ui: fix z indices and depth calculations

crates/bevy_render/src/camera/camera.rs

@@ -11,6 +11,20 @@ pub struct Camera {
     pub name: Option<String>,
     #[property(ignore)]
     pub window: WindowId,
+    #[property(ignore)]
+    pub depth_calculation: DepthCalculation,
+}
+
+#[derive(Debug)]
+pub enum DepthCalculation {
+    Distance,
+    ZDifference,
+}
+
+impl Default for DepthCalculation {
+    fn default() -> Self {
+        DepthCalculation::Distance
+    }
 }
 
 #[derive(Default)]
@@ -55,8 +69,11 @@ pub fn camera_system<T: CameraProjection + Component>(
 
     for (mut camera, mut camera_projection) in &mut query.iter() {
         if let Some(window) = windows.get(camera.window) {
-            camera_projection.update(window.width as usize, window.height as usize);
-            camera.projection_matrix = camera_projection.get_projection_matrix();
+            if changed_window_ids.contains(&window.id) {
+                camera_projection.update(window.width as usize, window.height as usize);
+                camera.projection_matrix = camera_projection.get_projection_matrix();
+                camera.depth_calculation = camera_projection.depth_calculation();
+            }
         }
     }
 }

crates/bevy_render/src/camera/projection.rs

@@ -1,10 +1,12 @@
 use bevy_math::{Mat4, PerspectiveRh};
 use bevy_property::{Properties, Property};
 use serde::{Deserialize, Serialize};
+use super::DepthCalculation;
 
 pub trait CameraProjection {
     fn get_projection_matrix(&self) -> Mat4;
     fn update(&mut self, width: usize, height: usize);
+    fn depth_calculation(&self) -> DepthCalculation;
 }
 
 #[derive(Debug, Clone, Properties)]
@@ -23,6 +25,10 @@ impl CameraProjection for PerspectiveProjection {
     fn update(&mut self, width: usize, height: usize) {
         self.aspect_ratio = width as f32 / height as f32;
     }
+
+    fn depth_calculation(&self) -> DepthCalculation {
+        DepthCalculation::Distance
+    }
 }
 
 impl Default for PerspectiveProjection {
@@ -84,6 +90,10 @@ impl CameraProjection for OrthographicProjection {
             }
         }
     }
+
+    fn depth_calculation(&self) -> DepthCalculation {
+        DepthCalculation::ZDifference
+    }
 }
 
 impl Default for OrthographicProjection {

crates/bevy_render/src/camera/visible_entities.rs

@@ -1,4 +1,4 @@
-use super::Camera;
+use super::{Camera, DepthCalculation};
 use crate::Draw;
 use bevy_core::FloatOrd;
 use bevy_ecs::{Entity, Query};
@@ -26,7 +26,7 @@ pub fn visible_entities_system(
     mut draw_query: Query<(Entity, &Draw)>,
     draw_transform_query: Query<(&Draw, &Transform)>,
 ) {
-    for (_camera, camera_transform, mut visible_entities) in &mut camera_query.iter() {
+    for (camera, camera_transform, mut visible_entities) in &mut camera_query.iter() {
         visible_entities.value.clear();
         let camera_position = camera_transform.value.w_axis().truncate();
 
@@ -40,7 +40,10 @@ pub fn visible_entities_system(
             let order = if let Ok(transform) = draw_transform_query.get::<Transform>(entity) {
                 let position = transform.value.w_axis().truncate();
                 // smaller distances are sorted to lower indices by using the distance from the camera
-                FloatOrd((camera_position - position).length())
+                FloatOrd(match camera.depth_calculation {
+                    DepthCalculation::ZDifference => camera_position.z() - position.z(),
+                    DepthCalculation::Distance => (camera_position - position).length(),
+                })
             } else {
                 let order = FloatOrd(no_transform_order);
                 no_transform_order += 0.1;

crates/bevy_ui/src/lib.rs

@@ -32,15 +32,20 @@ use update::ui_z_system;
 #[derive(Default)]
 pub struct UiPlugin;
 
+pub mod stage {
+    pub const UI: &'static str = "ui";
+}
+
 impl AppPlugin for UiPlugin {
     fn build(&self, app: &mut AppBuilder) {
         app.init_resource::<FlexSurface>()
-            .add_system_to_stage(stage::PRE_UPDATE, ui_focus_system.system())
+            .add_stage_before(bevy_app::stage::POST_UPDATE, stage::UI)
+            .add_system_to_stage(bevy_app::stage::PRE_UPDATE, ui_focus_system.system())
             // add these stages to front because these must run before transform update systems
-            .add_system_to_stage_front(stage::POST_UPDATE, flex_node_system.system())
-            .add_system_to_stage_front(stage::POST_UPDATE, ui_z_system.system())
-            .add_system_to_stage_front(stage::POST_UPDATE, widget::text_system.system())
-            .add_system_to_stage_front(stage::POST_UPDATE, widget::image_node_system.system())
+            .add_system_to_stage(stage::UI, widget::text_system.system())
+            .add_system_to_stage(stage::UI, widget::image_node_system.system())
+            .add_system_to_stage(stage::UI, ui_z_system.system())
+            .add_system_to_stage(stage::UI, flex_node_system.system())
             .add_system_to_stage(bevy_render::stage::DRAW, widget::draw_text_system.system());
 
         let resources = app.resources();

crates/bevy_ui/src/render/mod.rs

@@ -3,10 +3,17 @@ use bevy_asset::{Assets, Handle};
 use bevy_ecs::Resources;
 use bevy_render::{
     camera::ActiveCameras,
+    pass::{
+        LoadOp, Operations, PassDescriptor, RenderPassColorAttachmentDescriptor,
+        RenderPassDepthStencilAttachmentDescriptor, TextureAttachment,
+    },
     pipeline::*,
-    render_graph::{base, CameraNode, PassNode, RenderGraph, RenderResourcesNode, WindowSwapChainNode, WindowTextureNode},
+    render_graph::{
+        base, CameraNode, PassNode, RenderGraph, RenderResourcesNode, WindowSwapChainNode,
+        WindowTextureNode,
+    },
     shader::{Shader, ShaderStage, ShaderStages},
-    texture::TextureFormat, prelude::{Color, MainPass}, pass::{RenderPassColorAttachmentDescriptor, PassDescriptor, TextureAttachment, LoadOp, Operations, RenderPassDepthStencilAttachmentDescriptor},
+    texture::TextureFormat,
 };
 
 pub const UI_PIPELINE_HANDLE: Handle<PipelineDescriptor> =
@@ -121,11 +128,9 @@ impl UiRenderGraphBuilder for RenderGraph {
 
         // setup ui camera
         self.add_system_node(node::UI_CAMERA, CameraNode::new(camera::UI_CAMERA));
-        self.add_node_edge(node::UI_CAMERA, node::UI_PASS)
-            .unwrap();
+        self.add_node_edge(node::UI_CAMERA, node::UI_PASS).unwrap();
         self.add_system_node(node::NODE, RenderResourcesNode::<Node>::new(true));
-        self.add_node_edge(node::NODE, node::UI_PASS)
-            .unwrap();
+        self.add_node_edge(node::NODE, node::UI_PASS).unwrap();
         let mut active_cameras = resources.get_mut::<ActiveCameras>().unwrap();
         active_cameras.add(camera::UI_CAMERA);
         self

crates/bevy_ui/src/update.rs

@@ -12,7 +12,7 @@ pub fn ui_z_system(
     mut node_query: Query<(Entity, &Node, &mut LocalTransform)>,
     children_query: Query<&Children>,
 ) {
-    let mut window_z = 0.0;
+    let mut current_global_z = 0.0;
 
     // PERF: we can probably avoid an allocation here by making root_node_query and node_query non-overlapping
     let root_nodes = (&mut root_node_query.iter())
@@ -24,11 +24,11 @@ pub fn ui_z_system(
             &children_query,
             &mut node_query,
             entity,
-            Some(window_z),
-            Some(window_z),
+            Some(current_global_z),
+            Some(current_global_z),
             &mut update_node_entity,
         ) {
-            window_z = result;
+            current_global_z = result;
         }
     }
 }
@@ -36,18 +36,20 @@ pub fn ui_z_system(
 fn update_node_entity(
     node_query: &mut Query<(Entity, &Node, &mut LocalTransform)>,
     entity: Entity,
-    _parent_result: Option<f32>,
+    parent_result: Option<f32>,
     previous_result: Option<f32>,
 ) -> Option<f32> {
     let mut transform = node_query.get_mut::<LocalTransform>(entity).unwrap();
     let mut z = UI_Z_STEP;
-    if let Some(previous_z) = previous_result {
-        z += previous_z;
+    let parent_global_z = parent_result.unwrap();
+    if let Some(previous_global_z) = previous_result {
+        z += previous_global_z - parent_global_z;
     };
+    let global_z = z + parent_global_z;
 
     let mut position = transform.w_axis();
     position.set_z(z);
     transform.set_w_axis(position);
 
-    return Some(z);
+    return Some(global_z);
 }