All of the indirect lighting and shadowing from Stationary Lights is stored within the Lightmap. Direct shadows are stored within the Shadowmap. These lights make use of Distance Field Shadows, meaning that their shadows can remain crisp even with fairly low Lightmap Resolution on lit objects.
Direct Lighting
The direct lighting of stationary lights is rendered dynamically using deferred shading. This allows the brightness and color to be changeable at runtime, along with a light function or IES profile. The light will have high quality analytical specular just like a movable light. The direct lighting can be shown or hidden in game by modifying the Visible property of the light.
Direct lighting from a stationary light.
Direct Shadowing
Realtime shadowing of light sources has a major performance cost. A fully dynamic light with shadows will often cost twenty times (20x) as much to render than a dynamic light without shadows. For this reason, stationary lights have the ability to have static shadowing on static objects, but with some limitations.
Static Shadowing
On Opaque
Lightmass generates distance field shadow maps for stationary lights on static objects during the lighting rebuild. Distance field shadow maps provide very accurate shadow transitions even at low resolutions, and with very little runtime cost. Like lightmaps, distance field shadow maps require uniquely unwrapped UVs on all StaticMeshes using static lighting.
Accurate shadows of a StaticMeshActor onto opaque surfaces using distance field shadows.
Lighting must be built to display distance field shadows, otherwise whole scene dynamic shadows will be used for previewing.
Only 4 or fewer overlapping stationary lights can have static shadowing, because the lights must be assigned to different channels of a shadowmap texture. This is a graph coloring problem, so there are often fewer than 4 overlapping allowed due to topology. Shadowing cannot affect the overlap test, so the sunlight typically requires a channel from the entire level it is in, even the underground areas. Once the channel limit is reached, additional stationary lights will use whole scene dynamic shadows at a severe performance cost. The StationaryLightOverlap view mode can be used to visualize the overlap, which is updated dynamically as you modify the lights. Light icons are changed to a red X when they are not able to allocate a channel.
StationaryLightOverlap viewmode showing one too many overlapping lights. Notice the three lights behind the Spot Light, the radius of left and the center light can be seen overlapping the Spot Light's radius while the right one does not.
On Translucency
Translucency also receives shadowing very cheaply with Stationary lights - Lightmass precomputes a shadow depth map from static geometry which is applied to translucency at runtime. This form of shadowing is fairly coarse and only captures shadowing on the scale of meters.
The resolution of the static shadow depth map is controlled by StaticShadowDepthMapTransitionSampleDistanceX and StaticShadowDepthMapTransitionSampleDistanceY in BaseLightmass.ini, with a default setting of 100 meaning one texel every meter.
Dynamic Shadowing
Dynamic objects (like StaticMeshComponents and SkeletalMeshComponents with Mobility set to Movable) must integrate into the static shadowing of the world from distance field shadowmaps. This is accomplished with Per Object shadows. Each movable object creates two dynamic shadows from a stationary light: a shadow to handle the static world casting onto the object, and a shadow to handle the object casting onto the world. With this setup, the only shadowing cost for stationary lights comes from dynamic objects that it affects. This means the cost can vary from very little to a large amount, depending on how many dynamic objects there are. With enough dynamic objects, it is more efficient to use a Movable light instead.
In the scene below, the spheres are all movable, and they all receive shadows from the static world and cast their own shadows, which merge with the distance field shadows. The Per Object shadow frustums for each Movable component are also shown.
Per Object shadows used by movable components apply a shadowmap to the object's bounds, therefore the bounds must be accurate. For Skeletal meshes this means they should have a physics asset. For particle systems, any fixed bounding box must be large enough to contain all particles.
Directional light dynamic shadowing
Directional Stationary Lights are special because they support whole scene shadows through Cascaded Shadow Maps at the same time as static shadowing. This is very useful in levels with a lot of animating foliage; you want to have moving shadows around the player but do not want to pay the cost of having many cascades to cover a large view range. The dynamic shadows are faded into static shadows over distance, such that the transition is often indistinguishable. To set this up, just change the Dynamic Shadow Distance StationaryLight of a DirectionalLightStationary to be the range at which you want the fade to happen.
Movable components will still create PerObject shadows even when using Cascaded Shadow Maps on a directional light. This behavior is useful with small Dynamic Shadow Distances, but incurs unnecessary cost with larger distances. To disable PerObject shadows and save performance, disable Use Inset Shadows For Movable Objects on the light.
Indirect Lighting
Stationary lights store their indirect lighting in the lightmap just like a static light. Indirect lighting cannot be modified at runtime by changing brightness and color like direct lighting can. This means that even when a light has Visible unchecked, its indirect lighting will be put in the lightmap during the lighting build. IndirectLightingIntensity on the light can be used to scale or disable the indirect lighting from a given light at lighting build time.
However there's a post process volume setting called IndirectLightingIntensity which lets you scale the contribution of the lightmap for all lights, which can be changed at runtime from a Blueprint.
Use Area Shadows for Stationary Lights
In the 4.9, or later, release of Unreal Engine, Stationary Directional Lights have a new shadowing option located in the Lightmass options called Use Area Shadows for Stationary Lights.
To Enable the Use Area Shadows for Stationary Lights option, first select the Directional Light in your scene and make sure that it's Mobility is set to Stationary. Then in the Lightmass section of the Directional light, click on the box next the Use Area Shadows for Stationary Lights option to enable it. When the Use Area Shadows for Stationary Lights option is enabled, the Stationary Light will use area shadows for the precomputed shadow maps. Area shadows are shadows that get softer the further they are from shadow casters. In the following image you see the difference between the two shadowing methods.
Note that Area Shadows will only work on Stationary Lights and you might have to increase some objects lightmap resolution to get the same shadow quality and sharpness.