Previous: Hello SimpleApplication,
Next: Hello Assets.
When creating a 3D game, you start out with creating a scene and some objects. You place the objects (player tokens, obstacles, etc) in the scene, and move, resize, rotate, color, and animate them.
In this tutorial we will have a look at a simple 3D scene. You will learn that the 3D world is represented in a scene graph, and why the rootNode is important. You will learn how to create simple objects and how to transform them – move, scale, rotate. You will understand the difference between the two types of Spatials in the scene graph, Node and Geometry. For a visual introduction to the scene graph check out our Scene Graph for Dummies presentation.
package jme3test.helloworld;
import com.jme3.app.SimpleApplication;
import com.jme3.material.Material;
import com.jme3.math.Vector3f;
import com.jme3.scene.Geometry;
import com.jme3.scene.shape.Box;
import com.jme3.math.ColorRGBA;
import com.jme3.scene.Node;
/** Sample 2 - How to use nodes as handles to manipulate objects in the scene graph.
* You can rotate, translate, and scale objects by manipulating their parent nodes.
* The Root Node is special: Only what is attached to the Root Node appears in the scene. */
public class HelloNode extends SimpleApplication {
public static void main(String[] args){
HelloNode app = new HelloNode();
app.start();
}
@Override
public void simpleInitApp() {
// create a blue box at coordinates (1,-1,1)
Box( new Vector3f(1,-1,1), 1,1,1);
Geometry blue = new Geometry("Box", box1);
Material mat1 = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
mat1.setColor("Color", ColorRGBA.Blue);
blue.setMaterial(mat1);
// create a red box straight above the blue one at (1,3,1)
Box( new Vector3f(1,3,1), 1,1,1);
Geometry red = new Geometry("Box", box2);
Material mat2 = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
mat2.setColor("Color", ColorRGBA.Red);
red.setMaterial(mat2);
// create a pivot node at (0,0,0) and attach it to root
Node pivot = new Node("pivot");
rootNode.attachChild(pivot);
// attach the two boxes to the *pivot* node!
pivot.attachChild(blue);
pivot.attachChild(red);
// rotate pivot node: Both boxes have rotated!
pivot.rotate( 0.4f , 0.4f , 0.0f );
}
}
Build and run the code sample. You should see two colored boxes tilted at the same angle.
In this tutorial, you will learn some new terms:
So what exactly happens in this code snippet? Note that we are using the simpleInitApp()
method that was introduced in the first tutorial.
Box( new Vector3f(1,-1,1), 1,1,1); Geometry blue = new Geometry("Box", box1); Material mat1 = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md"); mat1.setColor("Color", ColorRGBA.Blue); blue.setMaterial(mat1);
Box( new Vector3f(1,3,1), 1,1,1); Geometry red = new Geometry("Box", box2); Material mat2 = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md"); mat2.setColor("Color", ColorRGBA.Red); red.setMaterial(mat2);
Node pivot = new Node("pivot"); rootNode.attachChild(pivot);
pivot.attachChild(blue); pivot.attachChild(red);
pivot.rotate( 0.4f , 0.4f , 0.0f );
What has happened? We have attached two box Geometries to a Node. Then we used the Node as a handle to grab the two boxes and transform (rotate) both, in one step. This is a common task and you will use this method a lot in your games when you move game characters around.
You work with two types of Spatials in your scenegraph: Nodes and Geometries. Here is the difference:
Geometry | Node | |
---|---|---|
Visibility: | A visible 3-D object. | An invisible "handle". |
Purpose: | A Geometry stores an object's looks. | A Node groups Geometries and other Nodes together. |
Examples: | A box, a sphere, player, a building, a piece of terrain, a vehicle, missiles, NPCs, etc… | The default rootNode , the guiNode (for on-screen text); a floor node, a custom vehicle-with-passengers node, an audio node, etc… |
Task? | Solution! |
---|---|
Create a Spatial | Create a shape and give it a Material. For instance a box shape:Box(Vector3f.ZERO, 1, 1, 1); Geometry thing = new Geometry("thing", mesh); Material mat = new Material(assetManager, "Common/MatDefs/Misc/ShowNormals.j3md"); thing.setMaterial(mat); |
Make an object appear in the scene | Attach the Spatial to the rootNode , or to any node that is attached to the rootNode.rootNode.attachChild(thing); |
Remove objects from the scene | Detach the Spatial from the rootNode , and from any node that is attached to the rootNode.rootNode.detachChild(thing); rootNode.detachAllChildren(); |
Find a Spatial in the scene by the object's name or ID | Look at the node's children.Spatial thing = rootNode.getChild("thing"); Spatial twentyThird = rootNode.getChild(22); |
Specify what should be loaded at the start | Everything you initialize and attach to the rootNode in the simpleInitApp() method is part of the scene at the start of the game. |
There are three types of 3D transformation: Translation (moving), Scaling (resizing), and Rotation (turning).
Task? | Solution! | X | Y | Z |
---|---|---|---|---|
Position and move objects | Translation: Specify the new location in three dimensions: right/left, up/down, forward/backward. Example 1. To move an object to specific coordinates, such as (0,40.2f,-2), use: thing.setLocalTranslation( new Vector3f( 0.0f, 40.2f, -2.0f ) ); thing.move( 0.0f, 40.2f, -2.0f ); | right/left | up/down | forward/ backward |
Resize objects | Scaling: To resize a Spatial, specify the scale factor in each dimension: length, height, width. A value between 0.0f and 1.0f will shrink the object; a value bigger than 1.0f will make it grow; and 1.0f will keep this dimension the same. Using the same value for each dimension scales an object proportionally, using different values stretches it. Example: Make it 10 times longer, one tenth of the height, same width: thing.setLocalScale( 10.0f, 0.1f, 1.0f ); thing.scale( 10.0f, 0.1f, 1.0f ); | length | height | width |
Turn objects | Rotation: 3-D rotation is a bit tricky (learn details here). In short: You can rotate around three axes, pitch, yaw, and roll. Important: You do not specify the rotation in degrees from 0° to 360°, but in radians from 0.0f to 6.28f (FastMath.PI*2) ! Example: To roll an object 180° around the z axis: thing.rotate( 0f , 0f , FastMath.PI ); If you do want to specify angles in degrees then multiply your degrees value with FastMath.DEG_TO_RAD thing.rotate( 0f , 0f , 180*FastMath.DEG_TO_RAD ); Tip: If your game idea calls for a serious amount of rotations, it is worth looking into quaternions, a data structure that can combine and store rotations efficiently. thing.setLocalRotation( new Quaternion(). fromAngleAxis(FastMath.PI/2, new Vector3f(1,0,0))); | pitch | yaw | roll |
If you get unexpected results, check whether you made the following common mistakes:
Problem? | Solution! |
---|---|
Created Geometry does not appear in scene | Have you attached it to (a node that is attached to) the rootNode? Does it have a Material? What is its translation (position)? Is it covered up by another Geometry? Is it too far from the camera? try cam.setFrustumFar(111111f); |
Spatial rotates wrong | Did you use radian values, and not degrees? (if you used degrees multiply them with FastMath.DEG_TO_RAD to get them converted to radians) Did you rotate the intended pivot node? Did you rotate around the right axis? |
Geometry has an unexpected Material | Did you reuse a Material from another Geometry and have inadvertently changed its properties? (if so, maybe consider cloning: mat2 = mat.clone(); ) |
You have learned that the 3D world is a Scene Graph of Spatials: Visible Geometries and invisible Nodes. You can transform Spatials, or attach them to nodes and transform the nodes.
Since standard shapes like spheres and boxes get old fast, continue with the next chapter where you learn to load assets, such as 3-D models.