public void UpdateOcclusion(Matrix View, Matrix Projection)
        {
            // The sun is infinitely distant, so it should not be affected by the
            // position of the camera. Floating point math doesn't support infinitely
            // distant vectors, but we can get the same result by making a copy of our
            // view matrix, then resetting the view translation to zero. Pretending the
            // camera has not moved position gives the same result as if the camera
            // was moving, but the light was infinitely far away. If our flares came
            // from a local object rather than the sun, we would use the original view
            // matrix here.
            Matrix infiniteView = View;

            infiniteView.Translation = Vector3.Zero;

            // Project the light position into 2D screen space.
            Viewport viewport = graphicsDevice.Viewport;

            Vector3 projectedPosition = viewport.Project(-LightDirection, Projection,
                                                         infiniteView, Matrix.Identity);

            // Don't draw any flares if the light is behind the camera.
            if ((projectedPosition.Z < 0) || (projectedPosition.Z > 1))
            {
                lightBehindCamera = true;
                return;
            }

            lightPosition = new Vector2(projectedPosition.X, projectedPosition.Y);
            lightBehindCamera = false;

            if (occlusionQueryActive)
            {
                // If the previous query has not yet completed, wait until it does.
                if (!occlusionQuery.IsComplete)
                    return;

                // Use the occlusion query pixel count to work
                // out what percentage of the sun is visible.
                const float queryArea = querySize * querySize;

                occlusionAlpha = Math.Min(occlusionQuery.PixelCount / queryArea, 1);
                Console.WriteLine(occlusionAlpha);
            }

            // Set renderstates for drawing the occlusion query geometry. We want depth
            // tests enabled, but depth writes disabled, and we disable color writes
            // to prevent this query polygon actually showing up on the screen.
            graphicsDevice.BlendState = ColorWriteDisable;
            graphicsDevice.DepthStencilState = DepthStencilState.DepthRead;

            // Set up our BasicEffect to center on the current 2D light position.
            basicEffect.World = Matrix.CreateTranslation(lightPosition.X,
                                                         lightPosition.Y, 0);

            basicEffect.Projection = Matrix.CreateOrthographicOffCenter(0,
                                                                        viewport.Width,
                                                                        viewport.Height,
                                                                        0, 0, 1);

            basicEffect.CurrentTechnique.Passes[0].Apply();

            // Issue the occlusion query.
            occlusionQuery.Begin();

            graphicsDevice.DrawUserPrimitives(PrimitiveType.TriangleStrip, queryVertices, 0, 2);

            occlusionQuery.End();

            occlusionQueryActive = true;
        }