Change UI coordinate system to have origin at top left corner (#6000)

# Objective
Fixes #5572

## Solution

Approach is to invert the Y-axis of the UI Camera by changing the UI projection matrix to render the UI upside down.

After that I'm trying to fix all issues, that pop up:
- interaction expected the "old" position
- images and text were displayed upside-down
- baseline of text was based on the top of the glyph instead of bottom

... probably a lot more.

---

Result when running examples:
<details>
    <summary>Button example</summary>

main branch:
![button main](https://user-images.githubusercontent.com/4232644/190856087-61dd1d98-42b5-4238-bd97-149744ddfeba.png)
this pr:
![button pr](https://user-images.githubusercontent.com/4232644/190856097-3f4bc97a-ed15-4e97-b7f1-2b2dd6bb8b14.png)

</details>

<details>
    <summary>Text example</summary>

m
![text main](https://user-images.githubusercontent.com/4232644/192142831-4cf19aa1-f49a-485e-af7b-374d6f5c396c.png)
ain branch: 


this pr:
![text pr fixed](https://user-images.githubusercontent.com/4232644/192142829-c433db3b-32e1-4ee8-b493-0b4a4d9c8e70.png)


</details>

<details>
    <summary>Text debug example</summary>

main branch:
![text_debug main](https://user-images.githubusercontent.com/4232644/192142822-940aefa6-e502-410b-8da4-5570f77b5df2.png)

this pr:
![text_debug pr fixed](https://user-images.githubusercontent.com/4232644/194547010-8c968f5c-5a71-4ffc-871d-790c06d48016.png)

</details>

<details>
    <summary>Transparency UI example</summary>

main branch:
![transparency_ui main](https://user-images.githubusercontent.com/4232644/190856172-328c60fe-3622-4598-97d5-2f1595db13b3.png)


this pr:
![transperency_ui pr](https://user-images.githubusercontent.com/4232644/190856179-a2dafb99-41ea-45a9-9dd6-400fa3ef24b9.png)

</details>

<details>
    <summary>UI example</summary>

**ui example**
main branch:
![ui main](https://user-images.githubusercontent.com/4232644/192142812-e20ba31a-6841-46d9-a785-4198cf22dc99.png)

this pr:
![ui pr fixed](https://user-images.githubusercontent.com/4232644/192142788-cc0b74e0-7710-4faa-b5a2-60270a5da77c.png)

</details>

## Changelog
UI coordinate system and cursor position was changed from bottom left origin, y+ up to top left origin, y+ down.

## Migration Guide
All flex layout should be inverted (ColumnReverse => Column, FlexStart => FlexEnd, WrapReverse => Wrap)
System where dealing with cursor position should be changed to account for cursor position being based on the top left instead of bottom left

crates/bevy_text/src/glyph_brush.rs

@@ -73,16 +73,17 @@ impl GlyphBrush {
             })
             .collect::<Result<Vec<_>, _>>()?;
 
-        let mut max_y = std::f32::MIN;
         let mut min_x = std::f32::MAX;
+        let mut min_y = std::f32::MAX;
         for sg in &glyphs {
             let glyph = &sg.glyph;
+
             let scaled_font = sections_data[sg.section_index].3;
-            max_y = max_y.max(glyph.position.y - scaled_font.descent());
             min_x = min_x.min(glyph.position.x);
+            min_y = min_y.min(glyph.position.y - scaled_font.ascent());
         }
-        max_y = max_y.floor();
         min_x = min_x.floor();
+        min_y = min_y.floor();
 
         let mut positioned_glyphs = Vec::new();
         for sg in glyphs {
@@ -119,7 +120,7 @@ impl GlyphBrush {
                 let size = Vec2::new(glyph_rect.width(), glyph_rect.height());
 
                 let x = bounds.min.x + size.x / 2.0 - min_x;
-                let y = max_y - bounds.max.y + size.y / 2.0;
+                let y = bounds.min.y + size.y / 2.0 - min_y;
                 let position = adjust.position(Vec2::new(x, y));
 
                 positioned_glyphs.push(PositionedGlyph {

crates/bevy_ui/src/flex/convert.rs

@@ -10,9 +10,8 @@ pub fn from_rect(
     taffy::geometry::Rect {
         start: from_val(scale_factor, rect.left),
         end: from_val(scale_factor, rect.right),
-        // NOTE: top and bottom are intentionally flipped. stretch has a flipped y-axis
+        top: from_val(scale_factor, rect.top),
-        top: from_val(scale_factor, rect.bottom),
+        bottom: from_val(scale_factor, rect.bottom),
-        bottom: from_val(scale_factor, rect.top),
     }
 }
 

crates/bevy_ui/src/lib.rs

@@ -1,7 +1,7 @@
 //! This crate contains Bevy's UI system, which can be used to create UI for both 2D and 3D games
 //! # Basic usage
 //! Spawn UI elements with [`entity::ButtonBundle`], [`entity::ImageBundle`], [`entity::TextBundle`] and [`entity::NodeBundle`]
-//! This UI is laid out with the Flexbox paradigm (see <https://cssreference.io/flexbox/> ) except the vertical axis is inverted
+//! This UI is laid out with the Flexbox paradigm (see <https://cssreference.io/flexbox/>)
 mod flex;
 mod focus;
 mod geometry;

crates/bevy_ui/src/render/mod.rs

@@ -12,7 +12,7 @@ use bevy_ecs::prelude::*;
 use bevy_math::{Mat4, Rect, UVec4, Vec2, Vec3, Vec4Swizzles};
 use bevy_reflect::TypeUuid;
 use bevy_render::{
-    camera::{Camera, CameraProjection, OrthographicProjection, WindowOrigin},
+    camera::Camera,
     color::Color,
     render_asset::RenderAssets,
     render_graph::{RenderGraph, RunGraphOnViewNode, SlotInfo, SlotType},
@@ -243,15 +243,12 @@ pub fn extract_default_ui_camera_view<T: Component>(
             camera.physical_viewport_rect(),
             camera.physical_viewport_size(),
         ) {
-            let mut projection = OrthographicProjection {
+            // use a projection matrix with the origin in the top left instead of the bottom left that comes with OrthographicProjection
-                far: UI_CAMERA_FAR,
+            let projection_matrix =
-                window_origin: WindowOrigin::BottomLeft,
+                Mat4::orthographic_rh(0.0, logical_size.x, logical_size.y, 0.0, 0.0, UI_CAMERA_FAR);
-                ..Default::default()
-            };
-            projection.update(logical_size.x, logical_size.y);
             let default_camera_view = commands
                 .spawn(ExtractedView {
-                    projection: projection.get_projection_matrix(),
+                    projection: projection_matrix,
                     transform: GlobalTransform::from_xyz(
                         0.0,
                         0.0,
@@ -464,24 +461,23 @@ pub fn prepare_uinodes(
             }
         }
 
-        // Clip UVs (Note: y is reversed in UV space)
         let atlas_extent = extracted_uinode.atlas_size.unwrap_or(uinode_rect.max);
         let uvs = [
             Vec2::new(
-                uinode_rect.min.x + positions_diff[0].x,
+                uinode_rect.min.x + positions_diff[3].x,
-                uinode_rect.max.y - positions_diff[0].y,
+                uinode_rect.min.y - positions_diff[3].y,
-            ),
-            Vec2::new(
-                uinode_rect.max.x + positions_diff[1].x,
-                uinode_rect.max.y - positions_diff[1].y,
             ),
             Vec2::new(
                 uinode_rect.max.x + positions_diff[2].x,
                 uinode_rect.min.y - positions_diff[2].y,
             ),
             Vec2::new(
-                uinode_rect.min.x + positions_diff[3].x,
+                uinode_rect.max.x + positions_diff[1].x,
-                uinode_rect.min.y - positions_diff[3].y,
+                uinode_rect.max.y - positions_diff[1].y,
+            ),
+            Vec2::new(
+                uinode_rect.min.x + positions_diff[0].x,
+                uinode_rect.max.y - positions_diff[0].y,
             ),
         ]
         .map(|pos| pos / atlas_extent);

crates/bevy_ui/src/ui_node.rs

@@ -303,11 +303,11 @@ pub enum FlexDirection {
     /// Same way as text direction along the main axis
     #[default]
     Row,
-    /// Flex from bottom to top
+    /// Flex from top to bottom
     Column,
     /// Opposite way as text direction along the main axis
     RowReverse,
-    /// Flex from top to bottom
+    /// Flex from bottom to top
     ColumnReverse,
 }
 

crates/bevy_winit/src/lib.rs

@@ -149,12 +149,9 @@ fn change_window(
                 }
                 bevy_window::WindowCommand::SetCursorPosition { position } => {
                     let window = winit_windows.get_window(id).unwrap();
-                    let inner_size = window.inner_size().to_logical::<f32>(window.scale_factor());
+
                     window
-                        .set_cursor_position(LogicalPosition::new(
+                        .set_cursor_position(LogicalPosition::new(position.x, position.y))
-                            position.x,
-                            inner_size.height - position.y,
-                        ))
                         .unwrap_or_else(|e| error!("Unable to set cursor position: {}", e));
                 }
                 bevy_window::WindowCommand::SetMaximized { maximized } => {
@@ -431,13 +428,8 @@ pub fn winit_runner_with(mut app: App) {
                     }
                     WindowEvent::CursorMoved { position, .. } => {
                         let mut cursor_moved_events = world.resource_mut::<Events<CursorMoved>>();
-                        let winit_window = winit_windows.get_window(window_id).unwrap();
-                        let inner_size = winit_window.inner_size();
 
-                        // move origin to bottom left
+                        let physical_position = DVec2::new(position.x, position.y);
-                        let y_position = inner_size.height as f64 - position.y;
-
-                        let physical_position = DVec2::new(position.x, y_position);
                         window
                             .update_cursor_physical_position_from_backend(Some(physical_position));
 

examples/games/game_menu.rs

@@ -152,9 +152,8 @@ mod game {
                     style: Style {
                         // This will center the current node
                         margin: UiRect::all(Val::Auto),
-                        // This will display its children in a column, from top to bottom. Unlike
+                        // This will display its children in a column, from top to bottom
-                        // in Flexbox, Bevy origin is on bottom left, so the vertical axis is reversed
+                        flex_direction: FlexDirection::Column,
-                        flex_direction: FlexDirection::ColumnReverse,
                         // `align_items` will align children on the cross axis. Here the main axis is
                         // vertical (column), so the cross axis is horizontal. This will center the
                         // children
@@ -420,7 +419,7 @@ mod menu {
                 NodeBundle {
                     style: Style {
                         margin: UiRect::all(Val::Auto),
-                        flex_direction: FlexDirection::ColumnReverse,
+                        flex_direction: FlexDirection::Column,
                         align_items: AlignItems::Center,
                         ..default()
                     },
@@ -533,7 +532,7 @@ mod menu {
                 NodeBundle {
                     style: Style {
                         margin: UiRect::all(Val::Auto),
-                        flex_direction: FlexDirection::ColumnReverse,
+                        flex_direction: FlexDirection::Column,
                         align_items: AlignItems::Center,
                         ..default()
                     },
@@ -587,7 +586,7 @@ mod menu {
                 NodeBundle {
                     style: Style {
                         margin: UiRect::all(Val::Auto),
-                        flex_direction: FlexDirection::ColumnReverse,
+                        flex_direction: FlexDirection::Column,
                         align_items: AlignItems::Center,
                         ..default()
                     },
@@ -678,7 +677,7 @@ mod menu {
                 NodeBundle {
                     style: Style {
                         margin: UiRect::all(Val::Auto),
-                        flex_direction: FlexDirection::ColumnReverse,
+                        flex_direction: FlexDirection::Column,
                         align_items: AlignItems::Center,
                         ..default()
                     },

examples/ui/font_atlas_debug.rs

@@ -82,12 +82,27 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>, mut state: ResM
     let font_handle = asset_server.load("fonts/FiraSans-Bold.ttf");
     state.handle = font_handle.clone();
     commands.spawn(Camera2dBundle::default());
-    commands.spawn(TextBundle::from_section(
+    commands
-        "a",
+        .spawn(NodeBundle {
-        TextStyle {
+            background_color: Color::NONE.into(),
-            font: font_handle,
+            style: Style {
-            font_size: 60.0,
+                position_type: PositionType::Absolute,
-            color: Color::YELLOW,
+                position: UiRect {
-        },
+                    bottom: Val::Px(0.0),
-    ));
+                    ..default()
+                },
+                ..default()
+            },
+            ..default()
+        })
+        .with_children(|parent| {
+            parent.spawn(TextBundle::from_section(
+                "a",
+                TextStyle {
+                    font: font_handle,
+                    font_size: 60.0,
+                    color: Color::YELLOW,
+                },
+            ));
+        });
 }

examples/ui/text.rs

@@ -44,7 +44,6 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
         .with_text_alignment(TextAlignment::TOP_CENTER)
         // Set the style of the TextBundle itself.
         .with_style(Style {
-            align_self: AlignSelf::FlexEnd,
             position_type: PositionType::Absolute,
             position: UiRect {
                 bottom: Val::Px(5.0),
@@ -72,11 +71,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                 font_size: 60.0,
                 color: Color::GOLD,
             }),
-        ])
+        ]),
-        .with_style(Style {
-            align_self: AlignSelf::FlexEnd,
-            ..default()
-        }),
         FpsText,
     ));
 }

examples/ui/text_debug.rs

@@ -35,7 +35,6 @@ fn infotext_system(mut commands: Commands, asset_server: Res<AssetServer>) {
             },
         )
         .with_style(Style {
-            align_self: AlignSelf::FlexEnd,
             position_type: PositionType::Absolute,
             position: UiRect {
                 top: Val::Px(5.0),
@@ -55,7 +54,6 @@ fn infotext_system(mut commands: Commands, asset_server: Res<AssetServer>) {
         )
         .with_text_alignment(TextAlignment::CENTER)
         .with_style(Style {
-            align_self: AlignSelf::FlexEnd,
             position_type: PositionType::Absolute,
             position: UiRect {
                 top: Val::Px(5.0),
@@ -115,7 +113,6 @@ fn infotext_system(mut commands: Commands, asset_server: Res<AssetServer>) {
             ),
         ])
         .with_style(Style {
-            align_self: AlignSelf::FlexEnd,
             position_type: PositionType::Absolute,
             position: UiRect {
                 bottom: Val::Px(5.0),

examples/ui/ui.rs

@@ -48,7 +48,6 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                         .spawn(NodeBundle {
                             style: Style {
                                 size: Size::new(Val::Percent(100.0), Val::Percent(100.0)),
-                                align_items: AlignItems::FlexEnd,
                                 ..default()
                             },
                             background_color: Color::rgb(0.15, 0.15, 0.15).into(),
@@ -76,7 +75,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
             parent
                 .spawn(NodeBundle {
                     style: Style {
-                        flex_direction: FlexDirection::ColumnReverse,
+                        flex_direction: FlexDirection::Column,
                         justify_content: JustifyContent::Center,
                         size: Size::new(Val::Px(200.0), Val::Percent(100.0)),
                         ..default()
@@ -109,7 +108,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                     parent
                         .spawn(NodeBundle {
                             style: Style {
-                                flex_direction: FlexDirection::ColumnReverse,
+                                flex_direction: FlexDirection::Column,
                                 align_self: AlignSelf::Center,
                                 size: Size::new(Val::Percent(100.0), Val::Percent(50.0)),
                                 overflow: Overflow::Hidden,
@@ -124,7 +123,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                                 .spawn((
                                     NodeBundle {
                                         style: Style {
-                                            flex_direction: FlexDirection::ColumnReverse,
+                                            flex_direction: FlexDirection::Column,
                                             flex_grow: 1.0,
                                             max_size: Size::UNDEFINED,
                                             ..default()
@@ -161,7 +160,6 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                                 });
                         });
                 });
-            // absolute positioning
             parent
                 .spawn(NodeBundle {
                     style: Style {
@@ -277,7 +275,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                         size: Size::new(Val::Percent(100.0), Val::Percent(100.0)),
                         position_type: PositionType::Absolute,
                         justify_content: JustifyContent::Center,
-                        align_items: AlignItems::FlexEnd,
+                        align_items: AlignItems::FlexStart,
                         ..default()
                     },
                     ..default()
@@ -318,8 +316,8 @@ fn mouse_scroll(
                 MouseScrollUnit::Line => mouse_wheel_event.y * 20.,
                 MouseScrollUnit::Pixel => mouse_wheel_event.y,
             };
-            scrolling_list.position += dy;
+            scrolling_list.position -= dy;
-            scrolling_list.position = scrolling_list.position.clamp(-max_scroll, 0.);
+            scrolling_list.position = scrolling_list.position.clamp(0., max_scroll);
             style.position.top = Val::Px(scrolling_list.position);
         }
     }