Cloud Tunnel
3.1. Collapse the tunnel (Cylinder01), add a TurboSmooth modifier, and set Iterations to 2. Then add a Noise modifier on top. Set the Noise Scale to 40, activate Fractal, set Iterations to 2, and set X, Y and Z Strength to 40.
3.2. Add a Push modifier and set it to -10 to make the walls bulge out in round shapes (if you think the polys are moving in the wrong direction when you use the Push modifier, keep in mind that we are watching the inside of a cylinder, and all the polys are backfacing).
The cylinder is now a representation of where the clouds will be.
3.3. Press "6" to open the Particle View. Change the Amount to 2000, and select Cylinder01 in Position Object. Your tunnel should now be populated with billboard particles. Make a test render of that, to see how you're doing.
Not bad, but we're still using traditional lighting, based on polygon angles - that's the only reason for the grey shades you can see in the render. This isn't a liable solution, because when the billboards turn to follow the camera, their angle relative to the light source will change, and so will their brightness.
3.4. But before we begin lighting the cloud tunnel, let's work on the color. We can't apply any polygon-mapped texture, but we can make large chunks of color variation. In the cloud material, click the Diffuse Color slot, and select Noise from the Map Browser.
3.5. Choose Fractal, High=0,8 Low=0,2 Levels=10 and Size=100. Instead of black, use 60, 90, 115. Turn the cloud material's Self-illumination up to 100, and render.
What we have created is a large world-space texture variation. The reason we can't use polygon-mapped bitmap textures with billboard particles is that when they turn to follow the camera, they will intersect in and out of each other, and the textures will "pop" in and out. But with a world-space texture, two intersecting polygons are guaranteed to have the exact same color information at the intersection point, so the intersection is invisible.
(Technically, world-space textures aren't perfect either, because when the billboards turn, their surface will move through the world-space texture information, and the texture will appear to "crawl" over the surface. But with a pattern this large, it's not a problem)
3.6. In the top viewport, create a Target Direct light that lights the cloud tunnel from the side.
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 all the particles are 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...
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