Building a 3D shot (stereoscopic and non-stereoscopic depth cues)

Building a 3D shot does not only mean knowing how to set up the stereo without making the audience's eyes diverge or without making too powerful out-of-screen effects.

One must also know how to use the cues that will ease the audience's understanding of the depth of the image. Let's have a look at these different elements:

- 3D is COMPARATIVE . So one should always try to maximize the number of objects at different depths, so he may build a progressive 3D effect: a bee in very powerful out-of-screen effect with just a very far away mountains background, without any objects between the two, will not work very well.

- That's why an out-of-screen title on a black background will not work very well: there is nothing to compare the title to. So it's better to use a textured background, and to play with different depths with the different titles, to maximize reference points.

- Texture and shadows are stereoscopic cues, they are additional “objects” to compare with each other. A completely white sky, or a dark interior set with completely black zones will not work very well, not as well as a blue sky with clouds or a dark interior where all details are visibles. Shadows are also a good indicator, that's why some cinematographer, when they work in 3D, like to use more directional lighting - to emphasize shadows a little more than in 2D.

- One must be careful with the “depth coherence” , and avoid “3D collisions”: for exemple the collision of a title or subtitle on the screen plane with an object in “out-of-screen effect” - or the very common Stereoscopic Window Violation (SWV), where the borders of the screen collide with an out-of-screen object.

Various other factors can help your brain understand depth: that's why, when you close an eye, you still got a good perception of depth. By using these cues when you compose your shot, you'll ease your audience's understanding of the scene - and the 3D effect as a whole will appear to be much stronger. Among these factors, we may list:

- Close objects mask far objects : So lateral or vertical dolly or crane shots will enhance the perception of depth, since the placement of objects in space will appear much clearer.

- Object's size decrease with distance: we may use that to “cheat” the depth of some shots.

- Texture gradation : bricks, grass, cobblestones, etc., are less and less define with distance.

- Air transparency : in smoky rooms or vast landscape shots, this depth cue plays in important role.

- Classic perspective : a very important cue to guide your audience'eyes by using roads, skyscrapers, streets, railroads…

You should never underestimate the importance of these factors: they can literally make or break a shot.