Scene Graphs

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The SceneGraph is used to manage the spatial representation of objects. It is a tree structure in which the transformations of the parent nodes are applied to the child nodes.

Car SceneGraph

Diagram

Usage

info

The assets used in this example can be downloaded here

We start by including the required namespaces.

using MonoGame.Extended;
using MonoGame.Extended.SceneGraphs;
using MonoGame.Extended.Sprites;
using MonoGame.Extended.ViewportAdapters;

Next, we declare our SceneGraph and The SceneNodes

private SceneNode _carNode;
private SceneNode _hoveredNode;
private SceneNode _leftWheelNode;
private SceneNode _rightWheelNode;
private SceneGraph _sceneGraph;

Which we initialize in the LoadContent function

_sceneGraph = new SceneGraph();

_carNode = new SceneNode("car-hull", GraphicsDevice.Viewport.Bounds.Center.ToVector2());

var carHullTexture = Content.Load<Texture2D>("car-hull");
var carHullSprite = new Sprite(carHullTexture);
_carNode.Entities.Add(new SpriteEntity(carHullSprite));

var carWheelTexture = Content.Load<Texture2D>("car-wheel");
var carWheelSprite = new Sprite(carWheelTexture);

_leftWheelNode = new SceneNode("left-wheel", new Vector2(-29, 17));
_leftWheelNode.Entities.Add(new SpriteEntity(carWheelSprite));

_rightWheelNode = new SceneNode("right-wheel", new Vector2(40, 17));
_rightWheelNode.Entities.Add(new SpriteEntity(carWheelSprite));

_carNode.Children.Add(_rightWheelNode);
_carNode.Children.Add(_leftWheelNode);
_sceneGraph.RootNode.Children.Add(_carNode);

Updating

First we declare a _speed field that is used to update the SceneGraph

private float _speed = 0.15f;

Then, we add the following code to the Update function to update the potition of the Car and the rotation of the Wheels

var keyboardState = Keyboard.GetState();
var mouseState = Mouse.GetState();

if (keyboardState.IsKeyDown(Keys.W))
    _speed += (float)gameTime.ElapsedGameTime.TotalSeconds * 0.5f;

if (keyboardState.IsKeyDown(Keys.S))
    _speed -= (float)gameTime.ElapsedGameTime.TotalSeconds * 0.5f;

_leftWheelNode.Rotation += _speed;
_rightWheelNode.Rotation = _leftWheelNode.Rotation;
_carNode.Position += new Vector2(_speed * 5, 0);

We check the collision detection of the car in the following way

const int maxX = 535;
if (_carNode.Position.X >= maxX)
{
    _speed = -_speed * 0.2f;
    _carNode.Position = new Vector2(maxX, _carNode.Position.Y);
}
const int minX = 265;
if (_carNode.Position.X <= minX)
{
    _speed = -_speed * 0.2f;
    _carNode.Position = new Vector2(minX, _carNode.Position.Y);
}

Drawing

We use the following code in our Draw function to draw the SceneGraph and collision walls

GraphicsDevice.Clear(Color.CornflowerBlue);

_spriteBatch.Begin(samplerState: SamplerState.PointClamp);

_spriteBatch.Draw(_sceneGraph);
_spriteBatch.FillRectangle(0, 266, 800, 240, Color.DarkOliveGreen);
_spriteBatch.FillRectangle(200, 0, 5, 480, Color.DarkOliveGreen);
_spriteBatch.FillRectangle(595, 0, 5, 480, Color.DarkOliveGreen);

_spriteBatch.End();

Getting SceneNode at position

First we create a field _hoveredNode in which we store the Node.

private SceneNode _hoveredNode;

Which we then assign in the Update function

_hoveredNode = _sceneGraph.GetSceneNodeAt(new Vector2(mouseState.X, mouseState.Y));

Finally, We add the following code between _spriteBatch Begin and End to draw it

if (_hoveredNode != null)
{
    var boundingRectangle = _hoveredNode.BoundingRectangle;
    _spriteBatch.DrawRectangle(boundingRectangle, Color.Black);
    _spriteBatch.DrawString(_bitmapFont, _hoveredNode.Name, new Vector2(14, 2), Color.White);
}