How to create Clouds?
by Jonas Ussing Web:

In this tutorial you will learn to create a fly-through of a realistic cloud tunnel that renders relatively fast, using 3D Studio Max with no plugins or 3rd party renderers.

But first, a little theory... the rest of the tut is based on this, so no fair skipping ahead!

The Problem of Ray-Marching and Volumetrics

The basic physics of light moving through a gaseous cloud has nothing to do with traditional polygon lighting, and therein lies the main challenge when rendering clouds.

Cloud particles are translucent, and will be visible from all sides when illuminated.

However, light decays in strength as it passes through.

This will make the backside of a cloud illuminated only by how far through the cloud the sunlight has travelled.

Note that the angle of a cloud particle's surface is completely irrelevant, contrary to "normal" polygon lighting.

We will now move on, knowing that the photorealistic look of a cloud is dependent on decaying, non-polygon lighting.

The Cotton Teapot

Before moving onto a larger cloud formation, we'll take the theory from above and apply it to a simple shape.

1.1. Create a teapot with a radius of 40 in the Perspective viewport. Right-click, choose Properties, and de-select "Renderable". Press CTRL+C to create a camera in the viewport.

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