using microdisplacement for cloudscapes

seawolf · November 7, 2016, 6:10am

Hi guys,
The experimental microdisplacement feature opens new possibilities to create clouds. Already in September 2014, a pragmatic approach has been demonstrated by BlenderGuru. Clouds are created with displacements of simple objects. A volume shader is used for rendering. If you have large objects like clouds (hundreds of meters in size), static displacement is limited. Microdisplacement provides a way to create highly granular cloud surfaces as shown below.

Airships show size.

dvnobles · November 7, 2016, 7:16am

Very cool!

seawolf · November 7, 2016, 7:37am

Here are some experiences from creating the pictures above:

In addition to the volume shader, you need a transparent shader for the cloud surface. Otherwise, the object will not be rendered.
Be careful with the dicing parameter in the subsurface modifier. If it is too low, generating the microdisplacement will take lots of RAM (> 8GB in my case) forcing the system to swap to disk. That will take forever. I had to use a dicing parameter of 2 or 3.
If you link an object (e.g. a character) from a separate Blender file to your scene, it will be automatically microdisplaced if it has an subsurface modifier. As bump size will probably not fit, this might lead to strange looking artefacts.
Here is a material definition for the cloud:

Displacement is a cascade of Voronoi textures. needs some tweaking to find values for realisticly looking clouds:

one level of Voronoi texture

two levels

seawolf · November 7, 2016, 7:41am

Here are 4 levels of texture:

The last two levels do only subtly change the surface. But that is very important to create a realistic appearance.
Try it out.

cinnabarsonar · November 7, 2016, 3:22pm

Looks nice, how easy is it to render?

seawolf · November 8, 2016, 12:51am

The cumulus cloud picture in the first post (1600x1200) took about 30 min. on a GTX1080 at 200 samples. Needs about 2GB VRAM while rendering and 6GB main RAM for calculating the microdisplacement before rendering. The cloud volumes are homogeneous and therefore render quite fast. You have to crank up transparency in the light path parameters to something like 40 to avoid black artifacts.