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 dependant 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.
1.2. Press "6” to open Particle View. Create a flow with a Birth event, Position Object, Shape Facing and Material Static.
1.3. In the Birth Event, set Emit Stop to 0, and Amount to 1000. In Position Object, select the teapot as Emitter Object. In Shape Facing, select the camera in the scene as Lookat Object, set Size to 20 and select Random orientation. In the Display event, set Type to Geometry.
1.4. Now, in your favorite paint program, use a soft brush to make a popcorn shaped blob that looks something like this. Make sure it's white on black background.
1.5. In 3ds Max, create a material with a solid white color, that uses your popcorn image as Opacity Map. Drag this to the Material Static event in the Particle View.