Here’s how the human eye works. Light enters a hole in the front, passes through a lens and is focused on a light-sensitive surface at the back – the retina. A camera works in much the same way, but instead of a retina it uses either a charge-coupled device or a strip of film.
Unlike a camera, however, the human eye has a problem. There’s a place on the retina where the optic nerve is connected (the bit that carries all the information to the brain). Where there’s an optic nerve, there’s no room for light sensors. The result is something that all sighted people have, but which few of us are consciously aware of: a blind spot.
But that’s okay. If we don’t notice our blind spot, it must be pretty small, right?
Wrong. Although the size of the human blind spot varies from person to person, there’s evidence to suggest it can be up to 70 times the apparent size of the full moon as it appears in the night sky*. If you find that hard to visualise, try this bombshell: you and I are both blind across an area of our vision roughly the size of a DVD held at arm’s length.
I don’t know about you, but I find that mildly freaky.
Luckily for us, vision doesn’t happen in the eye. It happens in the mind. The blind spot might mean the brain gets a picture with a hole in the middle, but it turns out the brain is very good at filling in the missing detail.
Visual effects is all about filling in missing detail too. Like patching into a scene something that was never there in the first place – say, a ravening monster or a crashing aircraft. Possibly both. It might involve extending an existing background to hide the rig used to hold an actor off the ground. In other words, visual effects performs exactly the same sleight of hand as the human brain: it fools you into seeing something that isn’t really there.
In other words, we all carry in our heads our very own personal visual effects supervisor, an unsung hero who works constantly to bring inadequate stage footage up the standard required for theatrical release.
So how exactly does the brain perform its magic? One of the tricks it uses is to copy and paste information from the left eye to the right (and vice versa). Then there’s edge interpolation, whereby the brain assumes that any line passing through the blind spot does so unbroken. Also, the human eye is in constant motion. By scanning a scene, it ensures the sighted part of the retina gets to see everything at least some of the time.
The way moviegoers move their eyes is of particular interest to filmmakers, especially as eye tracking technology becomes more and more able to deliver meaningful data. In the future, eye tracking may be used commonly in test screenings to determine where the attention of audience members is directed on a shot-by-shot basis. Even before that stage, eye tracking has applications in the editing suite, such as quantifying how comfortable an assembly is to view. And let’s not forget product placement. Where’s the most effective place to put a can of soda in this shot? The eye tracking data will tell you.
Eye tracking may not yet be a standard weapon in the filmmaker’s arsenal, but advertisers are already using the product placement trick in a variety of media. In today’s movie theatre, it’s just the audience watching the screen. Soon, the screen may be watching them back.
Hmm. Maybe there’s a way to profit from all this. By my crude reckoning, blind spots could obscure up to 5% of the average moviegoer’s field of vision. So why not just leave 5% of the screen blank? Plus, if the eyes of the audience are wandering all over the place, why not dial down the level of detail in the parts of the frame you know they’ll be ignoring? Imagine the benefits of saying to your VFX supe, “Don’t bother feeding this part of the scene into your deep compositing pipeline. Those poor schmucks in the audience won’t see it anyway.” Just imagine! Shorter rendering times! Cost savings! If you can’t see the advantages you must be blind!
But then, in a sense, we’re all blind, aren’t we?