Add opacity factor to TextShape

crates/egui/src/painter.rs

@@ -469,6 +469,29 @@ impl Painter {
         }
     }
 
+    /// Paint text that has already been laid out in a [`Galley`].
+    ///
+    /// You can create the [`Galley`] with [`Self::layout`].
+    ///
+    /// The entire [`Galley`] will be rendered with the given opacity in gamma space.
+    #[inline]
+    pub fn galley_with_opacity_factor(
+        &self,
+        pos: Pos2,
+        galley: Arc<Galley>,
+        opacity_factor: f32,
+        fallback_color: Color32,
+    ) {
+        if !galley.is_empty() && opacity_factor > 0.0 {
+            self.add(Shape::galley_with_opacity_factor(
+                pos,
+                galley,
+                opacity_factor,
+                fallback_color,
+            ));
+        }
+    }
+
     #[deprecated = "Use `Painter::galley` or `Painter::galley_with_override_text_color` instead"]
     #[inline]
     pub fn galley_with_color(&self, pos: Pos2, galley: Arc<Galley>, text_color: Color32) {

crates/epaint/src/shape.rs

@@ -288,6 +288,19 @@ impl Shape {
             .into()
     }
 
+    /// The entire [`Galley`] will be rendered with the given opacity in gamma space.
+    #[inline]
+    pub fn galley_with_opacity_factor(
+        pos: Pos2,
+        galley: Arc<Galley>,
+        opacity_factor: f32,
+        fallback_color: Color32,
+    ) -> Self {
+        TextShape::new(pos, galley, fallback_color)
+            .with_opacity_factor(opacity_factor)
+            .into()
+    }
+
     #[inline]
     #[deprecated = "Use `Shape::galley` or `Shape::galley_with_override_text_color` instead"]
     pub fn galley_with_color(pos: Pos2, galley: Arc<Galley>, text_color: Color32) -> Self {
@@ -745,6 +758,10 @@ pub struct TextShape {
     /// This only affects the glyphs and will NOT replace background color nor strikethrough/underline color.
     pub override_text_color: Option<Color32>,
 
+    /// If set, the text will be rendered with the given opacity in gamma space
+    /// Affects everything: backgrounds, glyphs, strikethough, underline, etc.
+    pub opacity_factor: f32,
+
     /// Rotate text by this many radians clockwise.
     /// The pivot is `pos` (the upper left corner of the text).
     pub angle: f32,
@@ -762,6 +779,7 @@ impl TextShape {
             underline: Stroke::NONE,
             fallback_color,
             override_text_color: None,
+            opacity_factor: 1.0,
             angle: 0.0,
         }
     }
@@ -792,6 +810,13 @@ impl TextShape {
         self.angle = angle;
         self
     }
+
+    /// Render text with this opacity in gamma space
+    #[inline]
+    pub fn with_opacity_factor(mut self, opacity_factor: f32) -> Self {
+        self.opacity_factor = opacity_factor;
+        self
+    }
 }
 
 impl From<TextShape> for Shape {

crates/epaint/src/shape_transform.rs

@@ -56,6 +56,7 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             underline,
             fallback_color,
             override_text_color,
+            opacity_factor: _,
             angle: _,
         }) => {
             adjust_color(&mut underline.color);

crates/epaint/src/tessellator.rs

@@ -1474,13 +1474,18 @@ impl Tessellator {
             underline,
             override_text_color,
             fallback_color,
+            opacity_factor,
             angle,
         } = text_shape;
 
         if galley.is_empty() {
             return;
         }
 
+        if *opacity_factor <= 0.0 {
+            return;
+        }
+
         if galley.pixels_per_point != self.pixels_per_point {
             eprintln!("epaint: WARNING: pixels_per_point (dpi scale) have changed between text layout and tessellation. \
                        You must recreate your text shapes if pixels_per_point changes.");
@@ -1548,6 +1553,10 @@ impl Tessellator {
                             color = *fallback_color;
                         }
 
+                        if *opacity_factor <= 1.0 {
+                            color = color.gamma_multiply(*opacity_factor);
+                        }
+
                         crate::epaint_assert!(color != Color32::PLACEHOLDER, "A placeholder color made it to the tessellator. You forgot to set a fallback color.");
 
                         let offset = if *angle == 0.0 {