Fix various bugs with UI rounded borders (#13523)

# Objective

- Fixes #13503 
- Fix other various bugs I noticed while debugging above issue.

## Solution

- Change the antialiasing(AA) method. It was using fwidth which is the
derivative between pixels, but there were a lot of artifacts being added
from this. So just use the sdf value. This aa method probably isn't as
smooth looking, but better than having artifacts. Below is a
visualization of the fwidth.

![image](https://github.com/bevyengine/bevy/assets/2180432/4e475ad0-c9d0-4a40-af39-5f4422a78392)
- Use the internal sdf for drawing the background instead of the
external sdf and extract the border for these type of nodes. This fixed
2 bugs, one with the background coloring the AA pixels on the edge of
rounded borders. And also allows for the border to use a transparent
color.
- Don't extract borders if all the widths are zero.

## Testing

- played a bunch with the example in the linked issue.
This PR:

![image](https://github.com/bevyengine/bevy/assets/2180432/d7797e0e-e348-4daa-8646-554dc2032523)
Main:

![image](https://github.com/bevyengine/bevy/assets/2180432/4d46c17e-a12d-4e20-aaef-0ffc950cefe2)

- ran the `borders` and `rounded_borders` examples

---

## Changelog

- Fixed various antialiasing issues to do with rounded ui borders.

---------

Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: Andreas Weibye <[email protected]>

Author: 3 people

crates/bevy_ui/src/render/mod.rs

@@ -199,8 +199,11 @@ pub fn extract_uinode_background_colors(
             Option<&TargetCamera>,
             &BackgroundColor,
             Option<&BorderRadius>,
+            &Style,
+            Option<&Parent>,
         )>,
     >,
+    node_query: Extract<Query<&Node>>,
 ) {
     for (
         entity,
@@ -211,6 +214,8 @@ pub fn extract_uinode_background_colors(
         camera,
         background_color,
         border_radius,
+        style,
+        parent,
     ) in &uinode_query
     {
         let Some(camera_entity) = camera.map(TargetCamera::entity).or(default_ui_camera.get())
@@ -232,6 +237,23 @@ pub fn extract_uinode_background_colors(
             // so we have to divide by `UiScale` to get the size of the UI viewport.
             / ui_scale.0;
 
+        // Both vertical and horizontal percentage border values are calculated based on the width of the parent node
+        // <https://developer.mozilla.org/en-US/docs/Web/CSS/border-width>
+        let parent_width = parent
+            .and_then(|parent| node_query.get(parent.get()).ok())
+            .map(|parent_node| parent_node.size().x)
+            .unwrap_or(ui_logical_viewport_size.x);
+        let left =
+            resolve_border_thickness(style.border.left, parent_width, ui_logical_viewport_size);
+        let right =
+            resolve_border_thickness(style.border.right, parent_width, ui_logical_viewport_size);
+        let top =
+            resolve_border_thickness(style.border.top, parent_width, ui_logical_viewport_size);
+        let bottom =
+            resolve_border_thickness(style.border.bottom, parent_width, ui_logical_viewport_size);
+
+        let border = [left, top, right, bottom];
+
         let border_radius = if let Some(border_radius) = border_radius {
             resolve_border_radius(
                 border_radius,
@@ -259,14 +281,15 @@ pub fn extract_uinode_background_colors(
                 flip_x: false,
                 flip_y: false,
                 camera_entity,
-                border: [0.; 4],
+                border,
                 border_radius,
                 node_type: NodeType::Rect,
             },
         );
     }
 }
 
+#[allow(clippy::too_many_arguments)]
 pub fn extract_uinode_images(
     mut commands: Commands,
     mut extracted_uinodes: ResMut<ExtractedUiNodes>,
@@ -285,11 +308,25 @@ pub fn extract_uinode_images(
             Option<&TextureAtlas>,
             Option<&ComputedTextureSlices>,
             Option<&BorderRadius>,
+            Option<&Parent>,
+            &Style,
         )>,
     >,
+    node_query: Extract<Query<&Node>>,
 ) {
-    for (uinode, transform, view_visibility, clip, camera, image, atlas, slices, border_radius) in
-        &uinode_query
+    for (
+        uinode,
+        transform,
+        view_visibility,
+        clip,
+        camera,
+        image,
+        atlas,
+        slices,
+        border_radius,
+        parent,
+        style,
+    ) in &uinode_query
     {
         let Some(camera_entity) = camera.map(TargetCamera::entity).or(default_ui_camera.get())
         else {
@@ -342,6 +379,23 @@ pub fn extract_uinode_images(
             // so we have to divide by `UiScale` to get the size of the UI viewport.
             / ui_scale.0;
 
+        // Both vertical and horizontal percentage border values are calculated based on the width of the parent node
+        // <https://developer.mozilla.org/en-US/docs/Web/CSS/border-width>
+        let parent_width = parent
+            .and_then(|parent| node_query.get(parent.get()).ok())
+            .map(|parent_node| parent_node.size().x)
+            .unwrap_or(ui_logical_viewport_size.x);
+        let left =
+            resolve_border_thickness(style.border.left, parent_width, ui_logical_viewport_size);
+        let right =
+            resolve_border_thickness(style.border.right, parent_width, ui_logical_viewport_size);
+        let top =
+            resolve_border_thickness(style.border.top, parent_width, ui_logical_viewport_size);
+        let bottom =
+            resolve_border_thickness(style.border.bottom, parent_width, ui_logical_viewport_size);
+
+        let border = [left, top, right, bottom];
+
         let border_radius = if let Some(border_radius) = border_radius {
             resolve_border_radius(
                 border_radius,
@@ -366,7 +420,7 @@ pub fn extract_uinode_images(
                 flip_x: image.flip_x,
                 flip_y: image.flip_y,
                 camera_entity,
-                border: [0.; 4],
+                border,
                 border_radius,
                 node_type: NodeType::Rect,
             },
@@ -513,6 +567,11 @@ pub fn extract_uinode_borders(
 
         let border = [left, top, right, bottom];
 
+        // don't extract border if no border
+        if left == 0.0 && top == 0.0 && right == 0.0 && bottom == 0.0 {
+            continue;
+        }
+
         let border_radius = resolve_border_radius(
             border_radius,
             node.size(),

crates/bevy_ui/src/render/ui.wgsl

@@ -79,7 +79,7 @@ fn sd_rounded_box(point: vec2<f32>, size: vec2<f32>, corner_radii: vec4<f32>) ->
     // If 0.0 < y then select bottom left (w) and bottom right corner radius (z).
     // Else select top left (x) and top right corner radius (y).
     let rs = select(corner_radii.xy, corner_radii.wz, 0.0 < point.y);
-    // w and z are swapped so that both pairs are in left to right order, otherwise this second 
+    // w and z are swapped above so that both pairs are in left to right order, otherwise this second 
     // select statement would return the incorrect value for the bottom pair.
     let radius = select(rs.x, rs.y, 0.0 < point.x);
     // Vector from the corner closest to the point, to the point.
@@ -120,6 +120,12 @@ fn sd_inset_rounded_box(point: vec2<f32>, size: vec2<f32>, radius: vec4<f32>, in
     return sd_rounded_box(inner_point, inner_size, r);
 }
 
+// get alpha for antialiasing for sdf
+fn antialias(distance: f32) -> f32 {
+    // Using the fwidth(distance) was causing artifacts, so just use the distance.
+    return clamp(0.0, 1.0, 0.5 - distance);
+}
+
 fn draw(in: VertexOutput) -> vec4<f32> {
     let texture_color = textureSample(sprite_texture, sprite_sampler, in.uv);
 
@@ -145,32 +151,31 @@ fn draw(in: VertexOutput) -> vec4<f32> {
     // outside the outside edge, or inside the inner edge have positive signed distance.
     let border_distance = max(external_distance, -internal_distance);
 
-    // The `fwidth` function returns an approximation of the rate of change of the signed distance 
-    // value that is used to ensure that the smooth alpha transition created by smoothstep occurs 
-    // over a range of distance values that is proportional to how quickly the distance is changing.
-    let fborder = fwidth(border_distance);
-    let fexternal = fwidth(external_distance);
-
-    if enabled(in.flags, BORDER) {   
-        // The item is a border
+    // At external edges with no border, `border_distance` is equal to zero. 
+    // This select statement ensures we only perform anti-aliasing where a non-zero width border 
+    // is present, otherwise an outline about the external boundary would be drawn even without 
+    // a border.
+    let t = select(1.0 - step(0.0, border_distance), antialias(border_distance), external_distance < internal_distance);
 
-        // At external edges with no border, `border_distance` is equal to zero. 
-        // This select statement ensures we only perform anti-aliasing where a non-zero width border 
-        // is present, otherwise an outline about the external boundary would be drawn even without 
-        // a border.
-        let t = 1. - select(step(0.0, border_distance), smoothstep(0.0, fborder, border_distance), external_distance < internal_distance);
-
-        // Blend mode ALPHA_BLENDING is used for UI elements, so we don't premultiply alpha here.
-        return vec4(color.rgb, color.a * t);
-    }
+    // Blend mode ALPHA_BLENDING is used for UI elements, so we don't premultiply alpha here.
+    return vec4(color.rgb, color.a * t);
+}
 
-    // The item is a rectangle, draw normally with anti-aliasing at the edges.
-    let t = 1. - smoothstep(0.0, fexternal, external_distance);
+fn draw_background(in: VertexOutput) -> vec4<f32> {
+    let texture_color = textureSample(sprite_texture, sprite_sampler, in.uv);
+    let color = select(in.color, in.color * texture_color, enabled(in.flags, TEXTURED));
 
+    // When drawing the background only draw the internal area and not the border.
+    let internal_distance = sd_inset_rounded_box(in.point, in.size, in.radius, in.border);
+    let t = antialias(internal_distance);
     return vec4(color.rgb, color.a * t);
 }
 
 @fragment
 fn fragment(in: VertexOutput) -> @location(0) vec4<f32> {
-    return draw(in);
+    if enabled(in.flags, BORDER) {
+        return draw(in);    
+    } else {
+        return draw_background(in);
+    }
 }