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.
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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.
1.6. In the Material Editor, make sure the popcorn map is open. Click
on “Bitmap”, and select Mask from the Map Browser. Choose “Keep old map
as sub-map” in the “Replace Map” dialog. In the new Mask material, your
popcorn map is set as Map, so drag that do the Mask slot, and choose
“Swap”.
1.7. Click the Map slot, and choose Gradient Ramp from the Map Browser.
Set Gradient Type to Radial. Right-click the left flag, choose “Edit
properties” and change the color from black to white, right-click the
center flag and choose “Delete”, and right-click the right flag and
choose “Edit properties”.
1.8. Press the None slot to open the Map Browser and choose Smoke. In the Smoke Map settings, enter Size 5, and Exponent 1.
1.9.
Create a Target Direct light in the top viewport, so it lights the
teapot from behind, and place the target in the center of the teapot.
On the light’s “Intensity...” rollout, activate Use Far Attenuation,
and adjust the Near and Far values so the range covers the whole teapot
and the particles. Under Directional Parameters, adjust the
Hotspot/Beam so all the particles are covered. Now for the magic: in
the light’s Advanced Settings rollout, click “Ambient Only”. This will
make the light ignore the polygons’ angles, which is exactly what we
want.
1.10. Render that out to see the fluffiest teapot!
(hint: switch to Mental Ray to double render speed)
What we’ve done here is fake ray-marching. The fake part is that we
ignore how far the light has travelled inside the cloud, and instead
concentrate only on how far the light travels in space. For our
purposes, it’s close enough.
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Flight path:
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)
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..
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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 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.
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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.
نوشته شده
در دوشنبه یکم بهمن 1386 ساعت 2:15 توسط cracker
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