Frequently Asked Questions

Why 4.27 and not UE5?

Because for a large class of projects UE5's headline features cost more than they return. Nanite, Lumen and Chaos each carry a base cost whether or not you exploit them, and UE5's core class and tick overhead is higher. Vite takes 4.27's lower baseline and adds the rendering technology that was actually worth having.

The detailed argument, with numbers, is in Why NvRTX 4.27 and UE4 versus UE5 Cost Analysis.

Is Vite a drop-in replacement for stock 4.27?

Close, but not exactly. Vite changes a number of engine defaults for performance — overlap event handling, scalability settings, tick behaviour, ray tracing culling and others. A project moving from stock 4.27 will mostly work, but should read Engine Defaults to understand what changed.

Can I use UE5 marketplace assets?

Not directly. The Asset Downgrader plugin converts UE5 assets to 4.27, stripping data that has no 4.27 equivalent such as Nanite geometry. Features that do not exist in 4.27 cannot be ported, only removed. See Migrating from UE5.

What platforms does Vite support?

Windows 64-bit, primarily on DirectX 12. Other platforms inherit stock 4.27 support but are neither tested nor tuned. See Platforms.

What toolchain do I need?

Vite is currently developed against Visual Studio 2026 with MSVC 14.50 and Windows SDK 10.0.26100.

The build fails immediately on a fresh clone

Most often missing .NET Framework 4.5 reference assemblies, which recent Visual Studio installers no longer include by default. See Build Troubleshooting.

Do I need to build from source?

Only if you need to modify engine C++ or change a compile-time switch. For everything else an installed build is faster to get running and needs far less disk.

How long does a build take?

A source build is hours on typical hardware, and an installed build is longer. Shader compilation dominates the first launch afterwards. See Shader Compilation and PSO.

The engine takes too much disk space

Run the debloat suite in devops\. It moves platform binaries, templates, samples and optionally plugins out of the tree, with a dry-run mode and a recovery folder rather than deletion. See Debloat Guide.

I set a ray tracing console variable and nothing happened

This is the single most common question, and the answer is almost always VITE_RT_PSO_DEBLOAT. It defaults to 1, which compiles out the shader permutations for:

• RTXDI (r.RayTracing.SampledDirectLighting)

• Path tracing

• Ray-traced translucency and both caustics systems

• Per-pixel ray-traced global illumination

• The non-deferred reflection path, and with it reflected translucency and RT reflection captures

The console variables still exist and still set. Nothing renders. Rebuild the engine with VITE_RT_PSO_DEBLOAT=0 to use them. See Compile-Time Switches and Ray Tracing.

Which ray tracing features work out of the box?

Reflections, shadows, ambient occlusion and sky light. Those four are available in a default build; the rest are compiled out. Full table in Ray Tracing.

Which GI should I use?

For most projects, DDGI plus SSGI. DDGI provides the low-frequency indirect bounce and SSGI adds contact-scale detail its probe grid cannot resolve. Static DDGI is cheaper and correct if nothing moves. Per-pixel ray-traced GI is reference-only and compiled out by default. See Global Illumination.

Why SMAA instead of TAA?

TAA's ghosting, smearing and motion softness are structural, not tuning problems. SMAA is spatial and does not have them. It is Vite's recommended default and is enabled with r.Vite.SMAA.Mode. See Anti-Aliasing.

Which upscaler should I use?

DLSS if you are on NVIDIA hardware and shipping to NVIDIA users. FSR or XeSS for cross-vendor coverage. NIS as a cheap fallback with no temporal component. Most projects should offer more than one. See Upscalers.

Volumetric fog looks wrong with upscaling enabled

That is what VITE_DLSS_PATCH fixes. It adds r.VolumetricFog.UseUpScaledSizeVolumetricFog, which renders fog at output rather than internal resolution, along with translucency fixes. It defaults to 0, so it requires a rebuild. See Upscalers.

Does Vite have Nanite or Lumen?

No, and it will not. Those are UE5 systems built on UE5 assumptions. Vite's answers are hardware tessellation and LODs for geometry, and DDGI plus SSGI for global illumination.

Why PhysX instead of Chaos?

PhysX is faster and more predictable on the workloads Vite targets, and it keeps Apex Destruction and Apex Cloth — both removed in UE5 — available. See PhysX.

How do I get deterministic physics?

Rebuild with VITE_PHYSX_FIXED_TIMESTEP=1, then enable p.VitePhysXFixedTimestep.Enabled. Without the compile-time switch the console variables do nothing. See Fixed Timestep.

I need thousands of physics objects

Use the PhysX Instanced Subsystem. Conventional actor-per-body physics hits actor overhead, not solver limits, at a few thousand bodies. The subsystem removes that overhead by driving instanced mesh transforms directly.

Where do I start optimising?

stat unit, and identify which of Game, Draw or GPU is the largest. Optimising the wrong one produces no frame time improvement at all. See Profiling.

My game thread is the bottleneck

The usual suspects, in rough order of frequency: Character Movement Components, animation evaluation, Blueprint tick logic, and actor tick overhead generally. See 400 Characters CMC Bench and Performance Targets.

Shader compilation is taking forever

Check what your permutation count actually is before assuming it is unavoidable. Every shading model in use, every enabled plugin and every quality level multiplies. VITE_RT_PSO_DEBLOAT=1 already removes a large share of the ray tracing permutations, which is why it is the default. See Shader Compilation and PSO.

Shaders behave as though my change did not happen

Stale cache. Try recompileshaders changed or recompileshaders global first — both are far cheaper than a full wipe. If neither helps, run WipeShaderCache.bat. See Cache Management.

A feature is missing from the editor

Check whether its plugin is enabled. Most bundled plugins default to off. Only RTXGI, NRD and the custom splash screen are on by default. See Bundled Plugins.

Can I get plugin X bundled?

If it is 4.21–4.27 compatible, permissively licensed and worth its size and compile cost, propose it. UE5-only plugins are out of scope. See Proposed Plugins.

The debloat script removed a plugin I need

ExcludedPlugins.txt is aggressive and includes plugins many projects use. If you ran in move mode, the files are in ViteDebloat_Moved next to the engine folder. If you ran in delete mode, they are gone and you need to restore from source control. See Debloat Guide.

What gets a pull request rejected fastest?

Modifying an ABI: ray tracing payload bitfields, shader-visible enums, packed RHI or RenderCore bitmasks, reflection bitmask definitions, or any CPU/GPU shared struct layout. These are rejected regardless of the rest of the change. See Coding Guidelines.

Can I use recursion, virtuals or templates?

Recursion is banned. New virtuals need strict justification. Templates are discouraged unless they earn their compile-time and binary-size cost. New Blueprint-exposed functions need explicit approval.

How do I document my change?

In the same pull request. New console variables go on the relevant feature page, new VITE_* switches go in Compile-Time Switches, and changed defaults go in Engine Defaults. See Documentation Contributions.