Hi! I want to use gemwin (GEM library) on a stereo figure. What units do the point parameters, e.g. first three parameters of view (<view_x> <view_y> <view_z>) have? I am asking because I want to use a head mounted display and I want to present a steroe figure in a defined distance. Michael
Michael Mihocic wrote:
Hi! I want to use gemwin (GEM library) on a stereo figure. What units do the point parameters, e.g. first three parameters of view (<view_x> <view_y> <view_z>) have? I am asking because I want to use a
the position parameters are given in cubits (or was it fathoms?), which happens to be the same unit as [translate] works with.
head mounted display and I want to present a steroe figure in a defined distance.
also note that if you are loading a model, the coordinates of the vertices will be automagically converted from meters to fathoms (or was it cubits?).
happy measuring
nmfg.asdr. IOhannes
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
----- Original Message ----- From: "IOhannes m zmoelnig" zmoelnig@iem.at To: "Michael Mihocic" mihocic@kfs.oeaw.ac.at Cc: "pd-list list" pd-list@iem.at Sent: Tuesday, February 14, 2006 9:41 AM Subject: Re: [PD] GEM position parameters
Michael Mihocic wrote:
Hi! I want to use gemwin (GEM library) on a stereo figure. What units do the point parameters, e.g. first three parameters of view (<view_x> <view_y> <view_z>) have? I am asking because I want to use a
the position parameters are given in cubits (or was it fathoms?), which happens to be the same unit as [translate] works with.
head mounted display and I want to present a steroe figure in a defined distance.
also note that if you are loading a model, the coordinates of the vertices will be automagically converted from meters to fathoms (or was it cubits?).
happy measuring
nmfg.asdr. IOhannes
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__
On Tue, 14 Feb 2006, Michael Mihocic wrote:
What units do the point parameters, e.g. first three parameters of view (<view_x> <view_y> <view_z>) have? I am asking because I want to use a head mounted display and I want to present a steroe figure in a defined distance.
Whatever the units are in the 3-D space, they cancel out with the flattening: meters/meter, cubits/cubit, inches/inch, all those are non-units. This is because a point in 3-D space (x,y,z) becomes (k*x/z,k*y/z). The constant "k" can be imagined to be in whichever units you want, but in GEM it tends to be some fraction of the window's size in pixels.
PS: The formula I just gave assumes the simplest point of view of being at the origin looking at a screen that cuts the positive z axis at a right angle. There are other points of view but the units still cancel out.
_ _ __ ___ _____ ________ _____________ _____________________ ... | Mathieu Bouchard - tél:+1.514.383.3801 - http://artengine.ca/matju | Freelance Digital Arts Engineer, Montréal QC Canada
Mathieu Bouchard a écrit :
Whatever the units are in the 3-D space, they cancel out with the flattening: meters/meter, cubits/cubit, inches/inch, all those are non-units. This is because a point in 3-D space (x,y,z) becomes (k*x/z,k*y/z). The constant "k" can be imagined to be in whichever units you want, but in GEM it tends to be some fraction of the window's size in pixels.
The constant k is ... well, 4 is the width (or height, I forget) of the Gem window.
aalex
Strangely, the height.
Tom
On 2/14/06, Alexandre Quessy listes@sourcelibre.com wrote:
Mathieu Bouchard a écrit :
Whatever the units are in the 3-D space, they cancel out with the flattening: meters/meter, cubits/cubit, inches/inch, all those are non-units. This is because a point in 3-D space (x,y,z) becomes (k*x/z,k*y/z). The constant "k" can be imagined to be in whichever units you want, but in GEM it tends to be some fraction of the window's size in pixels.
The constant k is ... well, 4 is the width (or height, I forget) of the Gem window.
aalex
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