Shadow Optimization in Unreal Engine 5: A Guide to Archviz

When improving shadows in Unreal Engine 5 architectural scenes, you need to consider not only light quality but also scene performance and the geometry in use. Epic’s official documentation describes Virtual Shadow Maps (VSM) as a high-resolution method designed to work with Nanite. This guide covers the essentials, from project settings and soft-shadow controls to limiting the cost of lights.

1. Choose the shadow method that fits your project

First, determine whether your project uses Nanite. VSM is designed to work with Nanite’s virtualized geometry. Epic notes that VSM may not be suitable if Nanite isn’t used in the project or on the target platform. In projects that do use Nanite, traditional shadow maps aren’t supported alongside Stationary lights; in that case, use Movable lights or VSM.

You can choose the method under the Shadows section of Project Settings › Engine › Rendering. Use the Shadow Map Method setting to select either Virtual Shadow Maps or traditional Shadow Maps. Projects created before Unreal Engine 5.0 may need VSM enabled separately, either through the project setting or with the r.Shadow.Virtual.Enable variable. VSM is enabled by default in new projects.

VSM creates a virtual map at a resolution of 16K × 16K, divided into 128 × 128-pixel pages. It processes the pages needed for visible pixels and caches them between frames unless they’re invalidated. This approach focuses on visible areas rather than rendering the entire map again every frame.

2. Control soft shadows through the light source

Shadow Map Ray Tracing (SMRT), used with VSM, creates light-source-dependent softening and contact hardening along shadow edges. Source Radius affects Point and Spot lights, while Source Angle affects Directional lights. Increasing the source size softens shadows, but excessive values can affect performance and cause visual artifacts when the camera gets close to a shadow edge.

Epic’s documentation states that local lights have no Source Radius value set by default, while Directional lights have a low default value for Source Angle. The documentation also includes an example Point Light with a Source Radius of 10; this should be treated as an example setting, not a recommendation for every scene.

You can also adjust SMRT quality by changing the number of rays and samples per ray. At Epic’s Quality shadow scalability level, local and directional lights use eight rays by default. Reducing the ray count can make noise more visible; setting it to zero disables SMRT and switches to a hard shadow with a single sample. The documentation gives 4 to 8 as a suitable range for samples per ray.

r.Shadow.Virtual.SMRT.RayCountLocal
r.Shadow.Virtual.SMRT.RayCountDirectional
r.Shadow.Virtual.SMRT.SamplesPerRayLocal
r.Shadow.Virtual.SMRT.SamplesPerRayDirectional

3. Limit local lights’ influence and shadows

For Point, Spot, and Rect lights, Attenuation Radius limits the area affected by the light. Epic notes that this area isn’t intended to be physically accurate; setting limits is important for performance when lights cast dynamic shadows. A smaller radius can also reduce the number of objects affected by the light that need to be shadowed.

Each light’s Cast Shadow property toggles its contribution to shadows. Turning it off can help performance when the shadow doesn’t meaningfully contribute to the scene, the light covers a large area, or it affects highly detailed geometry. After making changes, check how they affect ground contact in the frame and how clearly objects read in the space.

VSM’s high resolution can reduce the need for Contact Shadows to create sharp contact shadows. Epic notes that this feature may be less accurate than shadows produced by VSM and isn’t recommended for general use. It may be worth considering for specific needs, such as getting lower-cost shadows from objects that don’t participate in suitable shadow maps.

4. A checklist for architectural and interior workflows

In close-up interior renders, soft shadows can be key to making furniture sit on the floor and defining transitions between surfaces. In exteriors and large scenes, Directional lights’ clipmap structure provides more detail in areas closer to the camera. Epic recommends using Directional lights where possible, since local lights illuminating very large areas can hit resolution limits.

Shadows from Static or Stationary lights in the editor are previews until lighting has finished building; they don’t represent the final result. You can build lighting with Build › Build All Levels. To hide the preview indicator, go to Show › Visualize › Preview Shadows Indicator in the Level Viewport.

Common mistakes

• Assuming VSM is automatically the best option in a project that doesn’t use Nanite.

• Increasing Source Radius or Source Angle too much, which can negatively affect performance and the appearance of shadows.

• Reducing the ray count without checking for noise in the shadows.

• Not limiting the Attenuation Radius and shadow contribution of lights to suit the needs of the scene.

• Mistaking preview shadows from Stationary or Static lights for the final, built result.

There’s no single value that works for these settings: scene geometry, light type, camera distance, and the intended image all need to be considered together. Start by choosing the right shadow method, then adjust the light source size and area of influence, checking the visual result as you go.