fix(render): draw transparent solids without depth writes

Once solids carried a real sub-1 alpha (after the ENC transparency fix),
the single depth-writing mesh pass made a transparent solid occlude the
geometry behind it: it wrote depth at its near surface, so anything farther
failed the LessEqual test and was never drawn — the transparent fill looked
like it erased what was behind it.

The batched mesh builder now routes each solid's triangles into an opaque or
a transparent index stream by its baked colour alpha (< 0.999). The opaque
fills draw first and write depth; the transparent fills draw last through a
new depth-write-disabled pipeline, so they blend over the geometry behind
them instead of culling it. The hidden-line depth prepass still rasterises
both streams so solid surfaces keep occluding hidden edges.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pipeline/mesh_gpu.rs

@@ -112,8 +112,14 @@ impl Highlight {
 
 pub struct MeshBatchChunk {
     pub vertex_buffer: wgpu::Buffer,
+    /// Opaque triangle indices (mesh colour alpha ≈ 1). Drawn with depth write.
     pub index_buffer: wgpu::Buffer,
     pub index_count: u32,
+    /// Transparent triangle indices (mesh colour alpha < 1). Drawn after the
+    /// opaque fills with depth write disabled so they blend over — rather than
+    /// erase — the geometry behind them.
+    pub transp_index_buffer: wgpu::Buffer,
+    pub transp_index_count: u32,
     pub wire_index_buffer: wgpu::Buffer,
     pub wire_index_count: u32,
 }
@@ -122,28 +128,34 @@ fn make_chunk(
     device: &wgpu::Device,
     verts: &[MeshVertex],
     indices: &[u32],
+    transp_indices: &[u32],
     wire_indices: &[u32],
 ) -> MeshBatchChunk {
+    // `create_buffer_init` with an empty slice yields a zero-sized buffer that
+    // some backends reject for INDEX usage; a chunk can legitimately hold only
+    // opaque or only transparent tris, so fall back to a 1-index stub (count
+    // stays 0, so the draw loop skips it).
+    let mk_index = |data: &[u32], label: &'static str| {
+        let stub = [0u32];
+        let src = if data.is_empty() { &stub[..] } else { data };
+        device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
+            label: Some(label),
+            contents: bytemuck::cast_slice(src),
+            usage: wgpu::BufferUsages::INDEX,
+        })
+    };
     let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
         label: Some("mesh.batch.vbuf"),
         contents: bytemuck::cast_slice(verts),
         usage: wgpu::BufferUsages::VERTEX,
     });
-    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
-        label: Some("mesh.batch.ibuf"),
-        contents: bytemuck::cast_slice(indices),
-        usage: wgpu::BufferUsages::INDEX,
-    });
-    let wire_index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
-        label: Some("mesh.batch.wire_ibuf"),
-        contents: bytemuck::cast_slice(wire_indices),
-        usage: wgpu::BufferUsages::INDEX,
-    });
     MeshBatchChunk {
         vertex_buffer,
-        index_buffer,
+        index_buffer: mk_index(indices, "mesh.batch.ibuf"),
         index_count: indices.len() as u32,
-        wire_index_buffer,
+        transp_index_buffer: mk_index(transp_indices, "mesh.batch.transp_ibuf"),
+        transp_index_count: transp_indices.len() as u32,
+        wire_index_buffer: mk_index(wire_indices, "mesh.batch.wire_ibuf"),
         wire_index_count: wire_indices.len() as u32,
     }
 }
@@ -168,6 +180,7 @@ pub fn build_mesh_batch(device: &wgpu::Device, sets: &[MeshLodSet]) -> (Vec<Mesh
     let mut chunks = Vec::new();
     let mut verts: Vec<MeshVertex> = Vec::new();
     let mut indices: Vec<u32> = Vec::new();
+    let mut transp_indices: Vec<u32> = Vec::new();
     let mut wire_indices: Vec<u32> = Vec::new();
     let mut total_tris = 0u64;
     for set in sets {
@@ -181,6 +194,9 @@ pub fn build_mesh_batch(device: &wgpu::Device, sets: &[MeshLodSet]) -> (Vec<Mesh
             color: mesh.color,
             position_low: mesh.verts_low.get(vi).copied().unwrap_or([0.0; 3]),
         };
+        // A solid whose baked colour is not fully opaque routes into the
+        // transparent index stream so it is drawn last, without depth writes.
+        let is_transp = mesh.color[3] < 0.999;
         let mesh_tris = mesh.indices.len() / 3;
         total_tris += mesh_tris as u64;
 
@@ -188,9 +204,10 @@ pub fn build_mesh_batch(device: &wgpu::Device, sets: &[MeshLodSet]) -> (Vec<Mesh
         // sub-chunks (corners expanded, no vertex sharing) so each buffer fits.
         if mesh.verts.len() > max_verts || mesh_tris > max_tris {
             if !verts.is_empty() {
-                chunks.push(make_chunk(device, &verts, &indices, &wire_indices));
+                chunks.push(make_chunk(device, &verts, &indices, &transp_indices, &wire_indices));
                 verts.clear();
                 indices.clear();
+                transp_indices.clear();
                 wire_indices.clear();
             }
             let tris_per = (max_verts / 3).min(max_tris).max(1);
@@ -207,7 +224,13 @@ pub fn build_mesh_batch(device: &wgpu::Device, sets: &[MeshLodSet]) -> (Vec<Mesh
                     si.extend_from_slice(&[b, b + 1, b + 2]);
                     swi.extend_from_slice(&[b, b + 1, b + 1, b + 2, b + 2, b]);
                 }
-                chunks.push(make_chunk(device, &sv, &si, &swi));
+                // The whole mesh shares one colour, so a sub-chunk is entirely
+                // opaque or entirely transparent.
+                if is_transp {
+                    chunks.push(make_chunk(device, &sv, &[], &si, &swi));
+                } else {
+                    chunks.push(make_chunk(device, &sv, &si, &[], &swi));
+                }
                 t = end;
             }
             continue;
@@ -219,25 +242,27 @@ pub fn build_mesh_batch(device: &wgpu::Device, sets: &[MeshLodSet]) -> (Vec<Mesh
             && (verts.len() + mesh.verts.len() > max_verts
                 || wire_indices.len() / 6 + mesh_tris > max_tris)
         {
-            chunks.push(make_chunk(device, &verts, &indices, &wire_indices));
+            chunks.push(make_chunk(device, &verts, &indices, &transp_indices, &wire_indices));
             verts.clear();
             indices.clear();
+            transp_indices.clear();
             wire_indices.clear();
         }
         let base = verts.len() as u32;
         for i in 0..mesh.verts.len() {
             verts.push(vtx(i));
         }
+        let fill = if is_transp { &mut transp_indices } else { &mut indices };
         for &idx in &mesh.indices {
-            indices.push(base + idx);
+            fill.push(base + idx);
         }
         for tri in mesh.indices.chunks_exact(3) {
             let (a, b, c) = (base + tri[0], base + tri[1], base + tri[2]);
             wire_indices.extend_from_slice(&[a, b, b, c, c, a]);
         }
     }
-    if !indices.is_empty() {
+    if !indices.is_empty() || !transp_indices.is_empty() {
-        chunks.push(make_chunk(device, &verts, &indices, &wire_indices));
+        chunks.push(make_chunk(device, &verts, &indices, &transp_indices, &wire_indices));
     }
     (chunks, total_tris)
 }

src/scene/pipeline/mod.rs

@@ -45,6 +45,8 @@ pub struct Pipeline {
     hatch_batched_pipeline: Option<wgpu::RenderPipeline>,
     image_pipeline: wgpu::RenderPipeline,
     mesh_pipeline: wgpu::RenderPipeline,
+    /// Depth-write-disabled variant of `mesh_pipeline` for non-opaque solids.
+    mesh_transparent_pipeline: wgpu::RenderPipeline,
     mesh_highlight_pipeline: wgpu::RenderPipeline,
     /// Wireframe variant of the mesh pipeline (LineList topology, same
     /// vertex layout / shader). Used when the active render mode is
@@ -532,6 +534,55 @@ impl Pipeline {
             cache: None,
         });
 
+        // Transparent variant — identical to `mesh_pipeline` but with depth
+        // writes disabled. Non-opaque solids are drawn after the opaque fills
+        // with this pipeline so they blend over the geometry behind them (which
+        // has already written depth) instead of erasing it.
+        let mesh_transparent_pipeline =
+            device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
+                label: Some("mesh.transparent.pipeline"),
+                layout: Some(&mesh_layout),
+                vertex: wgpu::VertexState {
+                    module: &mesh_shader,
+                    entry_point: Some("vs_main"),
+                    buffers: &[mesh_gpu::MeshVertex::layout()],
+                    compilation_options: wgpu::PipelineCompilationOptions::default(),
+                },
+                primitive: wgpu::PrimitiveState {
+                    topology: wgpu::PrimitiveTopology::TriangleList,
+                    cull_mode: None,
+                    ..Default::default()
+                },
+                depth_stencil: Some(wgpu::DepthStencilState {
+                    format: wgpu::TextureFormat::Depth32Float,
+                    depth_write_enabled: false,
+                    depth_compare: wgpu::CompareFunction::LessEqual,
+                    stencil: wgpu::StencilState::default(),
+                    bias: wgpu::DepthBiasState {
+                        constant: 1,
+                        slope_scale: 1.0,
+                        clamp: 0.0,
+                    },
+                }),
+                multisample: wgpu::MultisampleState {
+                    count: MSAA_SAMPLES,
+                    mask: !0,
+                    alpha_to_coverage_enabled: false,
+                },
+                fragment: Some(wgpu::FragmentState {
+                    module: &mesh_shader,
+                    entry_point: Some("fs_main"),
+                    targets: &[Some(wgpu::ColorTargetState {
+                        format,
+                        blend: Some(wgpu::BlendState::ALPHA_BLENDING),
+                        write_mask: wgpu::ColorWrites::ALL,
+                    })],
+                    compilation_options: wgpu::PipelineCompilationOptions::default(),
+                }),
+                multiview: None,
+                cache: None,
+            });
+
         // Highlight variant — same shader / vertex layout, but `depth_compare =
         // Always` and no depth write, so a hovered / selected solid's tint is
         // drawn on top of everything (it shows through occluding geometry) and
@@ -1003,6 +1054,7 @@ impl Pipeline {
             hatch_batched_pipeline,
             image_pipeline,
             mesh_pipeline,
+            mesh_transparent_pipeline,
             mesh_highlight_pipeline,
             mesh_wireframe_pipeline,
             mesh_depth_pipeline,
@@ -1568,14 +1620,25 @@ impl Pipeline {
             // per-solid LOD / frustum cull in the batched path (the batch is
             // resident in full); that is reintroduced separately if needed.
             if hidden_line {
+                // Depth-only prepass: every solid surface occludes hidden edges,
+                // so both the opaque and the transparent tris write depth here.
                 pass.set_pipeline(&self.mesh_depth_pipeline);
                 for c in &self.gpu_mesh_batch {
-                    if c.index_count == 0 {
-                        continue;
-                    }
                     pass.set_vertex_buffer(0, c.vertex_buffer.slice(..));
-                    pass.set_index_buffer(c.index_buffer.slice(..), wgpu::IndexFormat::Uint32);
+                    if c.index_count != 0 {
-                    pass.draw_indexed(0..c.index_count, 0, 0..1);
+                        pass.set_index_buffer(
+                            c.index_buffer.slice(..),
+                            wgpu::IndexFormat::Uint32,
+                        );
+                        pass.draw_indexed(0..c.index_count, 0, 0..1);
+                    }
+                    if c.transp_index_count != 0 {
+                        pass.set_index_buffer(
+                            c.transp_index_buffer.slice(..),
+                            wgpu::IndexFormat::Uint32,
+                        );
+                        pass.draw_indexed(0..c.transp_index_count, 0, 0..1);
+                    }
                 }
                 pass.set_pipeline(&self.mesh_wireframe_pipeline);
                 for c in &self.gpu_mesh_batch {
@@ -1592,21 +1655,43 @@ impl Pipeline {
             } else {
                 if mesh_wireframe {
                     pass.set_pipeline(&self.mesh_wireframe_pipeline);
-                } else {
+                    for c in &self.gpu_mesh_batch {
-                    pass.set_pipeline(&self.mesh_pipeline);
+                        if c.wire_index_count == 0 {
-                }
+                            continue;
-                for c in &self.gpu_mesh_batch {
+                        }
-                    let (ibuf, icount) = if mesh_wireframe {
+                        pass.set_vertex_buffer(0, c.vertex_buffer.slice(..));
-                        (&c.wire_index_buffer, c.wire_index_count)
+                        pass.set_index_buffer(
-                    } else {
+                            c.wire_index_buffer.slice(..),
-                        (&c.index_buffer, c.index_count)
+                            wgpu::IndexFormat::Uint32,
-                    };
+                        );
-                    if icount == 0 {
+                        pass.draw_indexed(0..c.wire_index_count, 0, 0..1);
-                        continue;
+                    }
+                } else {
+                    // Opaque fills first (they write depth).
+                    pass.set_pipeline(&self.mesh_pipeline);
+                    for c in &self.gpu_mesh_batch {
+                        if c.index_count == 0 {
+                            continue;
+                        }
+                        pass.set_vertex_buffer(0, c.vertex_buffer.slice(..));
+                        pass.set_index_buffer(c.index_buffer.slice(..), wgpu::IndexFormat::Uint32);
+                        pass.draw_indexed(0..c.index_count, 0, 0..1);
+                    }
+                    // Transparent fills last, with depth writes disabled, so they
+                    // blend over the opaque geometry behind them instead of
+                    // culling it via the depth buffer.
+                    pass.set_pipeline(&self.mesh_transparent_pipeline);
+                    for c in &self.gpu_mesh_batch {
+                        if c.transp_index_count == 0 {
+                            continue;
+                        }
+                        pass.set_vertex_buffer(0, c.vertex_buffer.slice(..));
+                        pass.set_index_buffer(
+                            c.transp_index_buffer.slice(..),
+                            wgpu::IndexFormat::Uint32,
+                        );
+                        pass.draw_indexed(0..c.transp_index_count, 0, 0..1);
                     }
-                    pass.set_vertex_buffer(0, c.vertex_buffer.slice(..));
-                    pass.set_index_buffer(ibuf.slice(..), wgpu::IndexFormat::Uint32);
-                    pass.draw_indexed(0..icount, 0, 0..1);
                 }
                 // Selection / hover highlight: tinted copies of the picked
                 // solids, drawn last with the Always-depth highlight pipeline so