Making of Clouds

THE PROBLEM OF RAY-MARCHING AND VOLUMETRICS

THE COTTON TEAPOT

First, we'll create a camera that chases an airplane through a tunnel. Later, we'll turn that tunnel into clouds. If you think you've already spent too many hours of your life creating tunnel effects, move onto chapter 3, but skim chapter 2 to make a note of the dimensions used.

2.1. Delete the teapot and the light from before, but keep the camera, particle system and the material.
2.2. Create an s-shaped spline like this, using four bezier points. Note the shape in the Top and Left viewports. Be careful with the bezier handles – no sharp corners! And since we don't want any jittering in the camera, increase the interpolation to 20, and de-check Optimize. (actually, we do want jittering, but not this kind... more on this later!)

2.3. The first point in my curve is at 0,0,0, and the last is at 50,400,-100. This isn't crucial, but the closer your scale is to this, the easier it will be to match the sizes of particles and procedural maps later in the tutorial.
2.4. Select the camera, and add a Path Constraint by going to the Motion rollout, expand the Assign Controller rollout, click Position, click Assign controller and double-click Path Constraint.
2.5. In the Path Parameters rollout, click Add Path and click the spline in a viewport. Now the camera follows the length of the path during the whole length of the animation, but we want it to have a distance to the camera target, so go to the final frame, activate Auto Key, set the Default Tangent to linear, and type in 90% in the %Along Path field.
2.6. Create a dummy, and add a Path Constraint to make it follow the same spline, but now (with Auto Key on) go to frame 0 and type in 10% in the %Along Path field. Turn off Auto Key.
2.7. Align the camera target to the dummy, and then link the camera target to the dummy. Play the animation, and double check that you now have a nice animation of a camera following the camera target at a constant distance.
2.8. Why use the dummy and not apply the Path Constraint directly to the camera target? Remember that jittering we were talking about? Select the camera target, and assign the Noise Position controller to it, instead of the Path Constraint. In the Noise Controller dialog, de-select Fractal Noise, enter 0,2 as Frequency, and 2,0 as X, Y and Z Strength. This gives a nice turbulent bounce to the camera.
2.9. Create a cylinder with Radius 45, Height 440, Height Segments 40, Sides 15, and align it to the path (position and rotation) using the Align tool. Right-click the cylinder, go to Properties and de-select Renderable. Also, make sure the cylinder's Backface Cull is turned off.

2.10. In the Modify panel, open the Modifier rollout, and select PathDeform (WSM). It's very imortant that you choose the World-Space Modifier version (WSM), and not just the normal PathDeform. Select the path.
2.11. Now you can see the cylinder stretching along the path. If your path coordinates deviated radically from the ones I described earlier, you will need to adjust your cylinder's Length, Radius etc. Play the animation in the camera viewport, and it should look something like a tunnel effect from an Amiga demo, but without the plasma!
2.12. For an extra touch, add a banking effect to the camera: set Default Tangents to auto, activate Auto Key, select the camera, and press F12. Go to frame 25, and set Roll to -25. Go to frame 75 and set Roll to 25. Go to frame 100 and set Roll to 0.
2.13. If you want, now is the time to merge your favourite aircraft model into the scene, and add the same Path Constraint once more. Again, go to frame 0 and set the "%Along Path" to 10. Activate Follow and Bank, set the Bank Amount to 2, and the Smoothness to 1.

2.14. Also, load in a standard environment map, for instance, CHROMBLU.JPG from the Max library (not that we'll be seing a lot of background, but it's better than black)

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...

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.

That's it, I hope it's been informative. If you found this tutorial useful, please stop by my CGSociety portfolio or website:
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