Fog is
a term often used to represent a variety of
natural phenomenon within a 3D renderer. In
nature, fog consists of water vapor yet in Maya
you may use 'fog' to also represent dust, smoke,
air, smog, plasma, nebulae or even magical glows
and spells. To be able to yield such a variety
of effects from fog, one must have a clear understanding
of the volume shader associated with it. Specifically,
our focus in this discussion is 'light fog',
not volume primitives or environment fog. These
are related topics, yet they use different nodes
and are for another discussion.
In
the above image, we have a standard three-point
light rig using spotlights: a high key,
low fill and a couple back lights. All lights
have decay set to 'linear'.
When
lighting a scene which will require some
degree of 'fog' (or dust, steam, etc), I
will usually not begin setting up the fog
effects until the lighting is virtually
complete. This means that there will probably
be several lights, due to the fact that
all bounce light must be simulated. The
lights will all have some level of decay
and their intensities will have been determined.
In nature, particulate matter in the air
will be illuminated by all available light,
but Maya does not work this way. 'Fog' is
enabled on a per light basis. While this
is not 'natural', it offers a lot of flexibility.
Furthermore, we can create a light whose
sole purpose is to create fog... meaning
that the light will not illuminate scene
geometry.
When
light fog is enabled, a new shape node is
created and assigned a volume shader. The
'fog' exists as a volume rendering of the
interior of this spherical or conical shape.
The first thing you should do is scale the
shape, something discussed further in another
tutorial.
fog enabled on high key
Fog
has been enabled for the key and the spotlight
has been scaled. The result is 'fog' which
has some issues... it is a little bright and
is composed of extremely smooth gradations.
While it is possible in nature to have smoothly
illuminated fog, our goal is to illustrate
how we can simulate effects which are more
indicative of dust or smoke.
fog with intensity and spread modified.
At
this point the only change has been to the
'intensity' and 'spread' attributes in order
to produce a more subdued appearance. What
we will now do is change the quality of the
fog's density. If you look at the volume shader
assigned to the fog shape node, there are
attributes to control fog color and density.
While a light's color, in nature, is independent
of the color of the particulate matter in
the air, it is best to leave light fog color
as white. What happens in nature, is that
a white light through blue smoke would illuminate
objects with a blue light. Fog color in Maya,
however, does not change the color of the
cast light. But a light's color in Maya does
color the fog... therefore you need to determine
the correct coloration of the fog and the
light through the light's color attribute.
There is an alternate approach, however, which
we are not going to get into in this tutorial.
Technically, in nature, if a white light is
shined through blue smoke, the saturation
of the light's color will increase with distance
from the light. This is possible in Maya through
the use of 'color curves'. I'll leave it at
that.
In
regards to density, however, it is often necessary
to modify the appearance of the fog so that
it is not so evenly gradated. This is accomplished
by mapping fog density with solid 3D textures.
Solid fractal mapped to fog density
The
above image shows the result of mapping a
solid fractal to fog density. By animating
the texture node attributes, the fog can appear
to change due to turbulence, wind or other
phenomenon. All 3D textures also have a 3D
placement transform node that can be moved,
rotated, scaled and animated to design the
look or placement of the fog. In the above
image, the fractal transform was scaled, its
'frequency ratio' was lowered and its 'alpha
offset' was raised. Be aware that the mapped
density variation will not self-shadow. Therefore,
this technique should only be used to simulate
thin matter.
Ambient light added
Something
important to realize with fog in nature,
is that it becomes a lightsource itself,
once illuminated. Maya does not calculate
this type of reflected light, therefore
you will often need to add lightsources
in order to mimic the effect fog will have
on surrounding objects. In the above image,
an ambient light was added with 'ambient
shade' turned off and its color sampled
from the rendered color of the fog. This
addition helps place the character within
the fog volume.
fog variations, click images for larger sizes.
It
is also possible to combine multiple 3D textures
to further refine the fog density. By mapping
the 'alpha gain' or 'alpha offset' attributes
of the density map, other textures can be
used to darken or lighten areas. This is helpful
to reduce unnatural noise patterns. If fine
noise is required, a scaled down fractal will
work, but the result will look unnatural.
A scaled up fractal or cloud texture could
be mapped to the 'alpha gain' to further randomize
the density.
Aside
from fog density, one will sometimes need
to think about -where- the fog should be most
dense in the scene: near the ground? inside
a room but not outside the window? When fog
is associated with a light in Maya, the fog
will always appear in front of that light.
Our use of mapped density thus far is only
specific to where the light illumination is,
but not to where the light is in the scene.
image of fractal fog with ramp to control height.
In
the above image we have denser, patchy fog
near the ground with smoother fog towards
the top of the image nearer the lightsource.
This is accomplished by mapping planar projected
ramps to the fractal's 'alpha gain' and 'alpha
offset'.
3D placement nodes selected, hypershade graph.
The
placement node is scaled to the size of the
scene, or at least the bounding box of the
lightfog shape. Use 'smooth' interpolation
on the ramp, for a more natural gradation.
Alpha gain is mapped to control where the
fractal noise appears, while alpha offset
is mapped to control where smooth fog appears.
click to play animation
The
above animation shows the result of
animating a searchlight sweeping in
front of a blasted wall. Light rays
enter the room as shadows are cast into
the fog volume.
The
two ramps are mapped to fog density
so that fog only exists inside the room.
One ramp controls fog height (Y-axis),
the other controls fog density in the
Z-axis.
Be
aware that volume shadow occlusion only
works with depth-map shadows. This creates
problems if you have transparency mapped
objects. There is a workaround for this
that we will explore in another tutorial.
Another
rendering limitation is that you should
avoid animations where the camera enters,
exits or moves around within a lightfog
volume when 'light color' or 'fog density'
have been mapped. The results will not
be accurate as the Maya light fog engine
is, unfortunately, pseudo-volumetric.
click to play animation
The
above animation shows the result
of mapping a point light-fog's
density with a solid fractal,
animating the scale of the lightfog,
animating the fractal and placing
an animated image sequence on
to the point light's color. This
renders extremely fast (10 sec/frame
on a dual 2Ghz box) and looks
cool. In order to fly the camera
through it, however, we would
have to swap the lightfog with
a 'volume primitive' with a high
sample rate.
click to play animation
In
the above Zufuhr animation, I've used
light fog (blue), volume primitives
(warm) and particles (glow effect) together
to get the end result. Again, the advantage
of volume primitives is that they hold
up better if the camera is moving around/through
them. They also can be illuminated by
multiple scene lights. All the techniques
we've discussed in regards to mapping
density work well with volumes, just
be aware that their disadvantage is
that they take longer to render.
Conclusion?
Clearly
there are many more applications than
those we have discussed. By the way,
while these effects could also be accomplished
via rendered particle dynamics, our
aim is to utilize more render efficient
techniques whenever possible. Particles
offer a lot more flexibility in terms
of their movement and rendered appearance.
The drawback is that they take longer
to set-up, take longer to render and
require knowledge of Maya's dynamics
engine. Its up to you to learn through
experience when to use which technique
based on the requirements of a shot.
