I'm trying to implement a controlled trapezoid transformation in Adobe Flash's ActionScript using the built-in perspective projection facility. To give you an idea of how the effect looks like:
I've been searching for a solution for a few days now and, apart from the tessellation alternative which produces poor quality results, no one has tried to employ perspective projection to achieve this effect. It's maybe because it requires solving some math, which is something I hope you could help me with.
So Flash uses a simple frustum model with a viewpoint, projection plane, and
focal length, which is the distance from the viewpoint to the projection plane.
There is also fieldOfView but it should be of no importance as its meaning appears to be overridden by focalLength (at least it's value makes no difference in my ActionScript experiments as long as the focal length is specified).
The rectangular piece of graphics is located right on the projection plane so that the middle point of its top edge is located in the projection center. To achieve the effect, the rectangle is rotated by some
(Sorry, the forum wouldn't let me post more links to the drawings. Could anyone please edit it to put the last 2 imaged referred at the end of the source right here.)
The question is, what should be the values of
focal length and
angle so that
new height and
new bottom width satisfy some given values that are desired for the effect? Or speaking mathematically:
- a frustum with
- a rectangle with its
height, which is pinned to the projection plane (z=0) of so that the midpoint of the rectangle's top edge is located right in the projection center of the frustum
- the needed
new bottom widthof the rectangle after it's rotated and projected onto the projection plane
Unknowns to be found and applied to the perspective projection in order to get the desired
new height and
new bottom width:
focal lengthof the frustum
If I missed out any other parameters or added any redundant ones, please correct me.
Also, is it possible to control the effect just by the angle with which the sides of the rectangle should be projected on the projection plane (the angle between the projected rectangle's side and its top edge) provided that the
new height remains some constant value?