w.r.t. the J3D Viewing architecture, I've read much of this already (the
spec and the class API - which were as you said not very helpful) but it
looks like I have a lot more reading to do for the next few days...
In a bold/naive leap into the dark, I extracted view and projection matrices
from ViewInfo and compared results between
1) using the normal mode
2) going into compatibility mode and putting the obtained matrices back in
to see if my renderings would be the same in both modes.
ViewInfo vInfo = new ViewInfo(view);
Transform3D e2vw = new Transform3D();
Transform3D e2cc = new Transform3D();
vInfo.getEyeToVworld(c3d, e2vw, null); // view matrix
vInfo.getProjection(c3d, e2cc, null); // projection matrix
e2vw.invert();
// see if it looks the same in normal mode or compatibility mode
// if I get the matrices and set them in compatibility mode
view.setCompatibilityModeEnable(true);
view.setVpcToEc(e2vw);
view.setLeftProjection(e2cc);
Of course the resulting renderings were different.
> You may want to modify getProjection() to not perform
> the perspective depth scaling of Z since you'll be dropping it anyway.
> The transform from vworld coordinates to eye coordinates goes from
> virtual coordinates to physical coordinates, so you may be encountering
> scaling problems as well
I'm looking at these items you mentioned, but I suppose that reading
ViewInfo.java will clear a lot of things up for me. There may be more
questions coming next week! :)
Thanks for your help,
Jason
> -----Original Message-----
> From: Mark Hood [EMAIL PROTECTED]
> Sent: Thursday, June 19, 2003 9:03 PM
> To: [EMAIL PROTECTED]
> Subject: Re: [JAVA3D] projection coordinates
>
> > Date: Thu, 19 Jun 2003 18:17:36 -0400
> > From: "Young, Jason" <[EMAIL PROTECTED]>
> >
> > This is a general graphics question but there really is a J3D aspect to
> it:
> > Namely how does _JAVA3D_ convert the point??? This is something I'm
> > investigating right now. Here is what I've come up with, take it with a
> > grain of salt as I'm still working out the code. I'm eager to hear other
> > people's responses, as I'm unsure of the sublteties myself. Anyway,
> here's
> > what I do:
>
> You could call Canvas3D.getVworldToImagePlate() and transform the vworld
> point to a 3D image plate point. Project it into an AWT pixel by calling
> Canvas3D.getPixelLocationFromImagePlate().
>
> If you have a scene with a dynamically changing view point, then you may
> run into the problem of the Canvas3D methods returning information for
> the previous frame instead of the current frame. You can work around
> this by using the methods in com.sun.j3d.utils.universe.ViewInfo
> (available in Java 3D 1.3.1), which directly query the application scene
> graph instead of the internal representations. (The current Java 3D
> architecture injects 1 or 2 frames of lag between the application state
> and the internal representations).
>
> Read the source code for ViewInfo if you want to understand how to do
> this on your own. You can get the source code for the utilities in the
> Java 3D SDK release. Although it was written to work around
> synchronization issues, ViewInfo is essentially an alternative public
> implementation of the Java 3D view model. It computes the same
> transforms as the private core classes, modulo any bugs.
>
> For a complete understanding of the Java 3D view model, I recommend the
> following:
>
> 1) read the official but nearly incomprehensible documentation
> in the Java 3D Specification and class javadocs
>
> 2) read the unofficial, slightly less incomprehensible description of
> the view model in the Java 3D Configuration File documentation
> (get there from the class overview of ConfiguredUniverse
> and ConfigContainer)
>
> 3) read the ViewInfo source code to see how it all really works.
>
>
> > - take the (x,y,z) point, transform it to (x',y',z') by the view and
> > projection matrices (if in compatibility mode, this is easy, if not,
> well
> > I'm working on that)
>
> After transforming by the view and projection matrices and dividing by
> W, you'll end up in 3D Euclidean clipping coordinates (ranging from
> [-1.0 .. +1.0] in each dimension). The X and Y dimensions are mapped
> anisotropically to the width and height of the Canvas3D, so, knowing the
> dimensions of the canvas, you can do simple scale/offsets of the
> normalized X and Y clip coordinates, dropping the Z, to get your 2D
> pixel.
>
> You can use the inverse of the transform returned by
> ViewInfo.getEyeToVworld() to get the view matrix, and
> ViewInfo.getProjection() to get the projection transform from eye
> coordinates to clipping coordinates. You may want to modify
> getProjection() to not perform the perspective depth scaling of Z since
> you'll be dropping it anyway.
>
> > - find the point of intersection between the line L and the plane P,
> where L
> > is formed between (x',y',z') and (0,0,0), and P is the plane z -1
>
> This is exactly what Canvas3D.getPixelLocationFromImagePlate() does,
> except it uses the eye position in image plate coordinates instead of
> (0, 0, 0), and projects to the image plate Z = 0 plane.
>
> But you don't need to do this if you have the projection and view
> matrices.
>
> > I have code to do all this but my results are slightly off. I have a
> point
> > that is rendered by J3D on the edge of the screen but by my calculations
> > should be about 6 pixels below the edge therefore not rendered. There's
> a
> > subtlety here that I'm missing. I think J3D does an additional small
> > transform, but I have no way of knowing other than that my math and code
> are
> > triple-checked but my results are still different from J3D's.
>
> You may be having problems with the projection matrix. In Java 3D it's
> defined from the location of the eye in physical coordinates (meters)
> and the location of the Canvas3D in physical coordinates. With the
> standard defaults the eye is centered in the canvas in X and Y, with a Z
> offset that produces the current field of view across the width of the
> canvas. The projection frustum extends from the eye to the edges of the
> canvas, with the fore and aft clip planes relative to the eye in meters
> (although this can be changed with different policies). The canvas
> doesn't necessarily map to the location of either the fore or aft clip
> plane.
>
> The transform from vworld coordinates to eye coordinates goes from
> virtual coordinates to physical coordinates, so you may be encountering
> scaling problems as well. The whole physical/virtual transform is
> confusing to many people since Java 3D doesn't use a camera view model,
> but instead computes views based on the positions of the eyes relative
> to the canvases, in order to support the calibrations needed for virtual
> reality applications. Again, read the ViewInfo source code for more
> understanding, and we can help you out with any more specific questions
> you may have.
>
> -- Mark Hood
>
> From: Young, Jason
> Sent: Thursday, June 19, 2003 6:18 PM
> To: 'Discussion list for Java 3D API'
> Subject: RE: [JAVA3D] projection coordinates
>
>
> This is a general graphics question but there really is a J3D aspect to
> it: Namely how does _JAVA3D_ convert the point??? This is something I'm
> investigating right now. Here is what I've come up with, take it with a
> grain of salt as I'm still working out the code. I'm eager to hear other
> people's responses, as I'm unsure of the sublteties myself. Anyway, here's
> what I do:
>
> - take the (x,y,z) point, transform it to (x',y',z') by the view and
> projection matrices (if in compatibility mode, this is easy, if not, well
> I'm working on that)
>
> - find the point of intersection between the line L and the plane P, where
> L is formed between (x',y',z') and (0,0,0), and P is the plane z -1
>
> - calculate the image plate dimensions
>
> - if that point of intersection is in the image plate, it should appear on
> your screen. Convert the line/plane intersection to screen coordinates
> with(0,0) in upper left corner. (I use java's AffineTransform)
>
> I have code to do all this but my results are slightly off. I have a point
> that is rendered by J3D on the edge of the screen but by my calculations
> should be about 6 pixels below the edge therefore not rendered. There's a
> subtlety here that I'm missing. I think J3D does an additional small
> transform, but I have no way of knowing other than that my math and code
> are triple-checked but my results are still different from J3D's.
>
> Anybody else have some thoughts?
>
>
>
>
> -----Original Message-----
> From: Dola Woolfe [mailto:[EMAIL PROTECTED]
> Sent: Sunday, June 15, 2003 3:08 AM
> To: [EMAIL PROTECTED]
> Subject: [JAVA3D] projection coordinates
>
>
> Hi,
>
> Can I find the (i, j) coordinates on the screen of a
> point with (x, y, z) coordinates in real space?
>
> Thanks!
>
> Dola
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