Reflexium

NVIDIA Reflex, DLSS, AMD FSR 3.1 and frame generation for Minecraft Java's Vulkan renderer. Client-side Fabric mod for Minecraft 26.3.

What it does

• Reflex low latency through VK_NV_low_latency2: the driver sleeps the game just before input is read, so each frame starts as late as the GPU allows. Modes: Off, On, On + Boost.
• Frame pacing: while Reflex is on, Minecraft's Max Framerate (including the 60 FPS menu cap) is enforced by the Reflex sleep instead of vanilla's post-present sleep. No Reflex cap is inferred from the refresh rate; frame generation keeps its own output budget.
• Latency readout in F3: input-to-present time plus simulation, render submit and GPU time, straight from the driver. Works with Reflex off too, for A/B comparisons.
• DLSS and FSR 3.1 upscaling: renders the world below display resolution, reconstructs camera motion from depth, and applies NVIDIA DLSS or AMD FSR 3.1 before drawing the full-resolution HUD. Presets include DLAA (Native AA for FSR), Quality, Balanced, Performance, and Ultra Performance. See setup, limitations and validation.
• Frame generation (experimental): standard DLSS FG (2x), DLSS Multi Frame Generation (runtime-limited, up to 6x on the tested RTX 5070), and FSR FG (2x). Select Auto or the matching upscaler; turning upscaling off disables FG. Minecraft's prepared GUI is drawn onto every output frame, preserving sharp HUD text and background-dependent crosshair blending. See implementation and validation.

Requirements

• Minecraft 26.3, Fabric Loader 0.19.5+, Java 25
• Windows x64 for the bundled native runtimes. Reflex requires an NVIDIA driver exposing VK_NV_low_latency2; DLSS requires an RTX GPU. FSR also supports AMD and Intel Vulkan GPUs, which were not part of this acceptance run.
• Graphics API set to Vulkan (Options, Video Settings, Graphics API). On OpenGL the mod does nothing.
• Optional: Mod Menu for the settings screen

Fabric API is not required.

Upscaling

• DLSS: requires an NVIDIA RTX GPU. A missing nvngx_dlss.dll is downloaded from NVIDIA, hash-checked and enabled in the same session. Existing DLLs are preserved. The native NGX bridge is bundled.
• FSR 3.1: works on any Vulkan GPU (AMD, Intel, NVIDIA).
• Auto renders natively without a shader pack. With one, it picks DLSS when available, otherwise FSR 3.1, and returns to native when a measured comparison shows DLSS is not faster.
• Vitrail 0.12.x shader upscaling is supported, with Complementary Reimagined r5.9.3 tested. Reflexium owns render scale and jitter while active.
• Opaque moving entities and animated block entities receive per-vertex motion vectors. Shader-only deformation and translucent objects still have limitations; see validation.

Upscaling has a fixed reconstruction cost. Without shaders, native rendering can be faster even at lower upscaling qualities; DLAA also pays the full-resolution network cost. DLSS uses transient motion uniforms and retained scene depth; both upscalers skip depth reconstruction for an invisible outline. See per-pass timings, paired results and remaining limits. DLSS output gets the same RCAS sharpening as FSR, at the fsr_sharpness strength (0 disables it).

Settings

Settings appear in:

• Video Settings, below Graphics API, when the driver supports Reflex (vanilla screen).
• Sodium's Video Settings as a Reflexium page when Sodium is installed. Reese's Sodium Options and other Sodium screen replacements show the same page. Tested with Sodium 0.9.2 and Reese's Sodium Options 2.2.5 (which requires exactly Sodium 0.9.2).
• Mod Menu, and config/reflexium.properties (restart after editing).

Key	Values	Default
reflex_mode	off, on, on_boost	on
upscaler	off, auto, dlss, fsr	auto
upscale_quality	native_aa, quality, balanced, performance, ultra_performance	quality
dlss_model	auto, transformer, cnn	cnn
fsr_sharpness	0 to 100; also the DLSS sharpening strength	20
post_upscale_particles	true, false: draw vanilla particles at display resolution after upscaling	true
frame_generation	off, dlss, dlss_multi, fsr	dlss (only runs on supported GPUs with VSync off)
frame_generation_multiplier	0 (Auto) or 2 to 8, capped by runtime support	0
fg_force_multiplier	true, false: keep a fixed multiplier even when it exceeds the refresh budget	false
pack_shadow_resolution_cap	0 (pack value) to 8192	0
pack_shadow_distance_cap	0 (pack value) to 1024	0
betterf3_module_placed	true, false (set after Reflexium adds its BetterF3 module once)	false

Settings from the old nvidia-reflex-mod.properties are migrated automatically.

With BetterF3, which hides other mods' F3 lines, Reflexium adds a Reflexium module to the right column once. Move, recolor or remove it in BetterF3's module settings.

For bug reports, launch with -Dreflexium.logLatency=true to log the Reflex status and latency every 5 seconds.

Shaders

• Vitrail Shaders: verified with Vitrail 0.12.0-beta+mc26.3, Sodium 0.9.2+mc26.3 and Complementary Reimagined r5.9.3 on Windows/NVIDIA. Reflex sleep, frame limiting and driver latency readings work through the shared Vulkan backend. Install Vitrail's Minecraft 26.3 jar and its Sodium/Fabric API dependencies. See the compatibility check for evidence and repeat steps.
• DLSS and FSR run after Vitrail's final world pass and before the full-resolution GUI. Known Complementary packs have their own TAA disabled in memory while upscaling. Unsupported Vitrail versions pause shader upscaling.
• Aperture (the Iris team's Vulkan shader loader): shared Vulkan hooks are the intended integration point; runtime compatibility and DLSS integration remain unverified.
• Iris: the experimental Iris Vulkan build 1.12.0-snapshot+mc26.3-local is supported, including Distant Horizons (see Distant Horizons and shader loaders). Other Iris versions pause shader upscaling; stable OpenGL Iris never reaches the Vulkan path.

How it works

Hook	Purpose
VulkanBackend.createDevice	adds VK_NV_low_latency2, VK_KHR_present_id (+ presentId feature) and the DLSS NVX extensions when the GPU reports them
VulkanGpuSurface.configure	creates the swapchain with VkSwapchainLatencyCreateInfoNV and applies the sleep mode
VulkanQueue.Submission.close	tags every vkQueueSubmit2 with VkLatencySubmissionPresentIdNV
VulkanGpuSurface.present	tags the present with VkPresentIdKHR
Minecraft.run	Reflex sleep, simulation-start and input-sample markers before SDL event polling
Minecraft.runTick / renderFrame	simulation, render submit and present markers; skips vanilla's limiter when Reflex enforces the cap

Project layout

Client code lives in src/client/java/com/bambooz/reflexium:

Package	Contents
reflex	Reflex sleep, latency markers, frame cap and latency readout
upscale	Super resolution frame flow (UpscalingRenderer), jitter, camera history, mip bias, post-upscale passes
upscale.dlss, upscale.fsr	NGX and FSR native bridges, DLSS runtime download, FSR presentation scheduler
framegen	Frame generation renderer, present metering and the refresh-rate output policy
motion	Camera, entity, hand, cloud and shader pack motion vectors
vulkan	Device extensions, swapchain and queue hooks, native image handles
compat	Iris, Vitrail, Complementary, BSL, Distant Horizons, Sodium, Mod Menu and BetterF3
config, gui	config/reflexium.properties and the settings screens
debug, debug.server	F3 lines, profiler, measurement counters, and the dev-only HTTP harness (debugging)
mixin	Minecraft, RenderPearl and optional-mod hooks

Native bridge sources are in native/, assertion checks in src/test/java, dev scripts in tools/ (index) and documentation in docs/ (index).

Building

./gradlew build

The jar is written to build/libs/Reflexium-<version>-fabric-<minecraft>.jar and copied into the Modrinth profile 26.3 Testing Instance when it exists. One release line per Minecraft version.

The build runs every assertion check before installing: resource JSON, native call signatures, camera reprojection, jitter phases, object-motion history, frame delivery, Reflex timing and the shader pack integrations. The list is in build.gradle (assertionChecks). To rebuild the bundled native bridge, use native\build.cmd with Visual Studio 2022 C++ Build Tools and the Windows SDK installed. NGX_SDK can point to an existing NVIDIA DLSS SDK checkout. See native build notes.

License

Reflexium source: MIT. The native bridge incorporates NVIDIA NGX under the NVIDIA RTX SDK license, included in META-INF/licenses/NVIDIA-DLSS.txt.