Centralize accent foreground contrast across Ferese UI

crates/ferese-theme/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "ferese-theme"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version = "1.95"
+
+[dependencies]
+libcosmic.workspace = true

crates/ferese-theme/src/lib.rs

@@ -0,0 +1,226 @@
+//! Ferese owns foreground contrast; toolkit defaults must not choose it separately.
+use cosmic::{
+    iced::{Background, Color},
+    theme,
+    widget::button,
+};
+
+pub fn luminance(color: Color) -> f32 {
+    let linear = |v: f32| {
+        if v <= 0.04045 {
+            v / 12.92
+        } else {
+            ((v + 0.055) / 1.055).powf(2.4)
+        }
+    };
+    0.2126 * linear(color.r) + 0.7152 * linear(color.g) + 0.0722 * linear(color.b)
+}
+
+pub fn contrast(a: Color, b: Color) -> f32 {
+    let (a, b) = (luminance(a), luminance(b));
+    (a.max(b) + 0.05) / (a.min(b) + 0.05)
+}
+
+/// Resolve a control fill against its surface before selecting its foreground.
+/// The returned fill is opaque, so wallpaper and translucent panels cannot change
+/// the contrast of a small, filled control.
+pub fn composite(fill: Color, surface: Color) -> Color {
+    let a = fill.a.clamp(0., 1.);
+    Color::from_rgb(
+        fill.r * a + surface.r * (1. - a),
+        fill.g * a + surface.g * (1. - a),
+        fill.b * a + surface.b * (1. - a),
+    )
+}
+
+/// Preserve the user's preferred color when it passes normal-text contrast.
+/// Otherwise black or white always provides at least 4.5:1 on an opaque sRGB fill.
+pub fn foreground(background: Color, preferred: Color) -> Color {
+    let preferred = composite(preferred, background);
+    if contrast(background, preferred) >= 4.5 {
+        return preferred;
+    }
+    if contrast(background, Color::BLACK) >= contrast(background, Color::WHITE) {
+        Color::BLACK
+    } else {
+        Color::WHITE
+    }
+}
+
+/// Resolve configured accent transparency before passing an opaque color to a toolkit.
+pub fn accent_color(accent: Color, surface: Color) -> cosmic::cosmic_theme::palette::Srgb {
+    rgba(composite(accent, surface)).color
+}
+
+/// Supply the same semantic on-accent colors to native toolkit controls.
+pub fn apply(native: &mut cosmic::cosmic_theme::Theme, preferred: Color) {
+    let surface: Color = native.primary(false).base.into();
+    for component in [&mut native.accent, &mut native.accent_button] {
+        let base = composite(component.base.into(), surface);
+        let on = foreground(base, preferred);
+        component.hover = rgba(readable_fill(
+            composite(component.hover.into(), surface),
+            base,
+            on,
+        ));
+        component.pressed = rgba(readable_fill(
+            composite(component.pressed.into(), surface),
+            base,
+            on,
+        ));
+        component.selected = rgba(composite(component.selected.into(), surface));
+        component.base = rgba(base);
+        component.on = rgba(on);
+        component.selected_text = rgba(foreground(
+            composite(component.selected.into(), surface),
+            on,
+        ));
+        component.disabled = rgba(composite(Color { a: 0.5, ..base }, surface));
+        component.on_disabled = rgba(foreground(composite(Color { a: 0.5, ..base }, surface), on));
+    }
+}
+
+// Native controls reuse one foreground for normal/hover/pressed. Keep their
+// hover feedback, but pull an unreadable fill toward the readable normal fill.
+fn readable_fill(candidate: Color, base: Color, on: Color) -> Color {
+    for step in 0..=20 {
+        let t = step as f32 / 20.;
+        let fill = Color::from_rgb(
+            candidate.r + (base.r - candidate.r) * t,
+            candidate.g + (base.g - candidate.g) * t,
+            candidate.b + (base.b - candidate.b) * t,
+        );
+        if contrast(fill, on) >= 4.5 {
+            return fill;
+        }
+    }
+    base
+}
+
+fn rgba(c: Color) -> cosmic::cosmic_theme::palette::Srgba {
+    cosmic::cosmic_theme::palette::Srgba::new(c.r, c.g, c.b, c.a)
+}
+
+#[derive(Clone, Copy)]
+enum State {
+    Active,
+    Hovered,
+    Pressed,
+    Disabled,
+}
+
+fn style(theme: &cosmic::Theme, state: State, focused: bool) -> button::Style {
+    let native = theme.cosmic();
+    let component = &native.accent_button;
+    let fill: Color = match state {
+        State::Active => component.base.into(),
+        State::Hovered => component.hover.into(),
+        State::Pressed => component.pressed.into(),
+        State::Disabled => Color {
+            a: 0.5,
+            ..component.base.into()
+        },
+    };
+    let background = composite(fill, native.primary(false).base.into());
+    let foreground = foreground(background, component.on.into());
+    button::Style {
+        background: Some(Background::Color(background)),
+        text_color: Some(foreground),
+        icon_color: Some(foreground),
+        border_radius: native.corner_radii.radius_xl.into(),
+        outline_width: if focused { 1. } else { 0. },
+        outline_color: foreground,
+        ..Default::default()
+    }
+}
+
+/// Filled accent buttons and selected rows share this per-state contrast policy.
+pub fn accent_button() -> theme::Button {
+    theme::Button::Custom {
+        active: Box::new(|focused, theme| style(theme, State::Active, focused)),
+        hovered: Box::new(|focused, theme| style(theme, State::Hovered, focused)),
+        pressed: Box::new(|focused, theme| style(theme, State::Pressed, focused)),
+        disabled: Box::new(|theme| style(theme, State::Disabled, false)),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn reference_contrast_values_use_linear_srgb() {
+        assert!((contrast(Color::BLACK, Color::WHITE) - 21.).abs() < 0.00001);
+        assert!((contrast(Color::from_rgb8(128, 128, 128), Color::BLACK) - 5.31721).abs() < 0.0001);
+    }
+    #[test]
+    fn foreground_preserves_readable_theme_text_and_rejects_low_contrast() {
+        let preferred = Color::from_rgb8(240, 242, 246);
+        assert_eq!(foreground(Color::BLACK, preferred), preferred);
+        assert_eq!(foreground(Color::WHITE, preferred), Color::BLACK);
+        assert_eq!(
+            foreground(Color::from_rgb8(61, 123, 230), preferred),
+            Color::BLACK
+        );
+    }
+    #[test]
+    fn all_accent_hues_and_opacities_have_readable_text() {
+        for r in (0..=255).step_by(17) {
+            for g in (0..=255).step_by(17) {
+                for b in (0..=255).step_by(17) {
+                    for surface in [Color::BLACK, Color::WHITE] {
+                        for alpha in [0., 0.2, 0.5, 1.] {
+                            let fill = composite(Color::from_rgba8(r, g, b, alpha), surface);
+                            let on = foreground(fill, Color::from_rgb8(120, 130, 140));
+                            assert!(contrast(fill, on) >= 4.5, "{fill:?}: {on:?}");
+                        }
+                    }
+                }
+            }
+        }
+    }
+    #[test]
+    fn native_button_states_keep_text_and_icons_readable() {
+        for accent in [
+            Color::BLACK,
+            Color::WHITE,
+            Color::from_rgb8(61, 123, 230),
+            Color::from_rgb8(255, 220, 0),
+        ] {
+            for mut native in [
+                cosmic::cosmic_theme::ThemeBuilder::dark()
+                    .accent(rgba(accent).color)
+                    .build(),
+                cosmic::cosmic_theme::ThemeBuilder::light()
+                    .accent(rgba(accent).color)
+                    .build(),
+            ] {
+                apply(&mut native, Color::from_rgb8(244, 247, 251));
+                for component in [&native.accent, &native.accent_button] {
+                    for fill in [component.base, component.hover, component.pressed] {
+                        assert!(contrast(fill.into(), component.on.into()) >= 4.5);
+                    }
+                    assert!(
+                        contrast(component.selected.into(), component.selected_text.into()) >= 4.5
+                    );
+                    assert!(
+                        contrast(component.disabled.into(), component.on_disabled.into()) >= 4.5
+                    );
+                }
+                let theme = cosmic::Theme::custom(std::sync::Arc::new(native));
+                for state in [
+                    State::Active,
+                    State::Hovered,
+                    State::Pressed,
+                    State::Disabled,
+                ] {
+                    let style = style(&theme, state, false);
+                    let Some(Background::Color(fill)) = style.background else {
+                        panic!("No fill")
+                    };
+                    assert_eq!(style.text_color, style.icon_color);
+                    assert!(contrast(fill, style.text_color.unwrap()) >= 4.5);
+                }
+            }
+        }
+    }
+}