[PD] GEM position parameters

[Frank Barknecht]

Hallo,
Michael Mihocic hat gesagt: // Michael Mihocic wrote:

> where or how can I find out if they are given in cubits or fathoms?
> (factor 4 difference between the units)
> 
> if someone else is interested:
> 1 cubit = 0.25 fathoms = 45.72 cm

I believe, IOhannes was just being funny when he mentioned fathoms and
cubits. In fact, the unit is furlongs. 

No, don't believe that either. The truth us: OpenGL and GEM don't have
any units equivalent to realworld units, they are unitless. No meters,
no fathoms, no furlongs, just (0,0,0) or (1.2, 4.1, -10) or so.

To translate to real world coordinates, you need to do your own
scaling from the unitless GEM window (which in the default view
stretches from about x=-4 at the left to x=+4 at the right side, if
y==0) to the view presented in your head mounted display.

Also see the OpenGL-FAQ 9.010: 
http://www.opengl.org/resources/faq/technical/transformations.htm

  What are OpenGL coordinate units?
  
  The short answer: Anything you want them to be.
  
  Depending on the contents of your geometry database, it may be
  convenient for your application to treat one OpenGL coordinate unit
  as being equal to one millimeter or one parsec or anything in
  between (or larger or smaller).
  
  OpenGL also lets you specify your geometry with coordinates of
  differing values. For example, you may find it convenient to model
  an airplane's controls in centimeters, its fuselage in meters, and a
  world to fly around in kilometers. OpenGL's ModelView matrix can
  then scale these different coordinate systems into the same eye
  coordinate space.
  
  It's the application's responsibility to ensure that the Projection
  and ModelView matrices are constructed to provide an image that
  keeps the viewer at an appropriate distance, with an appropriate
  field of view, and keeps the zNear and zFar clipping planes at an
  appropriate range. An application that displays molecules in micron
  scale, for example, would probably not want to place the viewer at a
  distance of 10 feet with a 60 degree field of view.
  
Ciao
-- 
 Frank Barknecht                 _ ______footils.org_ __goto10.org__