Release 0.10.1

* GPU tessellation: a pipeline the device cannot build falls back to CPU meshing instead of crashing (#1098)

Build the tessellation compute pipeline inside error scopes; TessGpu::new returns an error,
and the viewport logs it and meshes bodies on the CPU. A Windows CI job builds the pipeline
with D3D12's FXC on the WARP adapter.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* Tessellation shader: basis functions in one-dimensional local arrays, so D3D12's FXC compiles it (#1098)

FXC has to unroll loops that write an array inside a struct (Basis.n0/n1) or a 2-D array
(ndu) by a counter, and cannot with run-time trip counts (X3511). A2.2 basis functions in a
local 1-D array returned whole; derivatives from the degree p-1 functions; the pole step
offsets computed rather than indexed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>

.github/workflows/ci.yml

@@ -286,9 +286,31 @@ jobs:
           name: oracle-report
           path: docs/oracle.md
 
+  # D3D12's older shader compiler (FXC, used where dxcompiler.dll is absent) is stricter than
+  # DXC, Vulkan and Metal: a shader it cannot compile once crashed the app at startup. Build the
+  # GPU tessellation pipeline with FXC on the runner's WARP software adapter and compare its
+  # results with the CPU.
+  windows-fxc:
+    name: Windows D3D12 FXC shaders
+    needs: changes
+    if: needs.changes.outputs.native == 'true'
+    runs-on: windows-latest
+    timeout-minutes: 45
+    env:
+      WGPU_BACKEND: dx12
+      WGPU_DX12_COMPILER: fxc
+      SOLIDCRAFT_REQUIRE_GPU: "1"
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+        with:
+          key: windows-fxc
+      - run: cargo test -p solidcraft-gpu --profile ci --locked
+
   # Preserve the existing check name for branch protection, including docs-only PRs.
   ci:
-    needs: [changes, checks, clippy, build-tests, tests, doc-tests, oracle]
+    needs: [changes, checks, clippy, build-tests, tests, doc-tests, oracle, windows-fxc]
     if: always()
     runs-on: ubuntu-24.04
     steps:
@@ -302,6 +324,7 @@ jobs:
           TESTS: ${{ needs.tests.result }}
           DOCS: ${{ needs.doc-tests.result }}
           ORACLE: ${{ needs.oracle.result }}
+          WINDOWS_FXC: ${{ needs.windows-fxc.result }}
         run: |
           test "$CHANGES" = success
           test "$CHECKS" = success
@@ -311,6 +334,7 @@ jobs:
             test "$TESTS" = success
             test "$DOCS" = success
             test "$ORACLE" = success
+            test "$WINDOWS_FXC" = success
           fi
 
   wasm:

Cargo.lock

@@ -3281,7 +3281,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "eframe",
  "egui",
@@ -3299,7 +3299,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-cli"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "serde_json",
  "solidcraft-engine",
@@ -3308,7 +3308,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-doc"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "log",
  "serde",
@@ -3323,7 +3323,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-engine"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "log",
  "serde",
@@ -3339,7 +3339,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-geom"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "serde",
  "serde_json",
@@ -3348,9 +3348,10 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-gpu"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "log",
+ "naga",
  "pollster 0.4.0",
  "solidcraft-geom",
  "solidcraft-kernel",
@@ -3360,7 +3361,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-io"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "quick-xml",
  "serde",
@@ -3375,7 +3376,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-kernel"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "log",
  "rayon",
@@ -3393,7 +3394,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-mcp"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "serde_json",
  "solidcraft-engine",
@@ -3401,7 +3402,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-render"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "image",
  "serde",
@@ -3411,7 +3412,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-sketch"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "serde",
  "serde_json",
@@ -3422,7 +3423,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-ui-egui"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "egui",
  "egui-wgpu",
@@ -3439,7 +3440,7 @@ dependencies = [
 
 [[package]]
 name = "solidcraft-web"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "eframe",
  "egui",
@@ -4862,7 +4863,7 @@ checksum = "e450f9b2ed1dff33c94c12589a87338689467b9c4f5d8a5710bd09a847d2c8a7"
 
 [[package]]
 name = "xtask"
-version = "0.10.0"
+version = "0.10.1"
 dependencies = [
  "serde_json",
  "sha2",

Cargo.toml

@@ -5,7 +5,7 @@ members = ["crates/*", "apps/*", "xtask"]
 exclude = ["vendor"]
 
 [workspace.package]
-version = "0.10.0"
+version = "0.10.1"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 rust-version = "1.95"
@@ -61,6 +61,7 @@ web-time = "1"
 egui-wgpu = "0.36"
 # the wgpu egui-wgpu uses, for compute (crates/gpu); backends come from the apps' features
 wgpu = { version = "30", default-features = false, features = ["std", "wgsl"] }
+naga = { version = "30", default-features = false }
 pollster = "0.4"
 rfd = "0.17"
 

crates/gpu/Cargo.toml

@@ -21,3 +21,5 @@ wgpu = { workspace = true, features = ["metal", "vulkan", "dx12", "gles"] }
 
 [dev-dependencies]
 solidcraft-kernel = { workspace = true }
+# The HLSL the D3D12 backend compiles (tests check it suits FXC).
+naga = { workspace = true, features = ["wgsl-in", "hlsl-out"] }

crates/gpu/src/lib.rs

@@ -17,6 +17,37 @@ use solidcraft_geom::Vec3;
 use solidcraft_geom::tess::{EvalCurve, EvalSurface, TessPlan};
 use wgpu::util::DeviceExt;
 
+/// The tessellation compute shader (WGSL).
+pub const SHADER: &str = include_str!("tess.wgsl");
+
+/// The error a popped error scope caught. Native devices answer at once; a browser's answer
+/// comes later, and is not waited for (no error is assumed).
+fn popped(f: impl std::future::Future<Output = Option<wgpu::Error>>) -> Option<wgpu::Error> {
+    let mut f = std::pin::pin!(f);
+    match f.as_mut().poll(&mut std::task::Context::from_waker(std::task::Waker::noop())) {
+        std::task::Poll::Ready(e) => e,
+        std::task::Poll::Pending => None,
+    }
+}
+
+/// An adapter and device without a window (native only). The environment may pick the backend
+/// and the D3D12 shader compiler (`WGPU_BACKEND`, `WGPU_DX12_COMPILER`).
+#[cfg(not(target_arch = "wasm32"))]
+fn open_headless() -> Result<(wgpu::Adapter, wgpu::Device, wgpu::Queue)> {
+    let instance = wgpu::Instance::new(wgpu::InstanceDescriptor::new_without_display_handle_from_env());
+    let adapter = pollster::block_on(
+        instance.request_adapter(&wgpu::RequestAdapterOptions { power_preference: wgpu::PowerPreference::HighPerformance, ..Default::default() }),
+    )
+    .map_err(|_| GpuError::NoAdapter)?;
+    let (device, queue) = pollster::block_on(adapter.request_device(&wgpu::DeviceDescriptor {
+        label: Some("solidcraft compute"),
+        required_limits: wgpu::Limits::downlevel_defaults().using_resolution(adapter.limits()),
+        ..Default::default()
+    }))
+    .map_err(|e| GpuError::Device(e.to_string()))?;
+    Ok((adapter, device, queue))
+}
+
 /// Most vertices evaluated in one dispatch (bounds the buffers; larger plans go in batches).
 pub const MAX_BATCH: usize = 1 << 20;
 const WORKGROUP: u32 = 64;
@@ -29,6 +60,8 @@ pub enum GpuError {
     Device(String),
     #[error("the GPU evaluation failed: {0}")]
     Failed(String),
+    #[error("the GPU tessellation pipeline could not be created: {0}")]
+    Pipeline(String),
     #[error("not supported yet: {0}")]
     Unsupported(String),
 }
@@ -78,11 +111,47 @@ pub struct TessGpu {
 }
 
 impl TessGpu {
-    /// The pipeline on an existing device (`adapter`: its name, for reports).
+    /// The pipeline on an existing device (`adapter`: its name, for reports). An error (never
-    pub fn new(adapter: impl Into<String>, device: wgpu::Device, queue: wgpu::Queue) -> Self {
+    /// a panic) when the device cannot build it, for example where the D3D12 driver's older shader
+    /// compiler (FXC) rejects the shader: the caller then builds bodies on the CPU.
+    pub fn new(adapter: impl Into<String>, device: wgpu::Device, queue: wgpu::Queue) -> Result<Self> {
+        Self::with_shader(adapter, device, queue, SHADER)
+    }
+
+    /// [`TessGpu::new`] with another shader source: tests use it to make pipeline creation fail
+    /// as it does on a device whose compiler rejects the real shader.
+    #[doc(hidden)]
+    pub fn with_shader(adapter: impl Into<String>, device: wgpu::Device, queue: wgpu::Queue, shader: &str) -> Result<Self> {
+        // Errors here would go to the device's uncaptured-error handler, which panics by default:
+        // capture them, so a device that cannot run the shader only loses GPU tessellation.
+        let scopes = [wgpu::ErrorFilter::OutOfMemory, wgpu::ErrorFilter::Internal, wgpu::ErrorFilter::Validation].map(|f| device.push_error_scope(f));
+        let (pipeline, layout) = Self::build(&device, shader);
+        let mut failed = None;
+        for scope in scopes.into_iter().rev() {
+            if let Some(e) = popped(scope.pop()) {
+                failed.get_or_insert(e.to_string());
+            }
+        }
+        if let Some(e) = failed {
+            return Err(GpuError::Pipeline(e));
+        }
+        // A viewport records the evaluation into its own frame (see `TessGpu::record`).
+        #[cfg(target_arch = "wasm32")]
+        let _ = queue;
+        Ok(TessGpu {
+            adapter: adapter.into(),
+            device,
+            #[cfg(not(target_arch = "wasm32"))]
+            queue,
+            pipeline,
+            layout,
+        })
+    }
+
+    fn build(device: &wgpu::Device, shader: &str) -> (wgpu::ComputePipeline, wgpu::BindGroupLayout) {
         let module = device.create_shader_module(wgpu::ShaderModuleDescriptor {
             label: Some("solidcraft tessellation"),
-            source: wgpu::ShaderSource::Wgsl(include_str!("tess.wgsl").into()),
+            source: wgpu::ShaderSource::Wgsl(shader.into()),
         });
         let entry = |binding: u32, ty: wgpu::BufferBindingType| wgpu::BindGroupLayoutEntry {
             binding,
@@ -114,34 +183,14 @@ impl TessGpu {
             compilation_options: Default::default(),
             cache: None,
         });
-        // A viewport records the evaluation into its own frame (see `TessGpu::record`).
+        (pipeline, layout)
-        #[cfg(target_arch = "wasm32")]
-        let _ = queue;
-        TessGpu {
-            adapter: adapter.into(),
-            device,
-            #[cfg(not(target_arch = "wasm32"))]
-            queue,
-            pipeline,
-            layout,
-        }
     }
 
     /// A device of its own, without a window (native only).
     #[cfg(not(target_arch = "wasm32"))]
     pub fn headless() -> Result<Self> {
-        let instance = wgpu::Instance::new(wgpu::InstanceDescriptor::new_without_display_handle());
+        let (adapter, device, queue) = open_headless()?;
-        let adapter = pollster::block_on(
+        Self::new(adapter.get_info().name, device, queue)
-            instance.request_adapter(&wgpu::RequestAdapterOptions { power_preference: wgpu::PowerPreference::HighPerformance, ..Default::default() }),
-        )
-        .map_err(|_| GpuError::NoAdapter)?;
-        let (device, queue) = pollster::block_on(adapter.request_device(&wgpu::DeviceDescriptor {
-            label: Some("solidcraft compute"),
-            required_limits: wgpu::Limits::downlevel_defaults().using_resolution(adapter.limits()),
-            ..Default::default()
-        }))
-        .map_err(|e| GpuError::Device(e.to_string()))?;
-        Ok(Self::new(adapter.get_info().name, device, queue))
     }
 
     /// The adapter's name, for reports.

crates/gpu/src/tess.wgsl

@@ -1,7 +1,7 @@
 // Display tessellation: surface points and normals at each vertex's (u, v).
 //
 // The GPU twin of solidcraft_geom::tess::EvalSurface::{derivs, eval}: the same surface forms,
-// the same algorithms (The NURBS Book A2.1 span search, A2.3 basis functions and first
+// the same algorithms (The NURBS Book A2.1 span search, A2.2 basis functions, A2.3 first
 // derivatives, the rational quotient rule), in f32.
 
 const PLANE: u32 = 0u;
@@ -60,11 +60,6 @@ struct Derivs {
     ok: bool,
 };
 
-struct Basis {
-    n0: array<f32, 8>,
-    n1: array<f32, 8>,
-};
-
 fn vec3_at(o: u32) -> vec3<f32> {
     return vec3<f32>(data[o], data[o + 1u], data[o + 2u]);
 }
@@ -101,42 +96,58 @@ fn span_of(k0: u32, p: u32, n: u32, t: f32) -> u32 {
     return lo;
 }
 
-// Non-zero basis functions of degree p on span `span` and their first derivatives (A2.3, n = 1).
+// The arrays below are one-dimensional locals, written by loop counters and returned whole:
-fn basis(k0: u32, span: u32, p: u32, t: f32) -> Basis {
+// D3D12's older shader compiler (FXC) must unroll a loop that writes an array inside a struct
-    var ndu: array<array<f32, 8>, 8>;
+// (or a two-dimensional one) by a counter, and it cannot unroll loops whose trip count is only
-    var left: array<f32, 8>;
+// known at run time, so such a loop fails to compile there ("unable to unroll loop").
-    var right: array<f32, 8>;
+
-    ndu[0][0] = 1.0;
+// The non-zero basis functions of degree p on span `span` (The NURBS Book A2.2).
+fn basis_funs(k0: u32, span: u32, p: u32, t: f32) -> array<f32, W> {
+    var n: array<f32, W>;
+    n[0] = 1.0;
     for (var j = 1u; j <= p; j = j + 1u) {
-        left[j] = t - data[k0 + span + 1u - j];
-        right[j] = data[k0 + span + j] - t;
         var saved = 0.0;
         for (var r = 0u; r < j; r = r + 1u) {
-            ndu[j][r] = right[r + 1u] + left[j - r];
+            // right[r + 1] and left[j - r] of A2.2 (span ≥ p ≥ j, so no underflow).
+            let right = data[k0 + span + r + 1u] - t;
+            let left = t - data[k0 + span + 1u + r - j];
+            let den = right + left;
             var temp = 0.0;
-            if (ndu[j][r] != 0.0) {
+            if (den != 0.0) {
-                temp = ndu[r][j - 1u] / ndu[j][r];
+                temp = n[r] / den;
             }
-            ndu[r][j] = saved + right[r + 1u] * temp;
+            n[r] = saved + right * temp;
-            saved = left[j - r] * temp;
+            saved = left * temp;
         }
-        ndu[j][j] = saved;
+        n[j] = saved;
     }
-    var b: Basis;
+    return n;
-    for (var j = 0u; j <= p; j = j + 1u) {
+}
-        b.n0[j] = ndu[j][p];
+
+// Their first derivatives, from the degree p - 1 functions on the same span (A2.3, n = 1).
+fn basis_ders(k0: u32, span: u32, p: u32, t: f32) -> array<f32, W> {
+    var d: array<f32, W>;
+    if (p == 0u) {
+        return d;
     }
+    let lower = basis_funs(k0, span, p - 1u, t);
     for (var r = 0u; r <= p; r = r + 1u) {
-        var d = 0.0;
+        var x = 0.0;
-        if (r >= 1u && ndu[p][r - 1u] != 0.0) {
+        if (r >= 1u) {
-            d = d + ndu[r - 1u][p - 1u] / ndu[p][r - 1u];
+            let den = data[k0 + span + r] - data[k0 + span + r - p];
+            if (den != 0.0) {
+                x = x + lower[r - 1u] / den;
+            }
         }
-        if (r < p && ndu[p][r] != 0.0) {
+        if (r < p) {
-            d = d - ndu[r][p - 1u] / ndu[p][r];
+            let den = data[k0 + span + r + 1u] - data[k0 + span + r + 1u - p];
+            if (den != 0.0) {
+                x = x - lower[r] / den;
+            }
         }
-        b.n1[r] = d * f32(p);
+        d[r] = x * f32(p);
     }
-    return b;
+    return d;
 }
 
 // Point and derivative of a rational function from its homogeneous value and derivative.
@@ -152,17 +163,19 @@ fn nurbs(s: Surface, u: f32, v: f32) -> Derivs {
     var r: Derivs;
     let su = span_of(s.knots_u, s.pu, s.nu, u);
     let sv = span_of(s.knots_v, s.pv, s.nv, v);
-    let bu = basis(s.knots_u, su, s.pu, u);
+    let bu0 = basis_funs(s.knots_u, su, s.pu, u);
-    let bv = basis(s.knots_v, sv, s.pv, v);
+    let bu1 = basis_ders(s.knots_u, su, s.pu, u);
+    let bv0 = basis_funs(s.knots_v, sv, s.pv, v);
+    let bv1 = basis_ders(s.knots_v, sv, s.pv, v);
     var a = vec4<f32>(0.0);
     var au = vec4<f32>(0.0);
     var av = vec4<f32>(0.0);
     for (var j = 0u; j <= s.pv; j = j + 1u) {
         for (var i = 0u; i <= s.pu; i = i + 1u) {
             let c = ctrl_at(s.ctrl + 4u * ((sv + j - s.pv) * s.nu + su + i - s.pu));
-            a = a + bu.n0[i] * bv.n0[j] * c;
+            a = a + bu0[i] * bv0[j] * c;
-            au = au + bu.n1[i] * bv.n0[j] * c;
+            au = au + bu1[i] * bv0[j] * c;
-            av = av + bu.n0[i] * bv.n1[j] * c;
+            av = av + bu0[i] * bv1[j] * c;
         }
     }
     r.ok = abs(a.w) > 0.0;
@@ -189,13 +202,14 @@ fn revolved(s: Surface, u: f32, v: f32) -> Derivs {
     }
     // The profile curve: a one-row B-spline.
     let span = span_of(s.knots_u, s.pu, s.nu, t);
-    let b = basis(s.knots_u, span, s.pu, t);
+    let b0 = basis_funs(s.knots_u, span, s.pu, t);
+    let b1 = basis_ders(s.knots_u, span, s.pu, t);
     var c = vec4<f32>(0.0);
     var dc = vec4<f32>(0.0);
     for (var i = 0u; i <= s.pu; i = i + 1u) {
         let h = ctrl_at(s.ctrl + 4u * (span + i - s.pu));
-        c = c + b.n0[i] * h;
+        c = c + b0[i] * h;
-        dc = dc + b.n1[i] * h;
+        dc = dc + b1[i] * h;
     }
     r.ok = abs(c.w) > 0.0;
     if (!r.ok) {
@@ -263,9 +277,11 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     if (!(length(n) > 1e-6 * scale) || !d.ok) {
         // A degenerate point (a pole): the normal from a step inside.
         let h = 1e-4 * (1.0 + max(abs(vert.u), abs(vert.v)));
-        let steps = array<vec2<f32>, 4>(vec2<f32>(h, h), vec2<f32>(-h, h), vec2<f32>(h, -h), vec2<f32>(-h, -h));
+        // Steps (h, h), (-h, h), (h, -h), (-h, -h), without an array indexed by k (FXC).
         for (var k = 0u; k < 4u; k = k + 1u) {
-            let e = derivs(s, vert.u + steps[k].x, vert.v + steps[k].y);
+            let du = select(h, -h, (k & 1u) != 0u);
+            let dv = select(h, -h, (k & 2u) != 0u);
+            let e = derivs(s, vert.u + du, vert.v + dv);
             let m = oriented(s, cross(e.du, e.dv));
             if (e.ok && length(m) > 0.0) {
                 n = m;

crates/gpu/src/tests.rs

@@ -4,12 +4,17 @@ use solidcraft_kernel::{Body, BoolOp};
 use super::*;
 
 /// The GPU, or `None` with a note when the machine has no adapter (CI runners without a GPU).
+/// A device that cannot build the pipeline fails the test: that is a shader the device rejects
+/// (as D3D12's FXC once did), not a missing GPU. `SOLIDCRAFT_REQUIRE_GPU=1` (the Windows CI job,
+/// on the WARP software adapter) fails without an adapter too.
 fn gpu() -> Option<TessGpu> {
     match TessGpu::headless() {
         Ok(g) => {
             eprintln!("GPU: {}", g.adapter_name());
             Some(g)
         }
+        Err(e @ GpuError::Pipeline(_)) => panic!("{e}"),
+        Err(e) if std::env::var_os("SOLIDCRAFT_REQUIRE_GPU").is_some() => panic!("a GPU is required here: {e}"),
         Err(e) => {
             eprintln!("skipping the GPU comparison: {e}");
             None
@@ -17,6 +22,80 @@ fn gpu() -> Option<TessGpu> {
     }
 }
 
+/// A shader the device rejects when the pipeline is created (its binding 1 is writable, the
+/// layout's read-only), standing in for one a device's compiler cannot build.
+const REJECTED: &str = "
+@group(0) @binding(1) var<storage, read_write> surfaces: array<u32>;
+@group(0) @binding(4) var<storage, read_write> out: array<vec4<f32>>;
+@compute @workgroup_size(64)
+fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
+    surfaces[gid.x] = 1u;
+    out[gid.x] = vec4<f32>(0.0);
+}
+";
+
+/// A pipeline the device cannot build is an error the caller can fall back from (to the CPU
+/// mesh), not a panic in the device's uncaptured-error handler; and the device stays usable.
+#[test]
+fn a_pipeline_the_device_rejects_is_an_error_not_a_crash() {
+    let (adapter, device, queue) = match open_headless() {
+        Ok(d) => d,
+        Err(e) => {
+            assert!(std::env::var_os("SOLIDCRAFT_REQUIRE_GPU").is_none(), "a GPU is required here: {e}");
+            eprintln!("skipping: {e}");
+            return;
+        }
+    };
+    let name = adapter.get_info().name;
+    match TessGpu::with_shader(name.clone(), device.clone(), queue.clone(), REJECTED) {
+        Err(GpuError::Pipeline(e)) => eprintln!("rejected as expected: {e}"),
+        Err(e) => panic!("unexpected error: {e}"),
+        Ok(_) => panic!("the rejected shader built a pipeline"),
+    }
+    // Not wgsl at all: also an error, not a panic.
+    assert!(matches!(TessGpu::with_shader(name.clone(), device.clone(), queue.clone(), "fn main( {"), Err(GpuError::Pipeline(_))));
+    // The same device then builds the real pipeline and evaluates on it.
+    let gpu = ok(TessGpu::new(name, device, queue));
+    let body = ok(solidcraft_kernel::torus(Vec3::ZERO, 6.0, 1.5));
+    let plan = ok(body.display_plan(0.01));
+    assert_eq!(ok(gpu.evaluate(&plan)).positions.len(), plan.evaluate().positions.len());
+}
+
+/// The shader as the D3D12 backend translates it (naga's HLSL for shader model 5.1, the FXC
+/// target, with wgpu's options) avoids what FXC cannot compile without unrolling a loop:
+/// arrays inside structs and two-dimensional arrays written by a loop counter. Runs everywhere;
+/// the Windows CI job compiles the real thing with FXC.
+#[test]
+fn the_hlsl_for_fxc_has_no_arrays_fxc_must_unroll_loops_for() {
+    use naga::back::hlsl;
+    let module = ok(naga::front::wgsl::parse_str(SHADER));
+    let info = ok(naga::valid::Validator::new(naga::valid::ValidationFlags::all(), naga::valid::Capabilities::default()).validate(&module));
+    let options = hlsl::Options {
+        shader_model: hlsl::ShaderModel::V5_1,
+        force_loop_bounding: true,
+        restrict_indexing: true,
+        fake_missing_bindings: true,
+        ..Default::default()
+    };
+    let pipeline = hlsl::PipelineOptions::default();
+    let mut out = String::new();
+    ok(hlsl::Writer::new(&mut out, &options, &pipeline).write(&module, &info, None));
+    assert!(out.contains("void main("), "{out}");
+    // Struct members that are arrays: `float n0_[8];` inside a `struct ... { };`.
+    let mut in_struct = false;
+    for line in out.lines() {
+        let l = line.trim();
+        if l.starts_with("struct ") && !l.ends_with(';') {
+            in_struct = true;
+        } else if l.starts_with("};") {
+            in_struct = false;
+        } else if in_struct {
+            assert!(!l.contains('['), "an array inside a struct: {l}");
+        }
+        assert!(!l.contains("]["), "a two-dimensional array: {l}");
+    }
+}
+
 fn ok<T, E: std::fmt::Debug>(r: std::result::Result<T, E>) -> T {
     match r {
         Ok(v) => v,

crates/ui-egui/src/gpu.rs

@@ -736,14 +736,30 @@ impl Batches {
     }
 }
 
+/// The tessellation pipeline (`shader`: its source), where display plans are evaluated: on a
+/// device that runs compute shaders with the storage buffers the evaluation binds (not WebGL2)
+/// and builds the pipeline. Elsewhere `None`, and bodies are built on the CPU: a device whose
+/// shader compiler rejects the shader (D3D12's FXC once did) only loses GPU evaluation.
+fn tessellation(adapter: &wgpu::Adapter, device: &wgpu::Device, queue: &wgpu::Queue, shader: &str) -> Option<solidcraft_gpu::TessGpu> {
+    let compute = adapter.get_downlevel_capabilities().flags.contains(wgpu::DownlevelFlags::COMPUTE_SHADERS)
+        && device.limits().max_storage_buffers_per_shader_stage >= 4;
+    if !compute {
+        return None;
+    }
+    let info = adapter.get_info();
+    match solidcraft_gpu::TessGpu::with_shader(info.name.clone(), device.clone(), queue.clone(), shader) {
+        Ok(t) => Some(t),
+        Err(e) => {
+            log::warn!("bodies are built on the CPU on {} ({:?}): {e}", info.name, info.backend);
+            None
+        }
+    }
+}
+
 /// Create the pipelines. Call once with the app's render state and its depth/stencil/MSAA
 /// settings. Without a stencil the section cap does not hide the inside of cut bodies.
 pub fn install(rs: &egui_wgpu::RenderState, depth_bits: u8, stencil_bits: u8, samples: u32) -> GpuTarget {
-    // Display plans are evaluated where the device runs compute shaders with the storage
+    let tess = tessellation(&rs.adapter, &rs.device, &rs.queue, solidcraft_gpu::SHADER);
-    // buffers the evaluation binds (not WebGL2): elsewhere bodies are built on the CPU.
-    let compute = rs.adapter.get_downlevel_capabilities().flags.contains(wgpu::DownlevelFlags::COMPUTE_SHADERS)
-        && rs.device.limits().max_storage_buffers_per_shader_stage >= 4;
-    let tess = compute.then(|| solidcraft_gpu::TessGpu::new(rs.adapter.get_info().name, rs.device.clone(), rs.queue.clone()));
     let t = GpuTarget { format: rs.target_format, depth: egui_wgpu::depth_format_from_bits(depth_bits, stencil_bits), samples, tess: tess.is_some() };
     let res = Resources::new(&rs.device, t, tess);
     rs.renderer.write().callback_resources.insert(res);
@@ -912,8 +928,29 @@ impl egui_wgpu::CallbackTrait for ViewportCallback {
 
 /// Render `scene` offscreen on a device of its own, through the viewport's real resources and
 /// frame callback (`prepare`, then `paint`), and read the pixels back: straight RGBA, `size` ×
-/// `size`. `None` without a GPU adapter. With `tess`, planned bodies are evaluated on the GPU.
+/// `size`. `None` without a GPU adapter. With `tess` (the tessellation shader), planned bodies
+/// are evaluated on the GPU where the device builds that pipeline, as [`install`] sets it up.
 /// `highlight` is drawn as the viewport's highlight scene (hover and selection).
+#[cfg(all(test, not(target_arch = "wasm32")))]
+fn offscreen_device() -> Option<(wgpu::Adapter, wgpu::Device, wgpu::Queue)> {
+    let instance = wgpu::Instance::new(wgpu::InstanceDescriptor::new_without_display_handle_from_env());
+    let adapter = pollster::block_on(instance.request_adapter(&Default::default())).ok()?;
+    let (device, queue) = pollster::block_on(adapter.request_device(&wgpu::DeviceDescriptor {
+        required_limits: wgpu::Limits::downlevel_defaults().using_resolution(adapter.limits()),
+        ..Default::default()
+    }))
+    .ok()?;
+    Some((adapter, device, queue))
+}
+
+/// Whether [`install`] would evaluate plans on the GPU with `shader`, on an offscreen device
+/// (`None` without a GPU adapter).
+#[cfg(all(test, not(target_arch = "wasm32")))]
+pub(crate) fn offscreen_tessellates(shader: &str) -> Option<bool> {
+    let (adapter, device, queue) = offscreen_device()?;
+    Some(tessellation(&adapter, &device, &queue, shader).is_some())
+}
+
 #[cfg(all(test, not(target_arch = "wasm32")))]
 pub(crate) fn render_offscreen(
     scene: GpuScene,
@@ -922,18 +959,12 @@ pub(crate) fn render_offscreen(
     back: [f32; 3],
     eye: [f32; 4],
     size: u32,
-    tess: bool,
+    tess: Option<&str>,
 ) -> Option<Vec<u8>> {
-    let instance = wgpu::Instance::new(wgpu::InstanceDescriptor::new_without_display_handle());
+    let (adapter, device, queue) = offscreen_device()?;
-    let adapter = pollster::block_on(instance.request_adapter(&Default::default())).ok()?;
-    let (device, queue) = pollster::block_on(adapter.request_device(&wgpu::DeviceDescriptor {
-        required_limits: wgpu::Limits::downlevel_defaults().using_resolution(adapter.limits()),
-        ..Default::default()
-    }))
-    .ok()?;
     let (format, depth) = (wgpu::TextureFormat::Rgba8Unorm, wgpu::TextureFormat::Depth24PlusStencil8);
-    let target = GpuTarget { format, depth: Some(depth), samples: 1, tess };
+    let compute = tess.and_then(|shader| tessellation(&adapter, &device, &queue, shader));
-    let compute = tess.then(|| solidcraft_gpu::TessGpu::new(adapter.get_info().name, device.clone(), queue.clone()));
+    let target = GpuTarget { format, depth: Some(depth), samples: 1, tess: compute.is_some() };
     let mut resources = egui_wgpu::CallbackResources::default();
     resources.insert(Resources::new(&device, target, compute));
     let none = || Arc::new(Mutex::new(None));

crates/ui-egui/src/viewport.rs

@@ -3316,8 +3316,8 @@ mod tests {
             let proj = projection(&app, rect);
             let e = proj.cam.eye().to_f32();
             let eye = [e[0], e[1], e[2], if proj.cam.fov > 0.0 { 1.0 } else { 0.0 }];
-            let planned = scene.planned.len();
+            let (planned, shader) = (scene.planned.len(), tess.then_some(solidcraft_gpu::SHADER));
-            (planned, scene.tris.len(), crate::gpu::render_offscreen(scene, None, proj.vp.to_f32(), proj.cam.back().to_f32(), eye, SIZE, tess))
+            (planned, scene.tris.len(), crate::gpu::render_offscreen(scene, None, proj.vp.to_f32(), proj.cam.back().to_f32(), eye, SIZE, shader))
         };
         let (planned, gpu_tris, gpu) = render(true);
         let (none, cpu_tris, cpu) = render(false);
@@ -3335,6 +3335,45 @@ mod tests {
         assert!(differ * 100 < drawn, "{differ} of {drawn} model pixels differ");
     }
 
+    /// A device that cannot build the tessellation pipeline (D3D12's FXC rejected the shader
+    /// at startup and the app panicked) still shows the model: the failure is caught, logged,
+    /// and bodies are built on the CPU and drawn. A shader the device rejects stands in for FXC.
+    /// Skipped without a GPU adapter.
+    #[cfg(not(target_arch = "wasm32"))]
+    #[test]
+    fn bodies_are_built_on_the_cpu_when_the_tessellation_pipeline_fails() {
+        const REJECTED: &str = "
+            @group(0) @binding(1) var<storage, read_write> surfaces: array<u32>;
+            @compute @workgroup_size(64)
+            fn main(@builtin(global_invocation_id) gid: vec3<u32>) { surfaces[gid.x] = 1u; }
+        ";
+        let Some(tess) = crate::gpu::offscreen_tessellates(REJECTED) else {
+            eprintln!("skipping: no GPU adapter");
+            return;
+        };
+        assert!(!tess, "the rejected shader built a pipeline");
+        let mut s = solidcraft_engine::Session::default();
+        s.run_script(&solidcraft_engine::sample::script()).unwrap();
+        let mut app = SolveApp::new(s, crate::Services::default());
+        app.viewport.no_pixels = true;
+        app.fit_view();
+        app.viewport.gpu = Some(GpuTarget { format: egui_wgpu::wgpu::TextureFormat::Rgba8Unorm, depth: None, samples: 1, tess });
+        const SIZE: u32 = 256;
+        let rect = Rect::from_min_size(Pos2::ZERO, vec2(SIZE as f32, SIZE as f32));
+        let scene = build_scene(&app);
+        assert!(scene.planned.is_empty(), "no body waits for the GPU evaluation");
+        assert!(!scene.tris.is_empty(), "the bodies are meshed on the CPU");
+        let proj = projection(&app, rect);
+        let e = proj.cam.eye().to_f32();
+        let eye = [e[0], e[1], e[2], if proj.cam.fov > 0.0 { 1.0 } else { 0.0 }];
+        let Some(img) = crate::gpu::render_offscreen(scene, None, proj.vp.to_f32(), proj.cam.back().to_f32(), eye, SIZE, Some(REJECTED)) else {
+            return;
+        };
+        let n = (SIZE * SIZE) as usize;
+        let drawn = img.chunks(4).filter(|p| p[..3] != [255, 255, 255]).count();
+        assert!(drawn > n / 20, "the model is drawn: {drawn} pixels");
+    }
+
     /// A hovered face lights solidly over a body drawn from its display plan on the GPU: the
     /// highlight uses the plan's triangles, not the reference mesh's other triangulation of the
     /// same face, which fought the model at equal depth and shimmered in wedges. Every face of a
@@ -3362,7 +3401,8 @@ mod tests {
             let proj = projection(&app, rect);
             let e = proj.cam.eye().to_f32();
             let eye = [e[0], e[1], e[2], if proj.cam.fov > 0.0 { 1.0 } else { 0.0 }];
-            crate::gpu::render_offscreen(scene, Some(highlight), proj.vp.to_f32(), proj.cam.back().to_f32(), eye, SIZE, tess)
+            let shader = tess.then_some(solidcraft_gpu::SHADER);
+            crate::gpu::render_offscreen(scene, Some(highlight), proj.vp.to_f32(), proj.cam.back().to_f32(), eye, SIZE, shader)
         };
         let lit = |plain: &[u8], hovered: &[u8]| -> Vec<bool> { plain.chunks(4).zip(hovered.chunks(4)).map(|(a, b)| a != b).collect() };
         let (Some(gpu_plain), Some(cpu_plain)) = (render(true, None), render(false, None)) else {