Hi guys

I have a ray to heightmap terrain(2Dtexture) collision project that acts as a light dimmer switch from point 0(bottom of terrain) to point 30 (the very top of the terrain) where if the mouse ray is at the bottom of the terrain (point 0) the background color is black and as you move up the terrain the background becomes lighter and lighter until it reaches its lightest point(point 30). the problem that Im having is trying to modify

the code(below) to change to a different color based on certain heights for example if from point 0-10 background blue, if from point 10-20 red, etc,...Ive provided the code below, if anyone could point me in the right direction on how this modification would be done it would be much appreciated.

Thankyou

The main game code

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;
using Microsoft.Xna.Framework.Net;
using Microsoft.Xna.Framework.Storage;

namespace COLLISION
{
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        GraphicsDevice device;
        BasicEffect basicEffect;
        QuakeCamera fpsCam;
        CoordCross cCross;
        Vector3 pointerPos;
        Vector2 pointerScreenPos;
        Texture2D crosshair;
        SpriteBatch spriteBatch;

        Terrain terrain;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {
            fpsCam = new QuakeCamera(GraphicsDevice.Viewport, new Vector3(-10, 50, 10), -MathHelper.PiOver4, -MathHelper.Pi/8.0f);
            pointerScreenPos = new Vector2(GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2);
            base.Initialize();
        }        

        protected override void LoadContent()
        {
            device = graphics.GraphicsDevice;
            spriteBatch = new SpriteBatch(device);
            basicEffect = new BasicEffect(device, null);            
            cCross = new CoordCross(device);
            crosshair = Content.Load<Texture2D>("cross");

            terrain = new Terrain(device, Content);            
        }

        protected override void UnloadContent()
        {
        }

        protected override void Update(GameTime gameTime)
        {
            GamePadState gamePadState = GamePad.GetState(PlayerIndex.One);
            if (gamePadState.Buttons.Back == ButtonState.Pressed)
                this.Exit();

            MouseState mouseState = Mouse.GetState();
            KeyboardState keyState = Keyboard.GetState();
            if (keyState.IsKeyDown(Keys.Escape))
                this.Exit(); // Exit with ESC key
#if XBOX
            pointerScreenPos.X += gamePadState.ThumbSticks.Left.X*2.0f;
            pointerScreenPos.Y -= gamePadState.ThumbSticks.Left.Y*2.0f;
#else
            pointerScreenPos = new Vector2(mouseState.X, mouseState.Y);
#endif
            Ray pointerRay = GetPointerRay(pointerScreenPos);

            Ray clippedRay = ClipRay(pointerRay, 30, 0);
            Ray shorterRay = LinearSearch(clippedRay);
            pointerPos = BinarySearch(shorterRay);
            
            base.Update(gameTime);
        }

        private Ray GetPointerRay(Vector2 pointerPosition)
        {
            Vector3 nearScreenPoint = new Vector3(pointerPosition.X, pointerPosition.Y, 0);
            Vector3 farScreenPoint = new Vector3(pointerPosition.X, pointerPosition.Y, 1);

            Vector3 near3DWorldPoint = device.Viewport.Unproject(nearScreenPoint, fpsCam.ProjectionMatrix, fpsCam.ViewMatrix, Matrix.Identity);
            Vector3 far3DWorldPoint = device.Viewport.Unproject(farScreenPoint, fpsCam.ProjectionMatrix, fpsCam.ViewMatrix, Matrix.Identity);

            Vector3 pointerRayDirection = far3DWorldPoint - near3DWorldPoint;
            Ray pointerRay = new Ray(near3DWorldPoint, pointerRayDirection);

            return pointerRay;
        }

        private Ray ClipRay(Ray ray, float highest, float lowest)
        {
            Vector3 oldStartPoint = ray.Position;

            float factorH = -(oldStartPoint.Y-highest) / ray.Direction.Y;
            Vector3 pointA = oldStartPoint + factorH * ray.Direction;

            float factorL = -(oldStartPoint.Y-lowest) / ray.Direction.Y;
            Vector3 pointB = oldStartPoint + factorL * ray.Direction;

            Vector3 newDirection = pointB - pointA;
            return new Ray(pointA, newDirection);
        }

        private Ray LinearSearch(Ray ray)
        {
            ray.Direction /= 50.0f;

            Vector3 nextPoint = ray.Position + ray.Direction;
            float heightAtNextPoint = terrain.GetExactHeightAt(nextPoint.X, -nextPoint.Z);
            while (heightAtNextPoint < nextPoint.Y)
            {
                ray.Position = nextPoint;

                nextPoint = ray.Position + ray.Direction;
                heightAtNextPoint = terrain.GetExactHeightAt(nextPoint.X, -nextPoint.Z);
            }
            return ray;
        }

        private Vector3 BinarySearch(Ray ray)
        {
            float accuracy = 0.01f;
            float heightAtStartingPoint = terrain.GetExactHeightAt(ray.Position.X, -ray.Position.Z);
            float currentError = ray.Position.Y - heightAtStartingPoint;
            int counter = 0;
            while (currentError > accuracy)
            {
                ray.Direction /= 2.0f;
                Vector3 nextPoint = ray.Position + ray.Direction;
                float heightAtNextPoint = terrain.GetExactHeightAt(nextPoint.X, -nextPoint.Z);
                if (nextPoint.Y > heightAtNextPoint)
                {
                    ray.Position = nextPoint;
                    currentError = ray.Position.Y - heightAtNextPoint;
                }
                if (counter++ == 1000) break;
            }
            return ray.Position;
        }

        protected override void Draw(GameTime gameTime)
        {
            float lowestHeight = 0;
            float highestHeight = 30;
            float interp = (pointerPos.Y - lowestHeight) / (highestHeight - lowestHeight);
            byte colorValue = (byte)(255.0f * interp);
            Color clearColor = new Color(colorValue, colorValue, colorValue);
            device.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, clearColor, 1, 0);

            cCross.Draw(fpsCam.ViewMatrix, fpsCam.ProjectionMatrix);

            //draw terrain
            terrain.Draw(Matrix.Identity, fpsCam.ViewMatrix, fpsCam.ProjectionMatrix);

            spriteBatch.Begin(SpriteBlendMode.AlphaBlend, SpriteSortMode.Deferred, SaveStateMode.SaveState);
            spriteBatch.Draw(crosshair, pointerScreenPos, null, Color.White, 0, new Vector2(7, 7), 1, SpriteEffects.None, 0);
            spriteBatch.End();

            Window.Title = pointerPos.Y.ToString();

            base.Draw(gameTime);
        }
    }
}

The terrain code

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Storage;

namespace COLLISION
{
    public class Terrain
    {
        private GraphicsDevice device;
        private ContentManager content;
        private BasicEffect basicEffect;

        private Texture2D grassTexture;
        private float[,] heightData;
        private VertexBuffer terrainVertexBuffer;
        private IndexBuffer terrainIndexBuffer;
        private VertexDeclaration terrainVertexDeclaration;
        
        public Terrain(GraphicsDevice device, ContentManager content)
        {
            this.device = device;
            this.content = content;
            basicEffect = new BasicEffect(device, null);
            
            Texture2D heightMap = content.Load<Texture2D>("heightmap");                                    
            LoadHeightData(heightMap);

            grassTexture = content.Load<Texture2D>("grass");
            
            VertexPositionNormalTexture[] terrainVertices = CreateTerrainVertices();
            int[] terrainIndices = CreateTerrainIndices();
            terrainVertices = GenerateNormalsForTriangleStrip(terrainVertices, terrainIndices);
            CreateBuffers(terrainVertices, terrainIndices);

            terrainVertexDeclaration = new VertexDeclaration(device, VertexPositionNormalTexture.VertexElements);
        }

        //constructor allowing a Terrain to be created without having a link to the device
        public Terrain(VertexBuffer vb, IndexBuffer ib, BasicEffect basicEffect, Texture2D grassTexture, float[,] heightData)
        {
            terrainVertexBuffer = vb;
            terrainIndexBuffer = ib;
            this.basicEffect = basicEffect;
            this.grassTexture = grassTexture;
            this.heightData = heightData;
        }

        private void LoadHeightData(Texture2D heightMap)
        {
            float minimumHeight = 255;
            float maximumHeight = 0;

            int width = heightMap.Width;
            int height = heightMap.Height;

            Color[] heightMapColors = new Color[width * height];
            heightMap.GetData<Color>(heightMapColors);

            heightData = new float[width, height];
            for (int x = 0; x < width; x++)
                for (int y = 0; y < height; y++)
                {
                    heightData[x, y] = heightMapColors[x + y * width].R;
                    if (heightData[x, y] < minimumHeight) minimumHeight = heightData[x, y];
                    if (heightData[x, y] > maximumHeight) maximumHeight = heightData[x, y];
                }

            for (int x = 0; x < width; x++)
                for (int y = 0; y < height; y++)
                    heightData[x, y] = (heightData[x, y] - minimumHeight) / (maximumHeight - minimumHeight) * 30.0f;
        }

        private VertexPositionNormalTexture[] CreateTerrainVertices()
        {
            int width = heightData.GetLength(0);
            int height = heightData.GetLength(1);
            VertexPositionNormalTexture[] terrainVertices = new VertexPositionNormalTexture[width * height];

            int i = 0;            
            for (int z = 0; z < height; z++)
            {
                for (int x = 0; x < width; x++)
                {
                    Vector3 position = new Vector3(x, heightData[x, z], -z);
                    Vector3 normal = new Vector3(0, 0, 1);
                    Vector2 texCoord = new Vector2((float)x / 30.0f, (float)z / 30.0f);

                    terrainVertices[i++] = new VertexPositionNormalTexture(position, normal, texCoord);
                }
            }

            return terrainVertices;
         }

        private int[] CreateTerrainIndices()
        {
            int width = heightData.GetLength(0);
            int height = heightData.GetLength(1);
            
            int[] terrainIndices = new int[(width)*2*(height-1)];
            
            int i = 0;
            int z = 0;
            while (z < height-1)
            {
                for (int x = 0; x < width; x++)
                {
                    terrainIndices[i++] = x + z * width;
                    terrainIndices[i++] = x + (z + 1) * width;
                }
                z++;

                if (z < height-1)
                {
                    for (int x = width - 1; x >= 0; x--)
                    {
                        terrainIndices[i++] = x + (z + 1) * width;
                        terrainIndices[i++] = x + z * width;
                    }
                }
                z++;
            }

            return terrainIndices;
        }

        private VertexPositionNormalTexture[] GenerateNormalsForTriangleStrip(VertexPositionNormalTexture[] vertices, int[] indices)
        {
            for (int i = 0; i < vertices.Length; i++)
                vertices[i].Normal = new Vector3(0, 0, 0);

            bool swappedWinding = false;
            for (int i = 2; i < indices.Length; i++)
            {
                Vector3 firstVec = vertices[indices[i - 1]].Position - vertices[indices[i]].Position;
                Vector3 secondVec = vertices[indices[i - 2]].Position - vertices[indices[i]].Position;
                Vector3 normal = Vector3.Cross(firstVec, secondVec);
                normal.Normalize();
                
                if (swappedWinding)
                    normal *= -1;

                if (!float.IsNaN(normal.X))
                {
                    vertices[indices[i]].Normal += normal;
                    vertices[indices[i - 1]].Normal += normal;
                    vertices[indices[i - 2]].Normal += normal;
                }

                swappedWinding = !swappedWinding;
            }

            for (int i = 0; i < vertices.Length; i++)
                vertices[i].Normal.Normalize();

            return vertices;
        }

        private void CreateBuffers(VertexPositionNormalTexture[] vertices, int[] indices)
        {
            terrainVertexBuffer = new VertexBuffer(device, VertexPositionNormalTexture.SizeInBytes * vertices.Length, BufferUsage.WriteOnly);
            terrainVertexBuffer.SetData(vertices);

            terrainIndexBuffer = new IndexBuffer(device, typeof(int), indices.Length, BufferUsage.WriteOnly);
            terrainIndexBuffer.SetData(indices);
        }

        public void Draw(Matrix worldMatrix, Matrix viewMatrix, Matrix projectionMatrix)
        {
            int width = heightData.GetLength(0);
            int height = heightData.GetLength(1);

            GraphicsDevice device = terrainVertexBuffer.GraphicsDevice;

            //draw terrain            
            basicEffect.World = worldMatrix;
            basicEffect.View = viewMatrix;
            basicEffect.Projection = projectionMatrix;
            basicEffect.Texture = grassTexture;
            basicEffect.TextureEnabled = true;
            
            basicEffect.EnableDefaultLighting();
            Vector3 lightDirection = new Vector3(1, -1, 1);
            lightDirection.Normalize();
            basicEffect.DirectionalLight0.Direction = lightDirection;
            basicEffect.DirectionalLight0.Enabled = true;
            basicEffect.AmbientLightColor = new Vector3(0.3f, 0.3f, 0.3f);
            basicEffect.DirectionalLight1.Enabled = false;
            basicEffect.DirectionalLight2.Enabled = false;
            basicEffect.SpecularColor = new Vector3(0, 0, 0);
            
            basicEffect.Begin();
            foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)
            {
                pass.Begin();

                device.Vertices[0].SetSource(terrainVertexBuffer, 0, VertexPositionNormalTexture.SizeInBytes);
                device.Indices = terrainIndexBuffer;
                device.VertexDeclaration = terrainVertexDeclaration;
                device.DrawIndexedPrimitives(PrimitiveType.TriangleStrip, 0, 0, width * height, 0,width*2*(height-1)-2);

                pass.End();
            }
            basicEffect.End();
        }

        private void DrawWithCustomEffect()
        {
            int width = heightData.GetLength(0);
            int height = heightData.GetLength(1);

            device.Vertices[0].SetSource(terrainVertexBuffer, 0, VertexPositionNormalTexture.SizeInBytes);
            device.Indices = terrainIndexBuffer;
            device.VertexDeclaration = terrainVertexDeclaration;
            device.DrawIndexedPrimitives(PrimitiveType.TriangleStrip, 0, 0, width * height, 0, width * 2 * (height - 1) - 2);
        }
        
        public float GetClippedHeightAt(float x, float z)
        {
            bool invalid = x < 0;
            invalid |= z < 0;
            invalid |= x > heightData.GetLength(0) - 1;
            invalid |= z > heightData.GetLength(1) - 1;
            if (invalid)
                return 10;            
            else
                return heightData[(int)x, (int)z];
        }

        public float GetExactHeightAt(float xCoord, float zCoord)
        {
            bool invalid = xCoord < 0;
            invalid |= zCoord < 0;
            invalid |= xCoord >= heightData.GetLength(0) - 1;
            invalid |= zCoord >= heightData.GetLength(1) - 1;
            if (invalid)
                return 10;
                        
            int xLower = (int)xCoord;
            int xHigher = xLower + 1;
            float xRelative = (xCoord - xLower) / ((float)xHigher - (float)xLower);

            int zLower = (int)zCoord;
            int zHigher = zLower + 1;
            float zRelative = (zCoord - zLower) / ((float)zHigher - (float)zLower);

            float heightLxLz = heightData[xLower, zLower];
            float heightLxHz = heightData[xLower, zHigher];
            float heightHxLz = heightData[xHigher, zLower];
            float heightHxHz = heightData[xHigher, zHigher];

            bool cameraAboveLowerTriangle = (xRelative + zRelative < 1);

            float finalHeight;
            if (cameraAboveLowerTriangle)
            {
                finalHeight = heightLxLz;
                finalHeight += zRelative * (heightLxHz - heightLxLz);
                finalHeight += xRelative * (heightHxLz - heightLxLz);
            }
            else
            {
                finalHeight = heightHxHz;
                finalHeight += (1.0f - zRelative) * (heightHxLz - heightHxHz);
                finalHeight += (1.0f - xRelative) * (heightLxHz - heightHxHz);
            }

            return finalHeight;
        }
    }
}