This is pretty straightforward. You can use gluProject
. It will take a given modelview, projection, and viewport transform, and a 3D point, and apply the inverse and spit out a 2D point in window coordinates for you (apologies for minor typos, just typing this here):
double myX = ..., myY = ..., myZ = ...; // your object's 3d coordinates
double[] my2DPoint = new double[2]; // will contain 2d window coords when done
double[] modelview = new double[16];
double[] projection = new double[16];
int[] viewport = new int[4];
gl.glGetDoublev(GL2.GL_MODELVIEW_MATRIX, modelview, 0);
gl.glGetDoublev(GL2.GL_PROJECTION_MATRIX, projection, 0);
gl.glGetIntegerv(GL2.GL_VIEWPORT, viewport, 0);
glu.gluProject(myX, myY, myZ, modelview, 0, projection, 0,
viewport, 0, my2DPoint, 0);
// now my2DPoint[0] is window x, and my2DPoint[1] is window y
After you do this, you'll have your 3D point in 2D window coordinates. Then simply switch your projection over to a 2D orthogonal projection, in window pixels, and draw your HUD in 2D space.
For performance, if you have multiple HUD items to draw per frame; just get the modelview/projection/viewport once per frame (or, even better, invalidate your cached ones if you change them and re-query only as needed) and reuse them in subsequent calls to gluProject
.