DirectX DXBC (vkd3d) Path

The DirectX backend is the part of ShadowDusk that most often gets misremembered, so the key fact first:

DirectX uses vkd3d-shader → DXBC, not DXC → DXBC.

Why DXC is wrong for DX11

DXC (the DirectX Shader Compiler, used on the OpenGL path to emit SPIR-V) only emits DXIL — shader model 6. MonoGame 3.8's DX11 runtime loads DXBC — shader model ≤ 5. So DXC's output simply will not load as a DX11 effect. DXC's ps_6_0/vs_6_0 (DXIL) output is retained only for the Vulkan / DX12-KNI SM6 profile, never for DX11.

The DXBC backends

DirectX compilation routes HLSL → DXBC through a backend behind the IDxbcShaderCompiler seam, selected by DxbcBackend:

DxbcBackend	Backend	Platform	Role
Vkd3d (default)	vkd3d-shader (the Wine project)	Linux / macOS / Windows	The shipping cross-platform backend — what makes the DX path compilable where mgfxc can't run; host-independent, so default output is byte-identical on every OS.
D3DCompiler	d3dcompiler_47	Windows-only	Correctness oracle — the reference output (opt-in).

var options = new CompilerOptions
{
    Target = PlatformTarget.DirectX,
    DxbcBackend = DxbcBackend.D3DCompiler,   // opt in to the Windows-only oracle
};

The vkd3d-shader natives for all four desktop RIDs ship inside the NuGet package (runtimes/<rid>/native, SHA-256-pinned builds hosted on the native-vkd3d-1.17 release) — consumers install nothing; the repo's restore script is only for building ShadowDusk from source. The opt-in d3dcompiler_47 oracle needs no restore but runs only on Windows.

Validation

Both backends are validated end-to-end: all 10/10 of the SM5 PS-only corpus shaders' DX .mgfx load in real MonoGame.Framework.WindowsDX and render pixel-equivalent to mgfxc — via both the d3dcompiler_47 oracle and the cross-platform vkd3d-shader backend. See Validation & Evidence Ladder.

Reflection from the same bytes

The emitted DXBC is also the reflection source, parsed by the pure-managed RdefReader (proven deeply equal to the Windows D3DReflect oracle for both backends' output), and a DxbcReflectionExtractor reflects both backends identically — so the MGFX writer gets the same parameter/cbuffer/sampler metadata regardless of which backend produced the bytes, on every OS.

In the browser

DirectX DXBC also compiles in the browser: the same pinned vkd3d-shader is compiled to WebAssembly and ships inside the ShadowDusk.Wasm package, producing .mgfx byte-identical to the desktop output. It is an export target — a browser cannot render DXBC — see DirectX & FNA in the Browser.