1. For this example we are going to create simple city landscape. So, create this plane as ground.
2. With boxes do something like this. And there we have our city.
3. Put VRay Sun light in to a scene like this and set intensity multiplier to 0.035.
4. Put VRay Physical camera in to a scene and set Type to video camera, f-number to 8 and film speed to 200.
5. Go to material editor. Click on Get material and chose VRaySky from the list. Click on sun node slot and pick on sun in the scene and set sun intensity multiplier to 0.035.
6. Here is a print screen of a render setup.
7. Drag the VRaySky map to the skylight slot.
8. Under the Color Mapping change type to Reinhard and set Gamma to 2.2.
9. When you click render you should get something like this. On my computer it last 30 seconds.
10. But we can cut our render time when we have animated camera. For nice circle camera path create sphere like I did in next picture and place Vray Physical camera on top of the sphere.
11. With selected sphere go to Hierarchy panel and click on Affect Pivot Only and move pivot to the middle of ground plane so that pivot of sphere is on top of the camera target.
12. With selected camera click on Select and Link. Hold left click and release it on the sphere. After that you can go frame 100 and click on Auto Key and rotate your sphere. When you finish with animation hide the sphere.
13. To create Irradiance map set mode to Multiframe Incremental in V-Ray Irradiance Map. Check Don’t delete and auto save for output file.
14. In VRay Global Switchers check Don’t render final image.This is very important.
15. Under the Common Parameters in Time Output Set Every Nth Frame to 10 and check Active Time Segment 0 to 100.
16. When rendering is done ( 10 minutes on my comp) go to V-Ray Irradiance map and chose From file Mode and Browse your irradiance map. Don’t forget to go back to Common Parameters and set Every Nth Frame to 1 and to uncheck Don’t render final image in VRay Global Switchers.
This way you will save a lot of render time. For this particular scene render time was 26 minutes with irradiance map and about 60 minutes without it.