Ray picking gives inversed results

Question

I'm trying to do a simple ray picking implementation to allow a user to select a cell in a grid rendered in 3D. I'm almost there, but I've ran into an issue which I can't seem to solve.

Consider the following image:

The line represents the ray. The circled part is where the mouse is, and the line's green tip is where it intersects with the grid (XZ plane). When I move the mouse around, the green tip moves almost opposite of the mouse.

What is causing this?

I am converting screen coordinates to OpenGL coordinates using gluUnproject.

After obtaining the direction and position of the mouse, I use the properties of similar triangles to obtain the XZ interception. The ratio between the direction y and position y is the same for both x and z axis. Hence multiplying the direction x, z by the ratio and using y=0 should give the location of the XZ plane intersection. Essentially:

Ray ray = ...
float ratio = ray.point.y / ray.dir.y;
Vector3 intercept = new Vector3(ray.dir.x * ratio, 0, ray.dir.z * ratio);

And yet, as the image shows, its not. Is there something wrong with my reasoning, or something I've overlooked?

Answer 1

A couple possible sources of error:

1) Is the double-negative on the Y-axis intentional?

Ray ray = getPickingRay(Mouse.getX(), Display.getHeight() - Mouse.getY());
... 
float winY = (float) viewport.get(3) - (float) cursorY;

2) You're ignoring ray.point.x and ray.point.z in the final line

Vec3d intercept = new Vec3d(ray.dir.x * ratio, 0, ray.dir.z * ratio);