add standard material depth bias to pipeline (#7847)

# Objective

the current depth bias only adjusts ordering, so it doesn't work for opaque meshes vs alpha-blend meshes, and it doesn't help when two meshes are infinitesimally offset from one another.

## Solution

pass the material's depth bias into the pipeline depth stencil `constant` field.

crates/bevy_pbr/src/material.rs

@@ -126,7 +126,8 @@ pub trait Material: AsBindGroup + Send + Sync + Clone + TypeUuid + Sized + 'stat
 
  #[inline]
  /// Add a bias to the view depth of the mesh which can be used to force a specific render order
- /// for meshes with equal depth, to avoid z-fighting.
+ /// for meshes with similar depth, to avoid z-fighting.
+ /// The bias is in depth-texture units so large values may be needed to overcome small depth differences.
  fn depth_bias(&self) -> f32 {
  0.0
  }
@@ -514,6 +515,7 @@ pub struct MaterialProperties {
  pub alpha_mode: AlphaMode,
  /// Add a bias to the view depth of the mesh which can be used to force a specific render order
  /// for meshes with equal depth, to avoid z-fighting.
+ /// The bias is in depth-texture units so large values may be needed to overcome small depth differences.
  pub depth_bias: f32,
 }
 

crates/bevy_pbr/src/pbr_material.rs

@@ -220,19 +220,14 @@ pub struct StandardMaterial {
  /// See [`AlphaMode`] for details. Defaults to [`AlphaMode::Opaque`].
  pub alpha_mode: AlphaMode,
 
- /// Re-arrange render ordering.
+ /// Adjust rendered depth.
  ///
  /// A material with a positive depth bias will render closer to the
  /// camera while negative values cause the material to render behind
  /// other objects. This is independent of the viewport.
  ///
- /// `depth_bias` only affects render ordering. This means that for opaque materials,
- /// `depth_bias` will only have any effect if two materials are overlapping,
- /// which only serves as a [z-fighting] resolver.
- ///
- /// `depth_bias` can however reorder [`AlphaMode::Blend`] materials.
- /// This is useful if your transparent materials are not rendering
- /// in the expected order.
+ /// `depth_bias` affects render ordering and depth write operations
+ /// using the `wgpu::DepthBiasState::Constant` field.
  ///
  /// [z-fighting]: https://en.wikipedia.org/wiki/Z-fighting
  pub depth_bias: f32,
@@ -420,13 +415,15 @@ impl AsBindGroupShaderType<StandardMaterialUniform> for StandardMaterial {
 pub struct StandardMaterialKey {
  normal_map: bool,
  cull_mode: Option<Face>,
+ depth_bias: i32,
 }
 
 impl From<&StandardMaterial> for StandardMaterialKey {
  fn from(material: &StandardMaterial) -> Self {
  StandardMaterialKey {
  normal_map: material.normal_map_texture.is_some(),
  cull_mode: material.cull_mode,
+ depth_bias: material.depth_bias as i32,
  }
  }
 }
@@ -449,6 +446,9 @@ impl Material for StandardMaterial {
  if let Some(label) = &mut descriptor.label {
  *label = format!("pbr_{}", *label).into();
  }
+ if let Some(depth_stencil) = descriptor.depth_stencil.as_mut() {
+ depth_stencil.bias.constant = key.bind_group_data.depth_bias;
+ }
  Ok(())
  }