Graphics Reference
In-Depth Information
fight sequence that we worked on from
The Incredible Hulk
, to convey a moody, dark
feeling. You have very intense bright lights and a lot of murky blacks. There are a lot
of tricky things to deal with!
Thad Beier, 3D stereoscopic supervisor at Digital Domain, in Venice, California, was
the stereo supervisor on
Transformers: Dark of the Moon
. He discusses the challenges
of modern stereo movies.
RJR: What are the different ways stereo movies are created?
Beier: There are two kinds of stereo movies: those that are shot natively in stereo
with two cameras like
Avatar
, and movies that are shot “flat” with a single camera
and converted to 3D like
Alice in Wonderland
.
Transformers: Dark of the Moon
is
unique in that it's half of each.
The main approach to 3D conversion is based on rotoscoping tools. Regions are
segmented into layers, each of which is assigned a depth. They may also add a little
bit of shape to each layer — for example, on a face, they might extrude the nose out
a little bit. Then the “slivers” of background that were occluded in each eye need
to be inpainted and the entire scene is re-rendered. The whole process is extremely
labor-intensive. It's a place where computer vision really can pay off. An automatic
algorithmdoesn't have to solve thewhole problemcompletely; I don't think it should,
since there's a lot of artistry involved in creating a good stereo composition.
RJR: What does a stereo rig for a feature film look like?
Beier: It depends on the movie, but typically you have two cameras, such as Sony
F35's, mounted at roughly right angles on a rig, viewing the scene through a beam-
splitter. Since the cameras and lenses are big and heavy, unfortunately the rig isn't
very rigid; if you rotate it slightly, it puts a tremendous amount of torque on the rig
and the cameras go out of alignment. There's no time on a busy set to recalibrate the
rig, so it has to be fixed in post-production to bring each frame pair back into a perfect
stereo alignment. There's a very expensive device on set doing real-time analysis of
the videos that will alert you if it sees substantial vertical or rotational disparity, but
it doesn't really fix the problem.
The critical parameter for stereo filming is called the
interocular distance
, the
equivalent physical separation between the cameras, which is often really small, like
three centimeters (it would be more accurate to call this the interaxial distance). This
is related to the distance between the viewer's eyes by similar triangles. That is, the
ratio between the camera interocular distance and the physical distance to a subject
should be the same as the ratio between the viewer's interocular distance and the
movie screen. It's important to get this parameter right, because you can't change it
in postproduction without messing up the parallax. The only thing you can do after
the fact is apply a corner-pin, or projective transformation, to rotate the camera's
view. This is important so that when the viewer focuses on far-away points in the
