How to achieve shadows with a single HDRI
in Unreal Engine 4.18 
Tutorial
Ok so this is my first ever tutorial so bear with me! ;)
You can try the following with your own scene or you can download the tutorial scene from here:
HDRI_shadows_tutorial.zip
(40Mb)
You will need the followings in your scene
- Lightmass Importance Volume covering your important area
- Lightmass Portal(s) covering your opening(s)
- Post Process Volume
+ for better result: Reflection Capture
You don't want autoexposure in your scene that will change regarding how bright area you're looking at, so
in your Post Process Volume / Lens / Exposure set both Min/Max Brightness to 1
Also for a clean result we'll turn off Ambient Occlusion by setting its Intensity to 0
And finally we'll set Post Process Volume as Unbound to effect the whole scene
Import your HDRI image into your scene.
In my scene I used 047-free-hdri-skies-com.hdr
In the HDRI's details uncheck sRGB and set Mip Gen Settings to NoMipmaps
Select your SkyLight, set it to Static.
Change Source Type from SLS Captured Scene to SLS Secified Cubemap.
Grab your HDRI and place it into the Cubemap slot.
Change Cubemap Resolution from 128 to 1024 and set the Intensity to 5 for a start.
If you want to visualize/see the HDRI in your scene as a "skydome",
please follow Rag3DVIZ's beautiful tutorial.
On the surfaces (walls), where you expect shadows to appear,
set the lightmap resolution to 256 or higher!
In my scene all walls have 1024 lightmap resolution on their inner(!) faces!
In your World Settings use the following setup:
Static Lighting Level Scale - DETAIL
Num Sky Lighting Bounces -
more bounce information, such as color bleed and lightness
Indirect Lighting Quality - QUALITY (cleaner result)
Indirect Lighting Smoothness - more detailed lighing information / less blurred
Set your Lighting Quality to Production and click Build.
This is my result:
...and from this on, it's only Post Processing!
I set Exposure to 2 to get more contrast.
And used a LUT file to get more contrast/style.
Great tutorial by Epic: Using Lookup Tables (LUTs) for Color Grading
All I used was Curves and Brightness/Contrast
Final results:
I hope it will be useful for someone...