Basic UI text shadows (#17559)

# Objective

Basic `TextShadow` support. 

## Solution

New `TextShadow` component with `offset` and `color` fields. Just insert
it on a `Text` node to add a shadow.
New system `extract_text_shadows` handles rendering.

It's not "real" shadows just the text redrawn with an offset and a
different colour. Blur-radius support will need changes to the shaders
and be a lot more complicated, whereas this still looks okay and took a
couple of minutes to implement.

I added the `TextShadow` component to `bevy_ui` rather than `bevy_text`
because it only supports the UI atm.
We can add a `Text2d` version in a followup but getting the same effect
in `Text2d` is trivial even without official support.

---

## Showcase

<img width="122" alt="text_shadow"
src="https://github.com/user-attachments/assets/0333d167-c507-4262-b93b-b6d39e2cf3a4"
/>
<img width="136" alt="g"
src="https://github.com/user-attachments/assets/9b01d5d9-55c9-4af7-9360-a7b04f55944d"
/>

crates/bevy_ui/src/lib.rs

@@ -167,6 +167,7 @@ impl Plugin for UiPlugin {
             .register_type::<Outline>()
             .register_type::<BoxShadowSamples>()
             .register_type::<UiAntiAlias>()
+            .register_type::<TextShadow>()
             .configure_sets(
                 PostUpdate,
                 (

crates/bevy_ui/src/render/mod.rs

@@ -10,7 +10,7 @@ mod debug_overlay;
 use crate::widget::ImageNode;
 use crate::{
     BackgroundColor, BorderColor, BoxShadowSamples, CalculatedClip, ComputedNode, DefaultUiCamera,
-    Outline, ResolvedBorderRadius, UiAntiAlias, UiTargetCamera,
+    Outline, ResolvedBorderRadius, TextShadow, UiAntiAlias, UiTargetCamera,
 };
 use bevy_app::prelude::*;
 use bevy_asset::{load_internal_asset, AssetEvent, AssetId, Assets, Handle};
@@ -105,6 +105,7 @@ pub enum RenderUiSystem {
     ExtractImages,
     ExtractTextureSlice,
     ExtractBorders,
+    ExtractTextShadows,
     ExtractText,
     ExtractDebug,
 }
@@ -134,6 +135,7 @@ pub fn build_ui_render(app: &mut App) {
                 RenderUiSystem::ExtractImages,
                 RenderUiSystem::ExtractTextureSlice,
                 RenderUiSystem::ExtractBorders,
+                RenderUiSystem::ExtractTextShadows,
                 RenderUiSystem::ExtractText,
                 RenderUiSystem::ExtractDebug,
             )
@@ -146,6 +148,7 @@ pub fn build_ui_render(app: &mut App) {
                 extract_uinode_background_colors.in_set(RenderUiSystem::ExtractBackgrounds),
                 extract_uinode_images.in_set(RenderUiSystem::ExtractImages),
                 extract_uinode_borders.in_set(RenderUiSystem::ExtractBorders),
+                extract_text_shadows.in_set(RenderUiSystem::ExtractTextShadows),
                 extract_text_sections.in_set(RenderUiSystem::ExtractText),
                 #[cfg(feature = "bevy_ui_debug")]
                 debug_overlay::extract_debug_overlay.in_set(RenderUiSystem::ExtractDebug),
@@ -714,8 +717,8 @@ pub fn extract_text_sections(
     text_styles: Extract<Query<&TextColor>>,
     camera_map: Extract<UiCameraMap>,
 ) {
-    let mut start = 0;
-    let mut end = 1;
+    let mut start = extracted_uinodes.glyphs.len();
+    let mut end = start + 1;
 
     let mut camera_mapper = camera_map.get_mapper();
     for (
@@ -743,6 +746,7 @@ pub fn extract_text_sections(
 
         let mut color = LinearRgba::WHITE;
         let mut current_span = usize::MAX;
+
         for (
             i,
             PositionedGlyph {
@@ -799,6 +803,105 @@ pub fn extract_text_sections(
     }
 }
 
+pub fn extract_text_shadows(
+    mut commands: Commands,
+    mut extracted_uinodes: ResMut<ExtractedUiNodes>,
+    default_ui_camera: Extract<DefaultUiCamera>,
+    texture_atlases: Extract<Res<Assets<TextureAtlasLayout>>>,
+    uinode_query: Extract<
+        Query<(
+            Entity,
+            &ComputedNode,
+            &GlobalTransform,
+            &InheritedVisibility,
+            Option<&CalculatedClip>,
+            Option<&UiTargetCamera>,
+            &TextLayoutInfo,
+            &TextShadow,
+        )>,
+    >,
+    mapping: Extract<Query<RenderEntity>>,
+) {
+    let mut start = extracted_uinodes.glyphs.len();
+    let mut end = start + 1;
+
+    let default_ui_camera = default_ui_camera.get();
+    for (
+        entity,
+        uinode,
+        global_transform,
+        inherited_visibility,
+        clip,
+        camera,
+        text_layout_info,
+        shadow,
+    ) in &uinode_query
+    {
+        let Some(camera_entity) = camera.map(UiTargetCamera::entity).or(default_ui_camera) else {
+            continue;
+        };
+
+        // Skip if not visible or if size is set to zero (e.g. when a parent is set to `Display::None`)
+        if !inherited_visibility.get() || uinode.is_empty() {
+            continue;
+        }
+
+        let Ok(extracted_camera_entity) = mapping.get(camera_entity) else {
+            continue;
+        };
+
+        let transform = global_transform.affine()
+            * Mat4::from_translation(
+                (-0.5 * uinode.size() + shadow.offset / uinode.inverse_scale_factor()).extend(0.),
+            );
+
+        let mut current_span = usize::MAX;
+        for (
+            i,
+            PositionedGlyph {
+                position,
+                atlas_info,
+                span_index,
+                ..
+            },
+        ) in text_layout_info.glyphs.iter().enumerate()
+        {
+            if *span_index != current_span {
+                current_span = *span_index;
+            }
+
+            let rect = texture_atlases
+                .get(&atlas_info.texture_atlas)
+                .unwrap()
+                .textures[atlas_info.location.glyph_index]
+                .as_rect();
+            extracted_uinodes.glyphs.push(ExtractedGlyph {
+                transform: transform * Mat4::from_translation(position.extend(0.)),
+                rect,
+            });
+
+            if text_layout_info.glyphs.get(i + 1).is_none_or(|info| {
+                info.span_index != current_span || info.atlas_info.texture != atlas_info.texture
+            }) {
+                extracted_uinodes.uinodes.push(ExtractedUiNode {
+                    render_entity: commands.spawn(TemporaryRenderEntity).id(),
+                    stack_index: uinode.stack_index,
+                    color: shadow.color.into(),
+                    image: atlas_info.texture.id(),
+                    clip: clip.map(|clip| clip.clip),
+                    extracted_camera_entity,
+                    rect,
+                    item: ExtractedUiItem::Glyphs { range: start..end },
+                    main_entity: entity.into(),
+                });
+                start = end;
+            }
+
+            end += 1;
+        }
+    }
+}
+
 #[repr(C)]
 #[derive(Copy, Clone, Pod, Zeroable)]
 struct UiVertex {

crates/bevy_ui/src/ui_node.rs

@@ -2762,3 +2762,23 @@ impl Default for BoxShadowSamples {
         Self(4)
     }
 }
+
+/// Adds a shadow behind text
+#[derive(Component, Copy, Clone, Debug, Reflect)]
+#[reflect(Component, Default, Debug)]
+pub struct TextShadow {
+    /// Shadow displacement in logical pixels
+    /// With a value of zero the shadow will be hidden directly behind the text
+    pub offset: Vec2,
+    /// Color of the shadow
+    pub color: Color,
+}
+
+impl Default for TextShadow {
+    fn default() -> Self {
+        Self {
+            offset: Vec2::splat(4.),
+            color: Color::linear_rgba(0., 0., 0., 0.75),
+        }
+    }
+}

examples/ui/button.rs

@@ -88,6 +88,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                         ..default()
                     },
                     TextColor(Color::srgb(0.9, 0.9, 0.9)),
+                    TextShadow::default(),
                 ));
         });
 }

examples/ui/text.rs

@@ -38,6 +38,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
             font_size: 67.0,
             ..default()
         },
+        TextShadow::default(),
         // Set the justification of the Text
         TextLayout::new_with_justify(JustifyText::Center),
         // Set the style of the Node itself.