> Date: Mon, 28 Oct 2002 19:34:24 -0500
> From: Kit Fuhrman <[EMAIL PROTECTED]>
>
> I'm working on a research project that needs to implement Java3D into a
> Head Mounted Projective Display(HMPD). In the HMPD there are two LCD,
> one for each eye. The disparity between objects is used to give the
> effect of depth. I have combed through the documentation a number of
> times and have become confused over Java3D's implementation of stereo
> viewing.
Your best bet is to try using the ConfiguredUniverse utility class available in
Java 3D 1.3. Read the documentation available in "The Java 3D Configuration
File" from the link referenced in the class overview for ConfiguredUniverse and
try to get something working. Then, if ConfiguredUniverse doesn't fully meet
your needs, set out to implement your application without using the utility.
I'll try to answer some of your questions briefly below.
> In my code I have two windows, each with a Canvas3D object. I use the
> setMonoscopicViewPolicy(View.XXXXX_EYE_VIEW) to define which eye is
> which. Then I use the physical body to set the inter-pupilary
> distance. Will this work correctly?
Yes, assuming everything else is specified correctly. If you're using a head
tracker you'll also need to set HeadToHeadTracker appropriately.
> Also the coordinates I set for the left and right eyes are supposedly
> in meters, but it seems like that is too much of a disparity for
> correct stereo viewing. Could this be a hardware issue or is their
> something in the Java3D that I'm not grasping?
All physical units in Java 3D are in meters for consistency. The typical
interpupillary distance is about 0.066 meters. If this appears to create too
much disparity, then some other aspect of the configuration is probably wrong.
These might include the screen (ImagePlate) dimensions, positions &
orientation, or perhaps the ScreenScale factor.
> Also what is the setMonoscopicViewPolicy's relation to the method below?
> java -Dj3d.stereo=PREFERRED HelloUniverse
> What is the difference? Is one for shutter glasses and the other for
> disparity left and right eye disparity viewing?
Yes, that's right. The j3d.stereo property selects whether or not
SimpleUniverse should try to get a stereo canvas. By "stereo canvas" we mean a
canvas that supports quad buffering, i.e., a separate color buffer and depth
buffer for each eye. These are then displayed in "field sequential" mode, so
that the frame buffer for the right eye is displayed while the shutter glasses
blank out the left eye and vice versa. This is the default CYCLOPEAN_EYE_VIEW
policy.
Unless you have a HMD that supports field-sequential stereo (the Sony LDI-D100B
is the only one I've come across), you'll need two separate video streams
generated by monoscopic canvases on separate screens for each eye, and
setMonoscopicViewPolicy({LEFT,RIGHT}_EYE_VIEW) is exactly how you do that.
You should create a full-screen Canvas3D for each physical screen, then
configure the Screen3D parameters of each Canvas3D according to the optics of
your head-mounted display. These would include PhysicalScreenWidth and
PhysicalScreenHeight. Note, however, that for a HMD, these parameters refer to
the *apparent* width and height of the screens as viewed through the HMD
optics. So if the HMD manufacturer says you get a field of view of 60 degrees
for each eye with a focal plane of 1 meter apparent distance, use those
specifications to compute the screen width and height for each eye at the focal
plane. You can't rely on the Java 3D default screen metrics (90 pixels/inch)
for a HMD.
You will also need to set HeadTrackerToLeftImagePlate and
HeadTrackerToRightImagePlate for the appropriate Screen3D's, even if you don't
use head tracking. Again, these are the *apparent* positions and orientations
of the screens as viewed through the HMD optics, and must be computed from the
manufacturer specifications. This allows Java 3D to compute the apparent
orientation and position of each screen relative to the eye positions specified
in the PhysicalBody. If you're not using head tracking, set HeadToHeadTracker
to identity and then the HeadTrackerTo{Left,Right}ImagePlate transforms
effectively become HeadTo{Left,Right}ImagePlate. The origin of the
PhysicalBody frame of reference is the origin of "head" coordinates, which is
the midpoint directly between the left and right eyes.
Most HMD optics have 100% overlap between the left and right images, so the
HeadTrackerTo{Left,Right}ImagePlate transforms should be the same.
Finally, be sure to set the View's ViewPolicy to HMD_VIEW to switch the Java 3D
viewing model from fixed-screen mode to head-mounted mode.
> Would it be better for me to simply use two different view platforms
> for the each eye?
No, the view platform is part of the frame of reference in which both eye and
screen positions are defined. When you move and orient the view platform, both
eyes and screens move along with it in the virtual world. If you use head
tracking, then head movements are effectively defined relative to the position
and orientation of the view platform.
> I've been working on this application for a while, and any help would
> be great appreciated.
HMD viewing is fairly complicated since it exposes a part of the Java 3D view
model that most programmers don't need to worry too much about: the
correspondence between the physical world and the virtual world. The physical
metrics such as interpupilary distance, image plate dimensions, and the screen
scaling are critical for success.
Stereo disparity problems in particular are usually related to an inappropriate
screen scale; the Java 3D default is set up for the typical desktop window
environment, mapping the [-1.0 .. +1.0] range in virtual coordinates to the
width of the Canvas3D. In monoscopic applications you can just move the view
platform closer or further away and get an image of suitable scale, but in
stereo you might wind up trying to converge your eyes on an object that's only
an inch away from your nose in physical coordinates. You can set the screen
scale explicitly by setting the View's ScreenScalePolicy to SCALE_EXPLICIT.
Hope that helps for now -- Mark Hood
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