8 files changed, 34 insertions, 5 deletions

diff --git a/niri-config/src/lib.rs b/niri-config/src/lib.rs
index 6e8bab2e..9bf62e13 100644
--- a/niri-config/src/lib.rs
+++ b/niri-config/src/lib.rs
@@ -697,6 +697,8 @@ pub struct WindowRule {
 
     #[knuffel(child, unwrap(argument))]
     pub draw_border_with_background: Option<bool>,
+    #[knuffel(child, unwrap(argument))]
+    pub opacity: Option<f32>,
 }
 
 // Remember to update the PartialEq impl when adding fields!
diff --git a/niri-visual-tests/src/cases/window.rs b/niri-visual-tests/src/cases/window.rs
index f8f7a1af..a8855f46 100644
--- a/niri-visual-tests/src/cases/window.rs
+++ b/niri-visual-tests/src/cases/window.rs
@@ -49,7 +49,7 @@ impl TestCase for Window {
         let location = Point::from(((size.w - win_size.w) / 2, (size.h - win_size.h) / 2));
 
         self.window
-            .render(renderer, location, Scale::from(1.))
+            .render(renderer, location, Scale::from(1.), 1.)
             .into_iter()
             .map(|elem| Box::new(elem) as _)
             .collect()
diff --git a/niri-visual-tests/src/test_window.rs b/niri-visual-tests/src/test_window.rs
index 4f89d15e..341f4fbf 100644
--- a/niri-visual-tests/src/test_window.rs
+++ b/niri-visual-tests/src/test_window.rs
@@ -146,6 +146,7 @@ impl LayoutElement for TestWindow {
         _renderer: &mut R,
         location: Point<i32, Logical>,
         scale: Scale<f64>,
+        alpha: f32,
     ) -> Vec<LayoutElementRenderElement<R>> {
         let inner = self.inner.borrow();
 
@@ -154,7 +155,7 @@ impl LayoutElement for TestWindow {
                 &inner.buffer,
                 location.to_physical_precise_round(scale),
                 scale,
-                1.,
+                alpha,
                 Kind::Unspecified,
             )
             .into(),
@@ -163,7 +164,7 @@ impl LayoutElement for TestWindow {
                 (location - Point::from((inner.csd_shadow_width, inner.csd_shadow_width)))
                     .to_physical_precise_round(scale),
                 scale,
-                1.,
+                alpha,
                 Kind::Unspecified,
             )
             .into(),
diff --git a/resources/default-config.kdl b/resources/default-config.kdl
index 485ab649..0878f213 100644
--- a/resources/default-config.kdl
+++ b/resources/default-config.kdl
@@ -414,6 +414,16 @@ animations {
     // Set this to `false` to draw them as borders around the window even for
     // windows which use client-side decorations.
     draw-border-with-background false
+
+    // Set the opacity of the window.
+    // This is applied on top of the window's own opacity, so semitransparent
+    // windows will become even more transparent.
+    // Opacity is applied to every surface of the window individually, so
+    // subsurfaces and pop-up menus will show window content behind them.
+    // Also, focus ring and border with background will show through
+    // semitransparent windows (see prefer-no-csd and
+    // the draw-border-with-background property above).
+    opacity 0.5
 }
 
 // Here's a useful example. Work around WezTerm's initial configure bug
diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index bc5c8dcf..bbca41c9 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -95,6 +95,7 @@ pub trait LayoutElement {
         renderer: &mut R,
         location: Point<i32, Logical>,
         scale: Scale<f64>,
+        alpha: f32,
     ) -> Vec<LayoutElementRenderElement<R>>;
 
     fn request_size(&self, size: Size<i32, Logical>);
@@ -1856,6 +1857,7 @@ mod tests {
             _renderer: &mut R,
             _location: Point<i32, Logical>,
             _scale: Scale<f64>,
+            _alpha: f32,
         ) -> Vec<LayoutElementRenderElement<R>> {
             vec![]
         }
diff --git a/src/layout/tile.rs b/src/layout/tile.rs
index 0609583e..faa17840 100644
--- a/src/layout/tile.rs
+++ b/src/layout/tile.rs
@@ -325,9 +325,15 @@ impl<W: LayoutElement> Tile<W> {
         view_size: Size<i32, Logical>,
         focus_ring: bool,
     ) -> impl Iterator<Item = TileRenderElement<R>> {
+        let alpha = if self.is_fullscreen {
+            1.
+        } else {
+            self.window.rules().opacity.unwrap_or(1.).clamp(0., 1.)
+        };
+
         let rv = self
             .window
-            .render(renderer, location + self.window_loc(), scale)
+            .render(renderer, location + self.window_loc(), scale, alpha)
             .into_iter()
             .map(Into::into);
 
diff --git a/src/window/mapped.rs b/src/window/mapped.rs
index a48ca636..52b9ac92 100644
--- a/src/window/mapped.rs
+++ b/src/window/mapped.rs
@@ -108,13 +108,14 @@ impl LayoutElement for Mapped {
         renderer: &mut R,
         location: Point<i32, Logical>,
         scale: Scale<f64>,
+        alpha: f32,
     ) -> Vec<LayoutElementRenderElement<R>> {
         let buf_pos = location - self.window.geometry().loc;
         self.window.render_elements(
             renderer,
             buf_pos.to_physical_precise_round(scale),
             scale,
-            1.,
+            alpha,
         )
     }
 
diff --git a/src/window/mod.rs b/src/window/mod.rs
index 63cdbe3f..ddf712b0 100644
--- a/src/window/mod.rs
+++ b/src/window/mod.rs
@@ -52,6 +52,9 @@ pub struct ResolvedWindowRules {
     ///
     /// `None` means using the SSD heuristic.
     pub draw_border_with_background: Option<bool>,
+
+    /// Extra opacity to draw this window with.
+    pub opacity: Option<f32>,
 }
 
 impl<'a> WindowRef<'a> {
@@ -82,6 +85,7 @@ impl ResolvedWindowRules {
             max_width: None,
             max_height: None,
             draw_border_with_background: None,
+            opacity: None,
         }
     }
 
@@ -153,6 +157,9 @@ impl ResolvedWindowRules {
                 if let Some(x) = rule.draw_border_with_background {
                     resolved.draw_border_with_background = Some(x);
                 }
+                if let Some(x) = rule.opacity {
+                    resolved.opacity = Some(x);
+                }
             }
 
             resolved.open_on_output = open_on_output.map(|x| x.to_owned());