UI texture slice texture flipping reimplementation (#15034)

# Objective

Fixes #15032

## Solution

Reimplement support for the `flip_x` and `flip_y` fields.
This doesn't flip the border geometry, I'm not really sure whether that
is desirable or not.
Also fixes a bug that was causing the side and center slices to tile
incorrectly.

### Testing

```
cargo run --example ui_texture_slice_flip_and_tile
```

## Showcase
<img width="787" alt="nearest"
src="https://github.com/user-attachments/assets/bc044bae-1748-42ba-92b5-0500c87264f6">
With tiling need to use nearest filtering to avoid bleeding between the
slices.

---------

Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Cargo.toml

@@ -2853,6 +2853,17 @@ description = "Illustrates how to use 9 Slicing in UI"
 category = "UI (User Interface)"
 wasm = true
 
+[[example]]
+name = "ui_texture_slice_flip_and_tile"
+path = "examples/ui/ui_texture_slice_flip_and_tile.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.ui_texture_slice_flip_and_tile]
+name = "UI Texture Slice Flipping and Tiling"
+description = "Illustrates how to flip and tile images with 9 Slicing in UI"
+category = "UI (User Interface)"
+wasm = true
+
 [[example]]
 name = "ui_texture_atlas_slice"
 path = "examples/ui/ui_texture_atlas_slice.rs"

crates/bevy_ui/src/render/ui_texture_slice_pipeline.rs

@@ -232,6 +232,8 @@ pub struct ExtractedUiTextureSlice {
     pub camera_entity: Entity,
     pub color: LinearRgba,
     pub image_scale_mode: ImageScaleMode,
+    pub flip_x: bool,
+    pub flip_y: bool,
 }
 
 #[derive(Resource, Default)]
@@ -294,6 +296,8 @@ pub fn extract_ui_texture_slices(
                 camera_entity,
                 image_scale_mode: image_scale_mode.clone(),
                 atlas_rect,
+                flip_x: image.flip_x,
+                flip_y: image.flip_y,
             },
         );
     }
@@ -546,7 +550,7 @@ pub fn prepare_ui_slices(
 
                     let color = texture_slices.color.to_f32_array();
 
-                    let (image_size, atlas) = if let Some(atlas) = texture_slices.atlas_rect {
+                    let (image_size, mut atlas) = if let Some(atlas) = texture_slices.atlas_rect {
                         (
                             atlas.size(),
                             [
@@ -560,6 +564,14 @@ pub fn prepare_ui_slices(
                         (batch_image_size, [0., 0., 1., 1.])
                     };
 
+                    if texture_slices.flip_x {
+                        atlas.swap(0, 2);
+                    }
+
+                    if texture_slices.flip_y {
+                        atlas.swap(1, 3);
+                    }
+
                     let [slices, border, repeat] = compute_texture_slices(
                         image_size,
                         uinode_rect.size(),
@@ -692,7 +704,7 @@ fn compute_texture_slices(
 ) -> [[f32; 4]; 3] {
     match image_scale_mode {
         ImageScaleMode::Sliced(TextureSlicer {
-            border,
+            border: border_rect,
             center_scale_mode,
             sides_scale_mode,
             max_corner_scale,
@@ -701,31 +713,48 @@ fn compute_texture_slices(
                 .min_element()
                 .min(*max_corner_scale);
 
+            // calculate the normalized extents of the nine-patched image slices
             let slices = [
-                border.left / image_size.x,
-                border.top / image_size.y,
-                1. - border.right / image_size.x,
-                1. - border.bottom / image_size.y,
+                border_rect.left / image_size.x,
+                border_rect.top / image_size.y,
+                1. - border_rect.right / image_size.x,
+                1. - border_rect.bottom / image_size.y,
             ];
 
+            // calculate the normalized extents of the target slices
             let border = [
-                (border.left / target_size.x) * min_coeff,
-                (border.top / target_size.y) * min_coeff,
-                1. - (border.right / target_size.x) * min_coeff,
-                1. - (border.bottom / target_size.y) * min_coeff,
+                (border_rect.left / target_size.x) * min_coeff,
+                (border_rect.top / target_size.y) * min_coeff,
+                1. - (border_rect.right / target_size.x) * min_coeff,
+                1. - (border_rect.bottom / target_size.y) * min_coeff,
             ];
 
-            let isx = image_size.x * (1. - slices[0] - slices[2]);
-            let isy = image_size.y * (1. - slices[1] - slices[3]);
-            let tsx = target_size.x * (1. - border[0] - border[2]);
-            let tsy = target_size.y * (1. - border[1] - border[3]);
+            let image_side_width = image_size.x * (slices[2] - slices[0]);
+            let image_side_height = image_size.y * (slices[2] - slices[1]);
+            let target_side_height = target_size.x * (border[2] - border[0]);
+            let target_side_width = target_size.y * (border[3] - border[1]);
 
-            let rx = compute_tiled_subaxis(isx, tsx, sides_scale_mode);
-            let ry = compute_tiled_subaxis(isy, tsy, sides_scale_mode);
-            let cx = compute_tiled_subaxis(isx, tsx, center_scale_mode);
-            let cy = compute_tiled_subaxis(isy, tsy, center_scale_mode);
+            // compute the number of times to repeat the side and center slices when tiling along each axis
+            // if the returned value is `1.` the slice will be stretched to fill the axis.
+            let repeat_side_x =
+                compute_tiled_subaxis(image_side_width, target_side_height, sides_scale_mode);
+            let repeat_side_y =
+                compute_tiled_subaxis(image_side_height, target_side_width, sides_scale_mode);
+            let repeat_center_x =
+                compute_tiled_subaxis(image_side_width, target_side_height, center_scale_mode);
+            let repeat_center_y =
+                compute_tiled_subaxis(image_side_height, target_side_width, center_scale_mode);
 
-            [slices, border, [rx, ry, cx, cy]]
+            [
+                slices,
+                border,
+                [
+                    repeat_side_x,
+                    repeat_side_y,
+                    repeat_center_x,
+                    repeat_center_y,
+                ],
+            ]
         }
         ImageScaleMode::Tiled {
             tile_x,
@@ -739,21 +768,21 @@ fn compute_texture_slices(
     }
 }
 
-fn compute_tiled_axis(tile: bool, is: f32, ts: f32, stretch: f32) -> f32 {
+fn compute_tiled_axis(tile: bool, image_extent: f32, target_extent: f32, stretch: f32) -> f32 {
     if tile {
-        let s = is * stretch;
-        ts / s
+        let s = image_extent * stretch;
+        target_extent / s
     } else {
         1.
     }
 }
 
-fn compute_tiled_subaxis(is: f32, ts: f32, mode: &SliceScaleMode) -> f32 {
+fn compute_tiled_subaxis(image_extent: f32, target_extent: f32, mode: &SliceScaleMode) -> f32 {
     match mode {
         SliceScaleMode::Stretch => 1.,
         SliceScaleMode::Tile { stretch_value } => {
-            let s = is * *stretch_value;
-            ts / s
+            let s = image_extent * *stretch_value;
+            target_extent / s
         }
     }
 }

examples/README.md

@@ -464,6 +464,7 @@ Example | Description
 [UI Texture Atlas](../examples/ui/ui_texture_atlas.rs) | Illustrates how to use TextureAtlases in UI
 [UI Texture Atlas Slice](../examples/ui/ui_texture_atlas_slice.rs) | Illustrates how to use 9 Slicing for TextureAtlases in UI
 [UI Texture Slice](../examples/ui/ui_texture_slice.rs) | Illustrates how to use 9 Slicing in UI
+[UI Texture Slice Flipping and Tiling](../examples/ui/ui_texture_slice_flip_and_tile.rs) | Illustrates how to flip and tile images with 9 Slicing in UI
 [UI Z-Index](../examples/ui/z_index.rs) | Demonstrates how to control the relative depth (z-position) of UI elements
 [Viewport Debug](../examples/ui/viewport_debug.rs) | An example for debugging viewport coordinates
 [Window Fallthrough](../examples/ui/window_fallthrough.rs) | Illustrates how to access `winit::window::Window`'s `hittest` functionality.

examples/ui/ui_texture_slice_flip_and_tile.rs

@@ -0,0 +1,78 @@
+//! This example illustrates how to how to flip and tile images with 9-slicing in the UI.
+
+use bevy::{prelude::*, winit::WinitSettings};
+use bevy_render::texture::{ImageLoaderSettings, ImageSampler};
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .insert_resource(UiScale(2.))
+        // Only run the app when there is user input. This will significantly reduce CPU/GPU use for UI-only apps.
+        .insert_resource(WinitSettings::desktop_app())
+        .add_systems(Startup, setup)
+        .run();
+}
+
+fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+    let image = asset_server.load_with_settings(
+        "textures/fantasy_ui_borders/numbered_slices.png",
+        |settings: &mut ImageLoaderSettings| {
+            // Need to use nearest filtering to avoid bleeding between the slices with tiling
+            settings.sampler = ImageSampler::nearest();
+        },
+    );
+
+    let slicer = TextureSlicer {
+        // `numbered_slices.png` is 48 pixels square. `BorderRect::square(16.)` insets the slicing line from each edge by 16 pixels, resulting in nine slices that are each 16 pixels square.
+        border: BorderRect::square(16.),
+        // With `SliceScaleMode::Tile` the side and center slices are tiled to to fill the side and center sections of the target.
+        // And with a `stretch_value` of `1.` the tiles will have the same size as the corresponding slices in the source image.
+        center_scale_mode: SliceScaleMode::Tile { stretch_value: 1. },
+        sides_scale_mode: SliceScaleMode::Tile { stretch_value: 1. },
+        ..default()
+    };
+
+    // ui camera
+    commands.spawn(Camera2dBundle::default());
+
+    commands
+        .spawn(NodeBundle {
+            style: Style {
+                width: Val::Percent(100.),
+                height: Val::Percent(100.),
+                justify_content: JustifyContent::Center,
+                align_content: AlignContent::Center,
+                flex_wrap: FlexWrap::Wrap,
+                column_gap: Val::Px(10.),
+                row_gap: Val::Px(10.),
+                ..default()
+            },
+            ..default()
+        })
+        .with_children(|parent| {
+            for ([width, height], flip_x, flip_y) in [
+                ([160., 160.], false, false),
+                ([320., 160.], false, true),
+                ([320., 160.], true, false),
+                ([160., 160.], true, true),
+            ] {
+                parent.spawn((
+                    NodeBundle {
+                        style: Style {
+                            width: Val::Px(width),
+                            height: Val::Px(height),
+                            ..default()
+                        },
+                        ..Default::default()
+                    },
+                    UiImage {
+                        texture: image.clone(),
+                        flip_x,
+                        flip_y,
+                        ..Default::default()
+                    },
+                    ImageScaleMode::Sliced(slicer.clone()),
+                ));
+            }
+        });
+}