P3D Re: Bruce's Challenge - III

["Greg Wageman" <gjw@xxxxxxxxxx>]

From: Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>

>By increasing the stereo base you introduce more depth.  More depth
>usually means closer.  So things look closer.  They look closer but
>their on-film size has not changed.  So they must be smaller.
>Another way to see that:  If the stereo pair is recorded with a stereo
>base wider than the eyes, upon viewing the observer is transformed
>into a giant whose eyes are spaced wider apart (to match the camera
>lens positions).  But it is usually easier to think of the scene
>shrinking into a small model than thinking ourselves grown into
>giants.


This is a highly subjective description of what happens.  I think Bruce
was looking for some more quantifiable information.  I have been
thinking about this myself, and here's what I've discovered.

The way I see it is that the perpendicular between the ocular axes
(lenses or eyes) forms the base of a triangle, with the viewed object at
the apex.  Draw a normal from the base to the object, and you have two
similar triangles (eye to nose to object).  If you increase the base of
this triangle (i.e. increase the interocular or interaxial),  you have
recreated the angular convergence for an object which would be much
closer using a normal interocular or interaxial.  So it appears that
enlarging the stereobase emphasizes the vergence depth cue.

I agree with the *result* the good Doctor describes, however: the
"Lilliputian" effect *is* subjective.  We "think" that, since the
objects are so "close" (based on vergence), they "must" therefore be
small, because in our learned experience we only view small objects with
such angles.