Multipass 3D Compositing

Multipass 3D Compositing

Some artists, including a majority of those who work
predominantly in 3D, labor under the delusion that you
should fi nalize the look of a computer-generated element
in one pass. Certainly, as it becomes more and more possible
to adjust the look of a 3D model in real time (via the
GPU, OpenGL) this becomes tempting.
However, it’s possible to do better by dividing the render of
a single element into multiple passes. This is different from
rendering in layers, which while also useful for compositing
is really only about separating foreground elements
from the background. Multipass rendering is the technique
of isolating individual surface qualities and creating a separate
render for each. By surface qualities I mean things like
specularity and wear and tear, also known as grunge. In his

excellent book Digital Lighting & Rendering, Second Edition
(Peachpit Press, 2006), Jeremy Birn calls out multiple benefi
ts yielded by rendering a model on multiple passes:
. Changes can be made with little or no re-rendering. If
a shadow is too dark or a glow is the wrong color, the
adjustment can be made right in After Effects.
. Integration often requires multiple passes where the
model interacts with the scene, casting a shadow on the
ground or being refl ected in water. If the cast shadow is
simply part of a single render you lose all control over
its appearance and cannot apply it as recommended in
the previous section.
. Refl ections, which often consume massive amounts of
time to process, can be rendered at lower quality and
blurred in After Effects.
. Bump Maps can be applied more selectively (held out
by another pass such as a highlight or refl ection pass).
. Glows can be created easily in 2D by simply blurring
and boosting exposure of a specular pass.
. Depth of Field can be controlled entirely in 2D by
using a Z pass as a matte for a blur adjustment layer.
. Less render power and time is required to render any
one pass than the entire shaded model, so a lower
powered computer can do more, and redoing any one
element takes far less time than redoing the entire
fi nished model.
Putting multiple passes to use is also surprisingly simple;
the artistry is in all of the minute decisions about which
combination of adjustments will bring the element to life.
 (on the next page) describes some common
render passes and how they are typically used.
Other passes might include: a Fresnel (or Incidence) pass
showing the sheen of indirect light and applied to an
adjustment layer with a Luma Matte (raise Output Black in
Levels to re-create sheen); a Grunge or Dirt map, applied as
a Luma Inverted Matte, allowing you to dial in areas of wear
and tear with Levels on an adjustment layer; a Light pass for
any self-illuminated details; and a Normal pass showing the

Note that none of these passes necessarily requires a

transparency (alpha) channel, and at the biggest oldschool

effects houses it is customary not to render them,

since multiple passes of edge transparency can lead to

image multiplication headaches.

The general rules for multipass compositing are simple:

. Use the Diffuse layer as the base.

. Apply color layers meant to illuminate the base layer,

such as specular and refl ection, via Add or Screen

blending modes.

. Apply color layers meant to darken the base layer, if

any, via Multiply or Darken blending modes.

. Apply grayscale maps as luma mattes for adjustment

layers. Apply Levels, Curves, and Hue/Saturation to

allow these mattes to infl uence the shading of the

object or scene.

. Control the strength of any layer using that layer’s

Opacity.

Note that multipass renders present an excellent case to

enable Blend Colors Using 1.0 Gamma in Project Settings,

whether or not you assign a Working Space (and whether

or not that working space is linearized).

Multipass rendering is only partially scientifi c and accurate;

successful use of multiple passes is a highly individualized

and creative sport. With the correct basic lighting setup

you can use multipass renders to place a given 3D element

in a variety of environments without the need for a complete

re-render.

Varied environments are themselves the subject of the following.