# Projecting 3D objects onto a 2D view

Flash Player 10 and later, Adobe AIR 1.5 and later

The PerspectiveProjection class in the flash.geom package provides a simple way of applying rudimentary perspective when moving display objects through 3D space.

If you do not explicitly create a perspective projection for your 3D space, the 3D engine uses a default PerspectiveProjection object that exists on the root and is propagated to all its children.

The three properties that define how a PerspectiveProjection object displays 3D space are:

• fieldOfView

• projectionCenter

• focalLength

Modifying the value of the fieldOfView automatically modifies the value of the focalLength and vice-versa, since they are interdependent.

The formula used to calculate the focalLength given the fieldOfView value is:

`focalLength = stageWidth/2 * (cos(fieldOfView/2) / sin(fieldOfView/2)`

Typically you would modify the fieldOfView property explicitly.

## Field of view

By manipulating the fieldOfView property of the PerspectiveProjection class, you can make a 3D display object approaching the viewer appear larger and an object receding from the viewer appear smaller.

The fieldOfView property specifies an angle between 0 and 180 degrees that determines the strength of the perspective projection. The greater the value, the stronger the distortion applied to a display object moving along its z-axis. A low fieldOfView value results in very little scaling and causes objects to appear to move only slightly back in space. A high fieldOfView value causes more distortion and the appearance of greater movement. The maximum value of 179.9999... degrees results in an extreme fish-eye camera lens effect. The maximum value of fieldOfView is 179.9999... and the minimum is 0.00001... Exactly 0 and 180 are illegal values.

## Projection center

The projectionCenter property represents the vanishing point in the perspective projection. It is applied as an offset to the default registration point (0,0) in the upper-left corner of the stage.

As an object appears to move further from the viewer, it skews towards the vanishing point and eventually vanishes. Imagine an infinitely long hall. As you look down the hall, the edges of the walls converge to a vanishing point far down the hall.

If the vanishing point is at the center of the stage, the hall disappears towards a point in the center. The default value for the projectionCenter property is the center of the stage. If, for example, you want elements to appear on the left of the stage and a 3D area to appear on the right, set the projectionCenter to a point on the right of the stage to make that the vanishing point of your 3D viewing area.

## Focal length

The focalLength property represents the distance between the origin of the viewpoint (0,0,0) and the location of the display object on its z-axis.

A long focal length is like a telephoto lens with a narrow view and compressed distances between objects. A short focal length is like a wide angle lens, with which you get a wide view with a lot of distortion. A medium focal length approximates what the human eye sees.

Typically the focalLength is re-calculated dynamically during perspective transformation as the display object moves, but you can set it explicitly.

## Default perspective projection values

The default PerspectiveProjection object created on the root has the following values:

• fieldOfView: 55

• perspectiveCenter: stagewidth/2, stageHeight/2

• focalLength: stageWidth/ 2 * ( cos(fieldOfView/2) / sin(fieldOfView/2) )

These are the values that are used if you do not create your own PerspectiveProjection object.

You can instantiate your own PerspectiveProjection object with the intention of modifying the projectionCenter and fieldOfView yourself. In this case, the default values of the newly created object are the following, based on a default stage size of 500 by 500:

• fieldOfView: 55

• perspectiveCenter: 250,250

• focalLength: 480.24554443359375