Change ui.disable() to modify opacity (#7282)

* Closes https://github.com/emilk/egui/pull/6765


Branched off of https://github.com/emilk/egui/pull/6765 by @tye-exe.

I needed to branch off to update snapshot images

---------

Co-authored-by: tye-exe <tye@mailbox.org>
Co-authored-by: Tye <131195812+tye-exe@users.noreply.github.com>

crates/egui/src/painter.rs

@@ -83,6 +83,7 @@ impl Painter {
     }
 
     /// If set, colors will be modified to look like this
+    #[deprecated = "Use `multiply_opacity` instead"]
     pub fn set_fade_to_color(&mut self, fade_to_color: Option<Color32>) {
         self.fade_to_color = fade_to_color;
     }

crates/egui/src/style.rs

@@ -1019,6 +1019,9 @@ pub struct Visuals {
 
     /// How to display numeric color values.
     pub numeric_color_space: NumericColorSpace,
+
+    /// How much to modify the alpha of a disabled widget.
+    pub disabled_alpha: f32,
 }
 
 impl Visuals {
@@ -1054,17 +1057,32 @@ impl Visuals {
     }
 
     /// When fading out things, we fade the colors towards this.
-    // TODO(emilk): replace with an alpha
     #[inline(always)]
+    #[deprecated = "Use disabled_alpha(). Fading is now handled by modifying the alpha channel."]
     pub fn fade_out_to_color(&self) -> Color32 {
         self.widgets.noninteractive.weak_bg_fill
     }
 
-    /// Returned a "grayed out" version of the given color.
+    /// Disabled widgets have their alpha modified by this.
+    #[inline(always)]
+    pub fn disabled_alpha(&self) -> f32 {
+        self.disabled_alpha
+    }
+
+    /// Returns a "disabled" version of the given color.
+    ///
+    /// This function modifies the opcacity of the given color.
+    /// If this is undesirable use [`gray_out`](Self::gray_out).
+    #[inline(always)]
+    pub fn disable(&self, color: Color32) -> Color32 {
+        color.gamma_multiply(self.disabled_alpha())
+    }
+
+    /// Returns a "grayed out" version of the given color.
     #[doc(alias = "grey_out")]
     #[inline(always)]
     pub fn gray_out(&self, color: Color32) -> Color32 {
-        crate::ecolor::tint_color_towards(color, self.fade_out_to_color())
+        crate::ecolor::tint_color_towards(color, self.widgets.noninteractive.weak_bg_fill)
     }
 }
 
@@ -1384,6 +1402,7 @@ impl Visuals {
             image_loading_spinners: true,
 
             numeric_color_space: NumericColorSpace::GammaByte,
+            disabled_alpha: 0.5,
         }
     }
 
@@ -2063,6 +2082,7 @@ impl Visuals {
             image_loading_spinners,
 
             numeric_color_space,
+            disabled_alpha,
         } = self;
 
         ui.collapsing("Background Colors", |ui| {
@@ -2191,6 +2211,8 @@ impl Visuals {
                 ui.label("Color picker type");
                 numeric_color_space.toggle_button_ui(ui);
             });
+
+            ui.add(Slider::new(disabled_alpha, 0.0..=1.0).text("Disabled element alpha"));
         });
 
         ui.vertical_centered(|ui| reset_button(ui, self, "Reset visuals"));

crates/egui/src/ui.rs

@@ -522,7 +522,7 @@ impl Ui {
         self.enabled = false;
         if self.is_visible() {
             self.painter
-                .set_fade_to_color(Some(self.visuals().fade_out_to_color()));
+                .multiply_opacity(self.visuals().disabled_alpha());
         }
     }
 
@@ -2963,8 +2963,8 @@ impl Ui {
 
         if is_anything_being_dragged && !can_accept_what_is_being_dragged {
             // When dragging something else, show that it can't be dropped here:
-            fill = self.visuals().gray_out(fill);
-            stroke.color = self.visuals().gray_out(stroke.color);
+            fill = self.visuals().disable(fill);
+            stroke.color = self.visuals().disable(stroke.color);
         }
 
         frame.frame.fill = fill;