Added assignment 6 directory and worksheet

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+/** CSci-4611 Assignment 6: Harold
+ */
+
+#ifndef BILLBOARDS_H_
+#define BILLBOARDS_H_
+
+#include <mingfx.h>
+using namespace mingfx;
+
+#include "ground.h"
+
+/** Billboards are strokes planted in the ground that rotate automatically to
+ face the viewer.  This class holds the entire collection of billboard strokes.
+ */
+class Billboards {
+public:
+    Billboards();
+    virtual ~Billboards();
+    
+    void Init(ShaderProgram *stroke3d_shaderprog);
+    
+    /// Projects a 2D normalized screen point (e.g., the mouse position in normalized
+    /// device coordinates) to a 3D point on a plane defined by an "origin", which can
+    /// really be any point coincident with the plane, and the plane normal.  Returns
+    /// true if the screen point projects onto the plane and stores the result in
+    /// plane_point.  Returns false if the screen point does not project onto the plane.
+    bool ScreenPtHitsPlane(const Matrix4 &view_matrix, const Matrix4 &proj_matrix,
+                         const Point3 &plane_origin, const Vector3 &plane_normal,
+                         const Point2 &normalized_screen_pt, Point3 *plane_point);
+    
+    
+    /// Checks to see if a ray starting at the eye point and passing through 2D
+    /// normalized screen point projects onto any of the billboards in the scene. If
+    /// so, returns the id of the closest billboard intersected.  If not, returns -1.
+    int IntersectBillboard(const Matrix4 &view_matrix, const Matrix4 &proj_matrix,
+                           const Point2 &normalized_screen_pt);
+    
+    
+    /// Adds a new stroke as a billboard by projecting it onto a plane parallel
+    /// to the filmplane that intersects with the anchor point, which should lie
+    /// on the ground.
+    void AddBillboardStroke(const Matrix4 &view_matrix, const Matrix4 &proj_matrix,
+                            const std::vector<Point2> &stroke2d, const Mesh &stroke2d_mesh,
+                            const Color &stroke_color, Ground *ground_ptr);
+    
+    /// Edits an existing billboard by adding an additional stroke to it.
+    void AddToBillboard(const Matrix4 &view_matrix, const Matrix4 &proj_matrix,
+                        int billboard_id, const Mesh &stroke2d_mesh,
+                        const Color &stroke_color);
+    
+    
+    /// Adjusts the transformation matrix used to draw each billboard so that the
+    /// billboard will face the camera.  Needs to be called each time the camera
+    /// is moved within the scene.
+    void UpdateBillboardRotations(const Point3 &current_eye_point);
+    
+    
+    /// Draws all of the billboards
+    void Draw(const Matrix4 &view_matrix, const Matrix4 &proj_matrix);
+    
+private:
+
+    // each billboard stores the following
+    struct Billboard {
+        Mesh mesh;
+        Color color;
+        Point3 anchor_pt;
+        AABB bounding_box;
+        Matrix4 transform;
+    };
+
+    // the array of all active billboards
+    std::vector<Billboard> billboards_;
+    
+    ShaderProgram *stroke3d_shaderprog_;
+};
+
+#endif