Reverse vsg::LineSegmentIntersector

[hasanshamsijeze]

I am using VSG, and I have a class called vsg::LineSegmentIntersector, which works by drawing a vertical line when a click occurs on the screen and returns any object that intersects with that line. This allows me to figure out which object was clicked on.

Now, I want to implement the opposite: I have a node and its coordinates, and I want to know from which point on the screen I need to draw a vertical line so that it intersects with the given object.

[robertosfield]

This really isn't a "reverse line segment intersector" but I understand what you mean.

The way I would tackle it:

1. Compute the position in eye coordinates by multiplying the object position by the modelview matrix, this model part of the matrix comes form accumulating the matrices in the scene graph, and the view comes from the Camera's ViewMatrix.
2. Create a vertical line geometry and place it under a vsg::AbsoluteTransform and place this in your view. Then update the absolute transforms translation to match the horizontal positing in eye space that you calculated in 1.
3. Each frame run calculation in 1. and update the absolute transform to make sure the positions keep matching as the view changes.