Forced Perspective – Theory and practice

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by Davide Mantovani, Marco Porcaro, Simone Villa


The Forced Perspective is a technique that uses optical illusion to make an object appear farther away, closer, larger or smaller than it actually is in reality: human perception is manipulated and deceived by the use of distances and scale ratios. It is mainly used in photography, film and architecture.

To make an actor look like a Hobbit of normal height, there are several options: you could use stunt doubles very low or very high, or computer graphics and sound stages. The last option, perhaps the one that gives the scene a more realistic look, is the Forced Perspective.

The Lord of The Rings Forced Perspective

In The Lord of the Rings, all the techniques mentioned above have been used, but director Peter Jackson’s favorite was just the last one. Thanks to Forced Perspective, it is not necessary to use stunt doubles and to replace the computerized face, dubbing or other gimmicks to insert the first actor in the scene. Moreover, even if of different heights, the actors interact and acttogether, on the same set and the same instant. To make a concrete example, we’ll refer again to The Lord of the Rings: a Hobbit is ¾ of a man height, which means that the human should be ¾ of the distance between the Hobbit and the camera. In a 2D photo or movie, our brain can estimate the size of subjects and objects through two parameters: the angular size and the context cues. Angular Size Forced Perspective 1 The angular size θ is calculated as follows: Angular Size Eq1

where L is an arc segment, that can be approximated as the height of the object in question, and the result is in radians. The following diagram shows the relationship between the two objects at the moment of the illusion.

angular_size_2 To find the L’ of the illusion-projected object, simply use the following formulas: Angular Size eq 2 Angular Size eq3   Simply, this equation compares L at the distance r + s with the image L’ at the hypothetical distance r. Therefore, if our aim is to decrease the height L to L/n, the calculated ratio will be defined by the following equation: Angular Size eq4 If we want to reduce by half the height L of an actor (L / 2, with n = 2), it would be enough to place him r · 2 meters (or feet) away from the camera. To further demonstrate these formulas and these relationships, we will analyze a picture, taken for the occasion.

LEGO FP 1 original

These two LEGO have, of course, the same height, yet the right one seems considerably smaller.

LEGO FP 1

Seen from above, the “set” is shown as follows, with the camera positioned on the right:

LEGO FP 2

This illusion is enhanced by the fact that the two characters seem to lean on the same surface, while there is a studied difference in height.

LEGO FP 3

There are significant points in favor of this technique, such as more realism, given that the actors can be in the same locations and not replaced, and the hic et nunc, the presence of the two characters interacting simultaneously.

However, there are some aspects to considerate and that can complicate the use of Forced Perspective.

First, both parties must be in focus, as if they were really facing each other. To maximize this parameter, the scene must take place in a brightly lit set and the camera iris must be very closed, in order to get the most depth of field (DOF).

The set lighting must be specifically designed to avoid shadows that could reveal the trick: it is better to opt for a front light and a whole system of spotlight distributed in order to clear unwanted shadows.

The most important problem is related to the movement of the camera. In fact, any change in point of view is likely to reveal the illusion. You can work around this issue, by a inversely proportional shift between the camera and the smaller subject.

The camera can move nearer the scene without ruining the effect if the movement has the same and opposite speed of the actor and objects in the background.

If you wish to pan the camera, however, it would be necessary to move in the opposite direction, with the same magnitude and speed, the part to be smaller.

These two solutions have been used extensively and with great skill in the entire trilogy of The Lord of the Rings, but it was not possible to do the same for any movie of The Hobbit saga. Why? The films are shot in 3D and, of course, with two Red cameras mounted on a 3D rig: this means that it’s impossible to synchronize and align Forced Perspective with both points of view simultaneously. The solution was the use of live compositing of the scenes, superimposed. Anyway, this operation was done in real time, so as to allow the actors to interact at the same time, although from different places.

hobbit-how-it-works-00-1212-xln

hobbit-how-it-works-00-1212-xln hobbit-how-it-works-00-1212-xln


Take a look to our other articles about FORCED PERSPECTIVE:


Sitography

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