Feature: frame generation for games with motion vectors (MTL4FXFrameInterpolator) #1509

Request

An opt-in frame-generation pass in the present path, built on macOS 26 / iOS 26's MTL4FXFrameInterpolator, scoped to games that hand the host usable motion vectors — in practice, the NVN2 DLSS path.

Why this is feasible here

The host already encodes Metal 4 MetalFX work in the right places: MTL4FXTemporalScaler for NVN2's DLSS and MTL4FXSpatialScaler for DM_UPSCALER=metalfx / DM_PRESENT_UPSCALE. The interpolator's inputs line up with what the DLSS eval already collects:

• colorTexture / prevColorTexture: the present thread sees both frames
• depthTexture / motionTexture: the DLSS path already takes the game's depth and motion textures plus jitter (the NGX feature flags give the MV layout)
• deltaTime: the present/vsync cadence
• uiTexture: optional; games that composite UI into the presented frame can leave it out and accept HUD artifacts, or the UI composite pass could feed it where one is identifiable
• shouldResetHistory: scene-cut / first-frame heuristics the present already tracks

For games without a motion source (NDSR-upscaler games like Bananza, Ocarina of Time), MV-less interpolation is the mushy case, so I'd scope this to interpolate only when the frame carried usable motion data — an env switch / launcher toggle like DM_PRESENT_UPSCALE, default off.

Use case

30 fps Switch 2 titles presenting at 60, or 60 → 120 on a 120 Hz screen. Caveats are the usual: one frame of added latency, interpolation artifacts, no benefit where MVs are absent.

Environment to test on

Apple silicon Mac, Xcode 27.2 / Metal 32023.922, building Switch 2 games from NUPs through the mac-app path.