Java3D transformation from 3D to current view 2D coordinate

Question

I'm trying to add a 2D overlay for a 3D scene in Java3D, part of this overlay is to draw a line from a 2D object to a corresponding point in the 3D scene...

Searched transformation from 3D to 2D and read those threads:

• Translate Java 3D coordinates to 2D screen coordinates

• 3D to 2D projection

from code inside walrus: https://github.com/CAIDA/walrus/blob/master/H3ViewParameters.java

copied a method to a class extending Canvas3D:

public Transform3D getObjectToEyeTransform() {
    Point3d m_eye = new Point3d();
    getCenterEyeInImagePlate(m_eye);
    Transform3D m_imageToEye = new Transform3D();
    m_imageToEye.set(new Vector3d(-m_eye.x, -m_eye.y, 0.0));
    Transform3D m_vworldToImage = new Transform3D();
    getVworldToImagePlate(m_vworldToImage);

    Transform3D transform = new Transform3D(m_imageToEye);
    transform.mul(m_vworldToImage);
    //transform.mul(m_objectTransform);
    return transform;
}

and then in my overlay in method postRender i try to do the following:

Transform3D viewTrans3d = getObjectToEyeTransform();
Vector3d point = new Vector3d(1,1,1);
viewTrans3d.invert();
viewTrans3d.transform(point);
this.getGraphics2D().drawLine(0, 0, (int)point.x, (int)point.y);

Getting very weird line, which do change in a quite logical pattern (when i rotate and tilt the view) but far from what i expect...

Questions:

1. commented the m_objectTransform matrix multiplication because i don't understand its purpose, any idea?
2. Why do i need to invert the transform matrix? without the invert the results are even weirder...
3. Is there a simpler way to do this??? sounds like something solved eons ago...

Answer 1

This can be done by using getVworldToImagePlate and then getPixelLocationFromImagePlate in the Canvas3D class. For example:

public Point2d getPosition2d(Point3d point) {
    Transform3D transform = new Transform3D();
    getVworldToImagePlate(transform);
    transform.mul(objectTransform);

    Point3d newPoint = new Point3d(point);
    transform.transform(newPoint);

    Point2d point2d = new Point2d();
    getPixelLocationFromImagePlate(newPoint, point2d);
    return point2d;
}

The objectTransform variable should be the transform of any TransformGroup in the scene that is applied to the 3d objects that are displayed. If you don't have any TransformGroup , then you can leave this out. Also, the transform shouldn't be inverted, just use it as it is.