OK, now I'm confused. My s1 was the hero's transformed center, and my s2 was the mouse click's position.

The hero''s transformed center is the origin. I have a direction vector for the keyboard that's always directly ahead of the hero, and I have the direction vector of the mouse click. What exactly should I feed into the method and for that matter, what version of the method? Yours?

Thanks a lot. What about my suggestion of adding a Math class? Could be quite useful and wouldn't make jPCT cumbersome in any way.

And include methods like static float calculateAngle(SimpleVector origin, SimpleVector p1, SimpleVector p2)? Until then, if somebody would tell me the code for that I'd be grateful! : -)

OK, that'll take me a little while. It's a little humbling discussing this with you, dude!

Here's a question: why does the cube translate to the camera's position? And then, why does the camera see something whose center is the same as its?

Cool. I await your solution, pal. Thanks.

I guess I misunderstood the documentation, because it says it will assume z to be 1. And the hero should move on the x and z planes. So I figured the z value would instead be placed on the y coordinate (some orientation problem someone with more spacial perception than I would understand!). But you're right, of course, about the delta. It should be calculated on the call to transform (between destination and hero.getTransformedCenter()). The following still doesn't work:
SimpleVector heroCenter3D = hero.getTransformedCenter();
SimpleVector destination = Interact2D.reproject2D3D(theCamera, buffer, mousePoint.x, mousePoint.y);
hero.translate(destination.x-heroCenter3D.x, destination.y-heroCenter3D.y, destination.z-heroCenter3D.z);
Note that the hero shouldn't even move in the y axis anyway, but I suppose I would just change the y and z values later (after I at least saw the hero appearing to be in the right place).

I'm trying to make an RPG with 2D backgrounds and a 3D hero. When the player clicks on the screen, the hero has to move that way. The following's what I did (for translation, rotation is something else). Is it wrong, because it's not working?
SimpleVector heroCenter = Interact2D.project3D2D(theCamera, buffer, hero.getTransformedCenter());
SimpleVector destination = Interact2D.reproject2D3D(theCamera, buffer, mousePoint.x-(int)heroCenter.x, mousePoint.y-(int)heroCenter.y, hero.getTransformedCenter().z);
destination.z = destination.y;
destination.y = 0;
hero.translate(destination);