Issue with skylight and static Lighting

Hi there,

I recently started working on a Landscape scene that uses static Lighting along with a SkyLight and noticed a strange behaviour in a combination of static point Lights and a sky light.

In the following i recreated the issue starting from the FirstPersonBP Template.

First, i created a closed box that should prevent all Light from entering (the sky lights intensity was increased to make the issue more visible), the box contains a small point light.

as expected, the interior is perfectly dark and not affected by the skylight at all (the point light was turned off here)

if the point light is set to movable or stationary, it behaves as expected

but as soon as its set to static, the lights intensity is affected by the skylight

this was tested with a static and stationary sky light on UE 4.10.1

Hello moep,

This is to be expected. By disabling your skylight you have disabled the ambient light and color that it provides.

Static: Sky lights support baked lighting support baked lighting. This lighting cannot be changed in game.
Stationary: means the light will only have it’s shadows and bounced lighting from static geometry baked by lightmass, all other lighting will be dynamic.
Movable : Does not use any form of precomputation. It captures components and lights of any mobility when setup to capture the scene as the sky lighting source.

You will have difficulty indoors with static lighting due in large part to lack of bounced lighting.

Hello ,

Thanks for your response, i understand the basic principles of the skylight system as well as the difference between static/stationary/movable lights but at the moment i dont see how this produces the effect i described. Just to clarify, my problem is that in this case the static skylight is illuminating an area that should not be iluminated (the interior of the box), but only if there is a static point light nearby. This seems odd from my perspective because the interior area is perfectly black (as it is supposed to be) when the sky light is the only light source and therefore the skylight should not affect the interior lighting.

The sky light provides an Ambient Fill light as opposed to an actual light source. Even if you have a completely enclosed area, the sky light is going to fill that space. This is more clearly seen when you toggle is lower hemisphere black on or off.

When you add a sky light you are adding a fill light for the entire scene. I ran a test with a cube made from six cubes. The walls are fairly thick. I added only a sky light to the scene and set this to static. I then baked lighting at production lighting quality. I am able to see a slight illumination in my scene. This is to be expected. I then added a static point light reduced the attenuation so that I would also see the affects of the static sky light. Again, light was behaving as I would expect.

The fill lighting of a sky light is based off of the environment is reflecting. IE if you have no BP_Skysphere or some influence for the sky light in your scene to reflect then your fill light will be black. I will post three screenshots below to demonstrate.

This first screenshot is with no Sky_Sphere. The skylight has literally nothing to influence the scene with. The environment is completely black.

This screenshot is with no source light and a Sky_Sphere within the scene. The sky sphere looks like evening.

You will notice the change in color and illumination of the inside of this box. These are both captured after lighting has been built.

The last screenshot is made with a directional light influencing the time of day of the sky light. All lights are still static.

Note the illumination and influence of the sky light but also note the color change which is influenced by the color of the sky sphere. What happens when you change this to stationary is that you are updating your lighting in real time and using light bounces as opposed to baking the lighting where the engine calls on the lightmass build. Therefore, you will see the update in the viewport. This explains why you are seeing the influence more defined when you switch from static lighting.