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c# opentk - converting How can I convert screen coordscoordinatess to world coordscoordinates in OpenTK?

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c# opentk - converting screen coords to world coords

I'm making a windows forms application with opengl view. I need to get the mouse coords converted to the opengl world coords. Well, my Y coord gets converted wrong. It's hard to explain, so here is the video: http://tinypic.com/r/23sal8k/6. I'm constantly pressing left mouse button and the red dot should be where the mouse is(but it isn't). Here is the code:

    private void glview_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            Point worldCoords = convertScreenToWorldCoords(e.X, e.Y);
            shitx = worldCoords.X;
            shity = worldCoords.Y;
        }
    }


    // functions:
    public static Point convertScreenToWorldCoords(int x, int y)
    {
        int[] viewport = new int[4];
        Matrix4 modelViewMatrix, projectionMatrix;
        GL.GetFloat(GetPName.ModelviewMatrix, out modelViewMatrix);
        GL.GetFloat(GetPName.ProjectionMatrix, out projectionMatrix);
        GL.GetInteger(GetPName.Viewport, viewport);
        Vector2 mouse;
        mouse.X = x;
        mouse.Y = viewport[3] - y;
        Vector4 vector = UnProject(ref projectionMatrix, modelViewMatrix, new Size(viewport[2], viewport[3]), mouse);
        Point coords = new Point((int)vector.X, (int)vector.Y);
        return coords;
    }
    public static Vector4 UnProject(ref Matrix4 projection, Matrix4 view, Size viewport, Vector2 mouse)
    {
        Vector4 vec;

        vec.X = 2.0f * mouse.X / (float)viewport.Width - 1;
        vec.Y = -(2.0f * mouse.Y / (float)viewport.Height - 1);
        vec.Z = 0;
        vec.W = 1.0f;

        Matrix4 viewInv = Matrix4.Invert(view);
        Matrix4 projInv = Matrix4.Invert(projection);

        Vector4.Transform(ref vec, ref projInv, out vec);
        Vector4.Transform(ref vec, ref viewInv, out vec);

        if (vec.W > float.Epsilon || vec.W < float.Epsilon)
        {
            vec.X /= vec.W;
            vec.Y /= vec.W;
            vec.Z /= vec.W;
        }

        return vec;
    }