# Explicit Windows d4r backend

Base: OptiScaler `45a2001303ddff632e279f77aef85ceede5832cb` (2026-09-30).
The modifications to OptiScaler are GPL-3.0, matching its upstream license.
No NVIDIA DLL is supplied or patched.

`0001` introduces `[DLSS] AllowExternalBackend=true`, default false. Set
`[Libraries] NvngxPath` to the absolute directory containing the Windows d4r
shim as `_nvngx.dll`. Discovery requires its GPU-free ABI-1 marker. Physical
adapter support is queried through the bridge's D3D12 requirements API after
DLL startup; vendor, LUID and HIP architecture are retained.

The configured `D4R_NGX_CORE` absolute path bypasses game-facing NGX DLL
redirection. The external backend loads its own NVAPI provider. An explicitly
selected external DLSS backend reports initialization/load errors instead of
silently switching to FSR. The patch fixes the upstream loader's filesystem
exception by making IsSubpath a lexical check, and retains a PDB for diagnosis.

Build with Microsoft Build Tools 2022, v143 and Windows SDK 10.0.26100:

```powershell
.\scripts\windows\build-optiscaler-windows.ps1
```

The script clones the pinned source if absent, checks/applies the patch,
initializes pinned submodules, invokes MSBuild and writes the DLL, PDB, license
and build metadata into `dist/optiscaler-windows-d4r`. Existing source is not
reset. The upstream packaging post-build event is disabled for this build.
The official bootstrapper is https://aka.ms/vs/17/release/vs_buildtools.exe;
install the `Microsoft.VisualStudio.Workload.VCTools`,
`Microsoft.VisualStudio.Component.VC.Tools.x86.x64` and
`Microsoft.VisualStudio.Component.Windows11SDK.26100` components. The audited
local build uses MSVC 14.44.35207 / Build Tools 17.14.

For standalone verification add
`-OptiScalerDll <absolute-dist-path>/OptiScaler.dll -CaptureExceptions`
to the documented K/M runner. Its private INI selects DLSS, disables frame
generation and keeps RestoreComputeSignature, RestoreGraphicSignature and
ExtendedStateRestore false. Use `-RequireNativeNetwork`; finite output alone
cannot distinguish DLSS from an unwanted FSR replacement.