3.7. Right-click Top -> Views -> Direct01 to see the scene from the light source's point of view. Under Directional Parameters, adjust the Hotspot/Beam until the particles are totally visible from the light source.
3.8. Render the scene from the light's point of view. Save the image, and load it into your favourite painting program.
Your particles will propably look all flat and weird because they're pointed at the camera and not the light source, but that's OK.
3.9. In the paint program, paint over the particles so they get covered with a gradient like this, and save the image. The idea is that anywhere in the cylinder, the cloud particles will have a gradient from top to bottom. We're faking ray-marching of light coming from the top.
I have used 170, 220, 240 for the bright color, 50, 90,110 for the dark color, and 80, 140, 170 as the background base
3.10. Back in 3ds Max, click the Map slot in the light's Projector Map rollout, and choose your paintover image. Remember to check "Ambient Only"!
3.11. NOTE: the purpose of this light source is to illuminate this particular cloud and nothing else, so if you have an aircraft in the scene, go to the light's Include list and add your cloud Particle Event, which is propably called "PF Source 01->Event 01". Then you're going to have to add new lights to light up your aircraft, so go ahead and add the same Particle Event to those lights' Exclude list.
3.12. Now it's time to test the light. Go to the cloud material and turn the Self-illumination to 0, and turn off the Noise Map in the Diffuse Color slot. Render!
(note: the first few frames might have particles right up in the camera because of the capped ends of the emitter cylinder, so delete those faces on the cylinder, or go forwards to a more interesting frame)
We're almost there! Now all we have to do is combine the world-space noise map with our light gradient.
3.13. Set Self-illumination to 50, and re-activate the Noise Map in the Diffuse Color slot. Render again! You're done!
If you will, adjust the Self-illumination and the Diffuse Map's Amount slider to blend between the Noise Map and the gradient light source.
Now render the whole animation while you make a pot of coffee...
Highlights on the sunny side
Remember that rimlight/halo look that clouds get when they're backlit? So far we've seen nothing like that in this tutorial, but there is a cheap and dirty way to fake it. Stay with me.
4.1. Copy the emitter (Cylinder01) in the Top viewport, and move the copy just a few units North.
4.2. Press "6" to open Particle View, and create a copy of Event 01.
4.3. In Event 02, change the Position Object to Cylinder02. You should see the double particles in the Top viewport like this. This should also warn you that the particle count has doubled, so prepare for slower renders.
4.4. Press "M" to open the Material Editor, and create a copy of the cloud material. Name this "cloud_highlight".
Change the Self-illumination to 75, and change the Diffuse Map's Amount slider to 25. This will make the highlight material brighter and more self-illuminated than the rest of the clouds, which is exactly what we want.
You can further enhance the effect by lowering the original cloud material's Self-illumination.
4.5. Drag the "cloud_highlight" material to Event 02's Material Static slot.
4.6. Render again to see the new look. At first glance, it may seem much better, but remember the disadvantages about textures and intersecting billboards? They'll start showing, once you render the whole animation.
Here is a like where you can see an animation of the whole thing in action:
That's it, I hope it's been informative. If you found this tutorial useful, please stop by my CGSociety portfolio or website: