Desktop/Dev/Cameras/Cameras/LookAtCamera.cs

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using System;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace Mindshifter
{
    public sealed class LookAtCamera : ICamera
    {
        private Matrix              mViewMatrix;
        private Projection          mProjection;

        private Vector3             mEyePosition;
        private Vector3             mLookAtPosition;

        private bool                mRequiresUpdate;

        public Matrix ViewMatrix
        {
            get 
            {
                if (mRequiresUpdate)
                    BuildMatrix();

                return mViewMatrix;
            }
        }

        public Matrix ProjectionMatrix
        {
            get { return mProjection.Matrix; }
        }

        public Projection Projection
        {
            get { return mProjection; }
        }

        public BoundingFrustum Frustum
        {
            get { return new BoundingFrustum(ViewMatrix * ProjectionMatrix); }
        }

        public Vector3 Position
        {
            get { return mEyePosition; }
            set 
            { 
                mEyePosition = value;
                mRequiresUpdate = true;
            }
        }

        public Vector3 LookAtPosition
        {
            get { return mLookAtPosition; }
            set 
            { 
                mLookAtPosition = value;
                mRequiresUpdate = true;
            }
        }

        public LookAtCamera() 
            : this(Vector3.Zero, -Vector3.UnitZ)
        {

        }

        public LookAtCamera(Vector3 eye, Vector3 lookAt) 
            : this(eye, lookAt, (float)Math.PI / 4f, 4f / 3f, 1f, 1000f)
        {
            mEyePosition = eye;
            mLookAtPosition = lookAt;
        }

        public LookAtCamera(Vector3 eye, Vector3 lookAt, float fov, float aspect, float near, float far)
        {
            mEyePosition = eye;
            mLookAtPosition = lookAt;

            mProjection = new Projection(fov, aspect, near, far);

            mRequiresUpdate = true;
        }

        public LookAtCamera(Vector3 eye, Vector3 lookAt, Rectangle rect, float near, float far)
        {
            mEyePosition = eye;
            mLookAtPosition = lookAt;

            mProjection = new Projection(rect.X, rect.Y, rect.Width, rect.Height, near, far);

            mRequiresUpdate = true;
        }

        private void BuildMatrix()
        {
            mViewMatrix = Matrix.CreateLookAt(mEyePosition, mLookAtPosition, Vector3.Up);
            mRequiresUpdate = false;
        }

        public void Update(float elapsedSeconds)
        {
            if (mRequiresUpdate)
                BuildMatrix();
        }

        public Ray GetPickRay(Vector2 position, Viewport viewport)
        {
            Vector3 near = viewport.Unproject(new Vector3(position, 0f), ProjectionMatrix,
                                ViewMatrix, Matrix.Identity);

            Vector3 far = viewport.Unproject(new Vector3(position, 1f), ProjectionMatrix,
                                ViewMatrix, Matrix.Identity);

            return new Ray(near, Vector3.Normalize(far - near));
        }
    }
}

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