
+                    (params.cursor_fg, params.cursor_bg)
+                };
 
                 let color = params
                     .config
@@ -585,7 +589,10 @@ impl crate::TermWindow {
                 CursorShape::BlinkingBlock | CursorShape::SteadyBlock,
                 CursorVisibility::Visible,
             ) => {
-                if self.config.force_reverse_video_cursor && params.cursor_is_default_color {
+                if self.config.force_reverse_video_cursor
+                    && params.cursor_is_default_color
+                    && params.fg_color.contrast_ratio(&params.bg_color) >= CURSOR_MIN_CONTRAST
+                {
                     (params.bg_color, params.fg_color, params.fg_color)
                 } else {
                     (
@@ -604,7 +611,10 @@ impl crate::TermWindow {
                 | CursorShape::SteadyBar,
                 CursorVisibility::Visible,
             ) => {
-                if self.config.force_reverse_video_cursor && params.cursor_is_default_color {
+                if self.config.force_reverse_video_cursor
+                    && params.cursor_is_default_color
+                    && params.fg_color.contrast_ratio(&params.bg_color) >= CURSOR_MIN_CONTRAST
+                {
                     (params.fg_color, params.bg_color, params.fg_color)
                 } else {
                     (params.fg_color, params.bg_color, params.cursor_bg)