S-3D cinema is more than a passing fad, and a new era of home ...

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S-3D cinema is more than a passing fad, and a new era of home ...

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• Publié par: yvevuqyb
• Nombre de lectures: 120
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

In association with Sony

Working with stereo 3D

»

Stereo 3D:

the new rules

S-3D cinema is more than a passing fad,

and a new era of home entertainment is set

to follow. Mark Ramshaw looks at the fresh

challenges, the new grammar and the wider

implications for the CG industry

espite a heritage that goes as far back

as the 19th century, the popularity of

stereoscopic imagery has waxed and

waned with alarming regularity. It feels like every

resurgence is ultimately curtailed by technical

hurdles and exploitative implementation. But the last

decade has finally seen many of the barriers to

effective stereo 3D capture and comfortable viewing

overcome, and creatives as well as money men are

at last waking up to the possibilities it offers.

2010 looks certain to be a pivotal year for S-3D,

as the overwhelming success of Avatar further

galvanises an already enthusiastic film industry, and

new technologies finally bring high-quality stereo

content into our homes for the first time.

The basic principle driving stereo 3D is simple

enough: the human brain typically perceives depth

within an image on a plane, such as one projected

onto a TV or cinema screen, using cues that include

changes in pattern sizes, light and shadow, relative

motion, apparent object sizes, haze, vertical

positioning and convergence of parallel lines. Stereo

3D adds another cue – stereopsis – by supplying

slightly different projections of the same scene

to each eye. Binocular disparity (the horizontal

difference between an object in the two versions

of the scene) determines how near or far an object

is perceived to be, with the disparity decreasing for

objects positioned further away. The flat screen then

has the illusion of being a window, with elements of

the scene visible through the window and even able

to float in front of it.

Where it gets more complicated is not only

rendering or capturing good-quality stereo 3D, but

also using it in a manner that is comfortable to view

and enhances the experience. It’s not just a matter

of adding an extra camera for a live shoot or a virtual

camera for a CG scene, then hoping for the best. For

those new to the medium, it pays to learn a little

more about the new rules of S-3D content creation.

“A new grammar for stereo 3D is necessary to

avoid compromising the 3D effect and also to

produce content that is comfortable to watch,” says

Philippe Gerard, co-founder of leading French stereo

3D direction, postproduction and conversion studio

3DLIZED. “Failure to be mindful of the grammar and

the results can prove nauseous for the audience.”

There are several reasons why care is needed –

not least the fact that there are inherent challenges

that the format presents to the viewer. In the real

world, eyes turn to look at, or converge on, the

object they want to focus on. For anything up close,

a large amount of vergence is required, whereas for

something on the horizon the vergence is zero.

The floating window

Stereo 3D introduces an effect called the vergence-

accommodation conflict. (Accommodation refers to

the act of focusing the eyes.) The problem arises

because the eyes are dealing with two separate

images on a fixed screen, while the brain thinks it

needs to focus on elements in the scene as if they

were located elsewhere in space – in front of or

the screen window effectively masking a closer

object as if it was further away. The obvious answer

is to avoid such conflicts, but that’s not always easy

for artists used to 2D conventions.

One solution, explains Ben Smith, creative

director of animation studio Red Star, is a floating

window. “By creating a rectangular surround that

sits closer to the viewer than the actual screen,

you’re able to change the audience’s perception of

where the screen sits, and so give the impression

that the cropped object now sits behind it.”

The concept dates back to the mid-20th century,

but it was only since stereographer Brian Gardner

realised that it needs to shift dynamically (with the

ability to change angle as well as depth) rather than

remain static that it has become an effective tool.

Camera challenges

Camera cuts are potentially just as challenging.

When watching a 2D movie, the human eye simply

needs to switch horizontal and vertical focus when

“Effects that use the depth in front of the screen are the

most complex of all. But it cannot be done without real

care, and it needs to be well integrated into the story”

Philippe Gerard, co-founder, 3DLIZED

the shot changes. With a stereo 3D movie, there’s

refocusing on the Z plane to consider. “Whenever

there’s a switch, you need to accommodate the

time it takes for the eyes of the audience to shift

their point of focus,” says Smith. “You can carefully

plan that with an all-CG production, but it’s more

complex – you really need to previz everything.”

“We often deal with customers who want to use

stereo 3D for explosions, smoke, battles and races.

The problem is that the third dimension requires

time to be processed by the human eye,” says

Gerard. “It’s one reason why more contemplative

sequences can be the most spectacular in S-3D.

If you look at Avatar, some of the most beautiful 3D

is to be found in the more ‘poetic’ sequences.”

Spacing out cuts is obviously preferable, but

not always practical. In such instances, blending

techniques can be used to reduce the difference

in 3D space between the point of focus in the first

and second shots. It’s even possible to ramp down

from stereo to an almost mono output just prior to

the cut, then ramp the sense of depth up through

the first few frames of the next shot – all without

the audience consciously registering any trickery.

Another potential pitfall arises with stereo that

works against rather than supports the 2D cues in

a scene. Objects with negative parallax will naturally

draw the audience’s attention, for example, so it’s

often preferable to have these in focus. Because the

human eye struggles to focus on much beyond one

behind that screen. One key problem for anybody

attempting to view stereo 3D arises from the fact

that a disparity between the real and the actual

position of the image causes conflicts for objects

cropped at the edge of the screen. If the object has

been positioned to float in front of the screen, you

end up with a window violation, with the edges of

D

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Images:

Coraline © Focus Features; Alice In Wonderland © Disney Enterprises, Inc;

Avatar © 2009 20th Century Fox; Meet the Robinsons © 2007 Walt Disney Pictures