Sky Light

The Sky Light captures the distant parts of your level and applies that to the scene as a light. That means the sky's appearance and its lighting/reflections will match, even if your sky is coming from atmosphere, or layered clouds on top of a skybox, or distant mountains. You can also manually specify a cubemap to use.

Scene Capture

The Sky Light will only capture the scene under certain circumstances:

• For Static Sky Lights, updates automatically happen when building lighting.

• For Stationary or Movable Sky Lights, updates happen once on load and only updates further when Recapture is called unless the Real Time Capture feature is enabled. Both are accessible via the Details panel, and Recapture can also be called via Blueprint in-game.

  

  Details panel Sky Light Recapture button

If you change the texture that the skybox is using, it will not automatically know to update. You'll be required to use one of the methods above for it to update.

A Sky Light should be used instead of the Ambient Cubemap to represent the sky's light because Sky Lights support local shadowing, which prevents indoor areas from getting lit by the sky.

Mobility

Similarly to other Light Types, the Sky Light can be set to one of the following Mobilities:

Static Sky Light

A Sky Light set to Static will be baked completely into the lightmap for static objects in the level and therefore costs nothing. When edits are made to the light's properties, the changes will not be visible until lighting has been rebuilt for the level.

When using a Static Sky Light, only Actors and Lights in the level that have their mobility set to Static or Stationary will be captured and used for lighting. Furthermore, only the emissive contribution of Materials can be captured with a Static Sky Light in order to avoid a feedback loop. For this reason, make sure any skybox has a Material that is set to Unlit.

Stationary Skylight

A Sky Light with Stationary Mobility gets baked shadowing from Lightmass. Once you place a Stationary Sky Light in your level, you have to rebuild lighting once to get the baked shadowing. You can then change the sky light as much as you want without rebuilding. The Sky Light shadowing that Lightmass bakes stores directional occlusion information in what is called a Bent Normal.

Only components and lights with Static or Stationary Mobility will be captured and used for lighting with a Stationary Sky Light.

Like all types of Stationary Lights, the color of the direct lighting can be changed at runtime through Blueprint or Sequencer. However the indirect lighting is baked into the lightmap and cannot be modified at runtime. The amount of indirect lighting can be controlled by IndirectLightingIntensity.

Movable Sky Light

A Sky Light set to Movable does not use any form of precomputation. It captures components and lights of any mobility when setup to capture the scene.

Distance Field Ambient Occlusion

This property requires that Mesh Distance Fields be enabled in the project settings.

Shadowing for a Movable Sky Light is provided by Distance Field Ambient Occlusion from Signed Distance Field Volumes that are generated around each rigid object. Distance Field Ambient Occlusion supports dynamic scene changes where the rigid meshes can be moved or hidden, and it will affect the occlusion.

Click image for full size.

Check out the How to use Distance Field Ambient Occlusion page for more information and examples.

Precomputed (Static or Stationary) Sky Light

When using baked lighting with a Static or Stationary Sky Light, the lighting and shadowing data will be stored in lightmaps. The sections below discuss some of these features supported by Lightmass.

Improved Static Sky Light Directionality

Prior to Unreal Engine 4.18, Static Sky Lights used to be represented by Lightmass with a 3rd Order Spherical Harmonic which didn't capture the detail that could be present in a sunrise or sunset. A filtered cubemap is used now which results in a higher resolution by default. Lightmass will also select the appropriate MIP of the cubemap texture
based on the size of the Final Gather rays to avoid any aliasing.

To see an example of this type of interaction, go to Engine Content > MapTemplates > Sky and select SunsetAmbientCubemap as it will show a great example.

The biggest difference can be seen in heavily occluded scenes with a Sky Light cubemap that has a lot of brightness and color variations.

Camera Obscura

With the improved directionality of Static Sky Lights, it is now possible to recreate a pinhole camera effect (Camera Obscura) with a small enough opening. The smaller the opening, the more directional sky lighting will become.

Bent Normal for Stationary Sky Lights

For Stationary Sky Lights, static and stationary lighting is baked separately from movable using Lightmass to store directional information in what is called a Bent Normal. This is the direction from the texel to the most un-occluded source of ambient light. Areas that are mostly occluded use this direction for sky lighting instead of the surface normal, which improves quality in cracks.

The first image shows Sky Light with Ambient Occlusion only. The second image shows Sky Light with Bent Normal occlusion. Notice how surfaces in cracks 'agree' on where the lighting is coming from.

For additional information visit the Bent Normal Map page.

Multiple Light Bounces

Multiple bounces of indirect light from Sky Lights is supported by adjusting the Num Sky Lighting Bounces setting in World Settings > Lightmass.

Setting for the number of sky light bounces to use.

For multiple Sky Light bounces to be noticeable, your Materials need to have a high diffuse value in most cases. For example, with a diffuse value of .18 (18% grey), the second sky light bounce will only contribute 18^3 = 0.006 of the light's emitted energy, which is very subtle. With a diffuse value of 0.5, the second skylight bounce will contribute 0.125 of the light's emitted energy and be noticeable.

Each added bounce increases non-distributed build times, so use with care when not using Swarm distribution.

Real Time Capture

The Real Time Capture mode provides dynamic and specular environment lighting, making possible dynamic time-of-day simulations with real-time reflections on scene elements.

This mode is available when the Sky Light’s mobility is set to Stationary and Dynamic and when Real Time Capture is enabled from the Sky Light component Details panel.

This mode supports the following components for captured reflections and shadowing:

• Sky Atmosphere
• Volumetric Clouds
• Height Fog
• Sky Dome Meshes with an unlit material tagged as IsSky

These are some additional things to keep in mind when using this mode:

• Volumetric Fog is not supported.
• It’s possible to render meshes only in real time capture mode by disabling them from rendering in the main view.
• It’s possible to have a different sky dome mesh for the main view as compared to the reflection of the real time capture when enabling Visible in Real Time Sky Captures on individual actors.
• If you want to render your sky dome using multiple meshes with IsSky=True in your Material, you should also set the command r.SkyLight.RealTimeReflectionCapture.DepthBuffer 1 in order to have correct depth ordering and intersection.
• Real Time Capture mode allows for ambient shadowing from Volumetric Clouds to be cast onto the world when Cloud Ambient Occlusion is enabled.
• Real Time Capture mode is faster and more efficient than the RecaptureSky Blueprint node because all the computations stay on the GPU without any use of CPU processing that can cause pauses, or slowdowns in gameplay.
• If you are using the volumetric clouds, performance can vary a lot depending on the complexity of the cloud material you’re using. You can control the performance tradeoff—while respecting visuals in your scene—by adjusting the Volumetric Cloud component’s Reflection Sample Count Scale and Shadow Reflection Sample Count Scale values.

Reflection Quality

The quality of reflections captured by the Sky Light is set by the property Cubemap Resolution. It uses power of two values (such as, 64, 128, 256, and so on) to specify a texture resolution for the captured scene.

Keep in mind that higher resolution values increase quality at the cost of performance. The specified value is applied to the highest cubemap mip (a texture’s level of detail).

Drag the slider to see decreased (64), default (128), and increased (256) skylight cubemap resolutions being used.

Note that Screen Space Reflections (SSR) has been disabled in these shots. Reflections are only the result of the Sky Light.

Time Slicing Optimization

Real Time Capture mode utilizes a performance optimization called Time Slicing to distribute the captured scene over several frames rather than performing a full scene process every frame. This optimization reduces the per frame cost that would make a full process real time capture every frame too expensive for use on some platforms.

For example, a capture performed on the PlayStation 4 using a 128x128x6 HDR cubemap with sky and volumetric clouds would look something like (measured in milliseconds):

• A Full process per frame for a 128x128 cubemap would cost around 1.465ms (divided up as the following):
  • Sky and Cloud costs 0.60 ms
  • Cubemap Mip Generation costs 0.05 ms
  • Specular Convolution costs 0.80 ms
  • Diffuse Irradiance costs 0.015 ms
• The same process but time sliced over 9-Frames Time Slice would cost, at most, 0.20 ms for the most expensive step.

Distributing a single frame’s capture over multiple frames has the benefit of keeping real time skylight capture within an affordable budget for your project, because time slicing is only as expensive as its most costly frame. It also means you can look for opportunities to push quality in other areas, such as the quality of the sky or clouds, because you have a little bit of room in the frame budget.

Time slicing is enabled by default, but can be set using the console command r.SkyLight.RealTimeReflectionCapture.TimeSlice.

Shadowing from Movable Objects

Soft shadowing from movable objects onto the environment can be achieved using Capsule Shadows for Skeletal Meshes or Distance Field Indirect Shadowing for rigid meshes. These require a separate setup to work properly.

Properties

There are two categories of Sky Light component properties, Sky Light and Light, although they are not differentiated in the UI. These can be accessed from the Actor Details panel. Sky Light properties are specific to Sky Light Actors, while Light properties are common to all Light Actors. Certain properties are only accesible by expanding the Advanced section, indicated in the table below.

Limitations

These features would be useful with SkyLights but are not yet implemented:

• Soft shadowing from movable objects (characters) onto the environment is not integrated with Sky Light and currently requires implementing separate features at additional rendering cost.
• The SkyLightComponent Blueprint function RecaptureSky() can be used, however it has a high processing cost and requires at least 120 ms, leading to a pause or slowdown in gameplay.