>
> Date: Thu, 23 Sep 1999 18:20:28 +0800
> From: Justin Couch <[EMAIL PROTECTED]>
> Subject: Re: view model (by trial and error)
>
> catching up on some past issues.
>
> Kevin Rushforth wrote:
>
> > >OK, bitch time :) Why is it that Java3D always seems to want to do
> > >things differently to every other Java API?
>
> Firstly appologies. I read back on this and everything really did come
> out wrong. I'll blame being highly stressed with my project release
> (available for hire, one slightly warn-out programmer. Will work for
> peanuts :P ), so I'll try to explain the point a little more clearly
> again.
>
> > Because we're not talking about 2D pixel coordinates. The values in
> > question are floating-point values specified in a right-handed 3D
> > coordinate system (image-plate coordinates) with the origin at the
> > lower left corner and Y increasing up, X increasing to the right, and Z
> > increasing out of the screen. Whenever Java 3D specifies anything in
> > pixel space, we always use the AWT convention of Y-down.
IMHO, the coordinate system named by AWT is wrong, wrong, wrong.
Using X-Y to refer to the AWT display coordinates implies that the
system is either a right handed or a left handed coordinate system.
It is neither. It is what can be called a "matrix" coordinate system
where the coordinates are expressed as row-column or even
vertical-horizontal or something, anything, besides X-Y. Such
misnaming of coordinates has existed for decades (I started in image
processing 20+ years ago) and has been causing confusion for decades.
One would think that after all these years we could at least come up
with an unambiguous and consistent way to name coordinates.
Defining "display space" as a right-handed coordinate system sounds
like a good thing to me because as Justin points out it keeps all the
spaces consistent -- world, view, and display. Getting AWT to change
won't happen. Is it worth changing in Java 3D? Perhaps not. We've
lived with such confusion all these decades and spent many an hour in
the wee morning trying to figure out why things are going up instead
of down. Why change now?
--jon
>
> I understand that, but the problem is that you generally are thinking
> about how you represent items in standard screen space when you are
> projecting things relative to the window. For example, you are rendering
> 2D stuff in the postSwap() method over the standard screen using the
> normal conventions, but you need to flick the thinking around to working
> out how to move the projections relative to the screen when trying to
> get things to match. Now granted, this is not something that a
> "beginner" J3D programmer would be doing, but I feel that we've got a
> real potential for problems here.
>
> As Jon mentioned, he wants it relative to the window so that the
> projection can be moved off center and then other things rendered around
> it (for example, using Mixed Mode rendering to put in the other
> objects). I'd like to know that I always set my "center of view" to be X
> pixels offset from the corner of the window (Canvas3D). This allows me
> to be consistent with the UI as the user may resize the Canvas through
> resizing the window. Considering a lot of the other problems that exist
> at the moment, re-calculating the exact floating point values from pixel
> coordinates is going to be a tricky prospect at best. At a minimum I
> could see jumping around of the 3D rendering due to the floating point
> inaccuracy (Is it possible to do this back projection from pixel to the
> camera coordinate system for the relative values? haven't checked)
>
> I'm not sure that the proposed solution will acheive the desired effect
> simply because you are not dealing with things in the user space and
> coordinate system (particularly in regard to the code/toolkit that Jon
> is working on).
>
--
____________________ Peculiar Technologies ____________________
Jon Barrilleaux 3800 Lake Shore Ave. Purveyors of
[EMAIL PROTECTED] Oakland, CA 94610 Alternate Reality
510.444.4370 voc Augmented Simulation
510.444.0231 fax www.augsim.com and 3D Solutions
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