Mogre Basic Tutorial 1

From Ogre Wiki

Beginner Tutorial 1: The SceneNode, Entity, and SceneManager constructs

Contents 1 Prerequisites 2 Introduction 3 Getting Started 3.1 Initial Code 3.2 Ogre vs Mogre Properties and Functions 4 How Ogre Works 4.1 SceneManager Basics 4.2 Entity Basics 4.3 SceneNode Basics 5 Your first Ogre application 6 Coordinates and Vectors 7 Adding another Object 8 Entities more in Depth 9 SceneNodes more in Depth 10 Things to Try 10.1 Scale 10.2 Rotations 11 Conclusions 11.1 Final Note 11.2 Source Code

Prerequisites

This tutorial assumes you have knowledge of C# programming and are able to setup and compile a Mogre application (if you have trouble setting up your application, see Mogre Basic Tutorial 0 for project setup). NO knowledge of Ogre is assumed for this tutorial outside of what is contained in the setup guide.

Introduction

In this tutorial I will be introducing you to the most basic Ogre constructs: SceneManager, SceneNode, and Entity objects. We will not cover a large amount of code; instead I will be focusing on the general concepts for you to begin learning Ogre.

As you go through the tutorial you should be slowly adding code to your own project and watching the results as we build it. There is no substitute for actual programming to get familiar with these concepts! Resist the urge to simply read along.

Getting Started

Initial Code

We will be using a pre-constructed code base for this tutorial. In a later tutorial we will go in depth explaining how Ogre applications work, and what is going on "Under the Hood" in MogreFramework. For now we will be starting at the most basic level, so create a C# windows application, and replace all the code in program.cs with the following code:

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using MogreFramework;
using Mogre;

namespace Tutorial01
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            try
            {
                OgreWindow win = new OgreWindow();
                new SceneCreator(win);
                win.Go();
            }
            catch (System.Runtime.InteropServices.SEHException)
            {
                if (OgreException.IsThrown)
                    MessageBox.Show(OgreException.LastException.FullDescription, "An Ogre exception has occurred!");
                else
                    throw;
            }
        }
    }

    class SceneCreator
    {
        public SceneCreator(OgreWindow win)
        {
            win.SceneCreating += new OgreWindow.SceneEventHandler(SceneCreating);
        }

        void SceneCreating(OgreWindow win)
        {
        }
    }
}

Compile and run the program in debug mode. You should see a splash screen with loading information, then a black window which contains nothing. If you are having trouble with this (or recieve an exception during startup, go back and double check you have followed all of the instructions in Mogre Basic Tutorial 0. If this does not solve your problem, post your question (with a detailed description of the problem) to the Mogre forums.

Ogre vs Mogre Properties and Functions

Throughout this series of tutorials I will be linking to the Ogre API reference. You should keep in mind that Mogre is a C# binding of the C++ Ogre graphics engine, and as such the functions have been changed to match the C# coding standards. Functions begin with an initial caps instead of lower case letters. Get and set functions have been removed entirely in favor of properties. For example, the C++ functions "setAmbientLight" and "getAmbientLight" have been changed to just be the "AmbientLight" property. When in doubt, use intellisense to figure out what a function call should be.

How Ogre Works

A broad topic. We will start with SceneManagers and work our way to Entities and SceneNodes. These three classes are the fundamental building blocks of Ogre applications.

SceneManager Basics

Everything that appears on the screen is managed by the SceneManager (fancy that). When you place objects in the scene, the SceneManager is the class which keeps track of their locations. When you create Cameras to view the scene (which we will cover in a later tutorial) the SceneManager keeps track of them. When you create planes, billboards, lights...and so on, the SceneManager keeps track of them.

There are multiple types of SceneManagers. There are SceneManagers that render terrain, there is a SceneManager for rendering BSP maps, and so on. You can see the various types of SceneManagers listed here. We will cover more about other SceneManagers as we progress through the tutorials.

Entity Basics

An Entity is one of the types of object that you can render on a scene. You can think of an Entity as being anything that's represented by a 3D mesh. A robot would be an entity, a fish would be an entity, the terrain your characters walk on would be a very large entity. Things such as Lights, Billboards, Particles, Cameras, etc would not be entities.

One thing to note about Ogre is that it separates renderable objects from their location and orientation. This means that you cannot directly place an Entity in a scene. Instead you must attach the Entity to a SceneNode object, and this SceneNode contains the information about location and orientation.

SceneNode Basics

As already mentioned, SceneNodes keep track of location and orientation for all of the objects attached to it. When you create an Entity, it is not ever rendered on the scene until you attach it to a SceneNode. Similarly, a SceneNode is not an object that is displayed on the screen. Only when you create a SceneNode and attach an Entity (or other object) to it is something actually displayed on the screen.

SceneNodes can have any number of objects attached to them. Let's say you have a character walking around on the screen and you want to have him generate a light around him. The way you do this would be to first create a SceneNode, then create an Entity for the character and attach it to the SceneNode. Then you would create a Light object and attach it to the SceneNode. SceneNodes may also be attached to other SceneNodes which allows you to create entire hierarchies of nodes. We will cover more advanced uses of SceneNode attachment in a later tutorial.

One major concept to note about SceneNodes is that a SceneNode's position is always relative to its parent SceneNode, and each SceneManager contains a root node to which all other SceneNodes are attached.

Your first Ogre application

Now go back to the code we created earlier. Find the SceneCreator.SceneCreating method. We will only be manipulating the contents of this method in this tutorial. The first thing we want to do is set the ambient light for the scene so that we can see what we are doing. We do this by setting the AmbientLight property and specifying what color we want. Note that the ColourValue constructor expects values for red, green, and blue in the range between 0 and 1. Add this line to SceneCreating:

            SceneManager mgr = win.SceneManager;
            mgr.AmbientLight = new ColourValue(1, 1, 1);

The next thing we need to do is create an Entity. We do this by calling the SceneManager's CreateEntity method:

            Entity ent = mgr.CreateEntity("Robot", "robot.mesh");

Ok several questions should pop up. First of all, where did the SceneManager come from, and what are the parameters we are calling the function with? The SceneManager we are using has already been set up for us by the MogreFramework assembly. We will delve into what goes on in the MogreFramework in a later tutorial. The first parameter to SceneManager's CreateEntity is the name of the Entity we are creating. All entities must have a unique name. You will get an error if you try to create two entities with the same name. The "robot.mesh" parameter specifies the mesh we want to use for the Entity. Again, the mesh that we are using has been preloaded for us by MogreFramework.

Now that we have created the Entity, we need to create a SceneNode to attach it to. Since every SceneManager has a root SceneNode, we will be creating a child of that node:

            SceneNode node = mgr.RootSceneNode.CreateChildSceneNode("RobotNode");

This statement first gets the RootSceneNode of the current SceneManager, and then it calls the CreateChildSceneNode method of the root SceneNode. The parameter to CreateChildSceneNode is the name of the SceneNode we are creating. Like the Entity class, no two SceneNodes can have the same name. We can omit the name parameter for SceneNodes if we choose to do so, and a default name will be assigned to the node.

Finally, we need to attach the Entity to the SceneNode so that the Robot has a location to be rendered at:

            node.AttachObject(ent);

And that's it! Run your application in debug mode. You should see a robot standing on the screen.

Coordinates and Vectors

Before we go any further, we need to talk about screen coordinates and Ogre Vector objects. Ogre (like many graphics engines) uses the x and z axis as the horizontal plane, and the y axis as your vertical axis. As you are looking at your monitor now, the x axis would run from the left side to the right side of your monitor, with the right side being the positive x direction. The y axis would run from the bottom of your monitor to the top of your monitor, with the top being the positive y direction. The z axis would run into and out of your screen, with out of the screen being the positive z direction.

Notice how our Robot is facing along the positive x direction? This is a property of the mesh itself, and how it was designed. Ogre makes no assumptions about how you orient your models. Each mesh that you load may have a different "starting direction" which it is facing.

Ogre uses the Vector class to represent both position and direction (there is no Point class). There are vectors defined for 2 (Vector2), 3 (Vector3), and 4 (Vector4) dimensions, with Vector3 being the most commonly used. If you are not familiar with Vectors, I suggest you brush up on it before doing anything serious with Ogre. The math behind Vectors will become very useful when you start working on complex programs.

Adding another Object

Now that you understand how the coordinate systems work, we can go back to our code. In the three lines that we have written, nowhere did we specify the exact location that we want our Robot to appear at. A large majority of the functions in Ogre have default parameters for them. For example, the SceneNode::createChildSceneNode method in Ogre has three parameters: the name of the SceneNode, the position of the SceneNode, and the initial rotation (orientation) the SceneNode is facing. The position, as you can see, has been set for us to the coordinates (0, 0, 0). Let's create another SceneNode, but this time we'll specify the starting location to be something other than the origin:

            Entity ent2 = mgr.CreateEntity("Robot2", "robot.mesh");
            SceneNode node2 = mgr.RootSceneNode.CreateChildSceneNode("RobotNode2", new Vector3(50, 0, 0));
            node2.AttachObject(ent2);

This should look familiar. We have done the exact same thing as before, with two exceptions. First of all, we have named the Entity and SceneNode something slightly different. The second thing we have done is specified that the starting position will be 50 units in the x direction away from the root SceneNode (remember that all SceneNode positions are relative to their parents). Run the program in debug mode. Now there are two robots side-by-side.

Entities more in Depth

The Entity class is very extensive, and I will not be covering how to use every portion of the object here...just enough to get you started. There are a few immediately useful methods and properties in Entity that I'd like to point out.

The first is Entity::setVisible and Entity::isVisible (which is wrapped into Entity.Visible in Mogre). You can set any Entity to be visible or not by simply setting "someEntity.Visible = false;". If you need to hide an Entity, but later display it, then call this function instead of destroying the Entity and later recreating it. Note that you don't need to "pool" Entities up. Only one copy of any object's mesh and texture are ever loaded into memory, so you are not saving yourself much by trying to save them. The only thing you really save is the creation and destruction costs for the Entity object itself, which is relatively low.

The Name property returns the name of Entity, and the ParentSceneNode property returns the SceneNode that the Entity is attached to.

SceneNodes more in Depth

The SceneNode class is very complex. There are a lot of things that can be done with a SceneNode, so we'll only cover some of the most useful.

You can get and set the position of a SceneNode with the Position attribute (this is always relative to the parent SceneNode). You can move the object relative to its current position by adding and subtracting from this Property.

SceneNodes not only set position, but they also manage scale and rotation of the object as well. You can set the scale of an object with Scale function. You can use the Yaw, Roll, and Pitch functions to rotate objects. You can use ResetOrientation to reset all rotations done to the object. You can also use the Orientation property, and the Rotate function for more advanced rotations. We will not be covering Quaternions until a much later tutorial though.

You have already seen the AttachObject function. These related functions and properties are also useful if you are looking to manipulate the objects that are attached to a SceneNode: NumAttachedObjects, GetAttachedObject (there are multiple versions of this function), DetachObject (also multiple versions), DetachAllObjects. There are also a whole set of functions for dealing with parent and child SceneNodes as well.

Since all positions/translating is done relative to the parent SceneNode, we can make two SceneNodes move together very easily. We currently have this code in application:

            Entity ent = mgr.CreateEntity("Robot", "robot.mesh");
            SceneNode node = mgr.RootSceneNode.CreateChildSceneNode("RobotNode");
            node.AttachObject(ent);

            Entity ent2 = mgr.CreateEntity("Robot2", "robot.mesh");
            SceneNode node2 = mgr.RootSceneNode.CreateChildSceneNode("RobotNode2", new Vector3(50, 0, 0));
            node2.AttachObject(ent2);

If we change the 5th line from this:

            SceneNode node2 = mgr.RootSceneNode.CreateChildSceneNode("RobotNode2", new Vector3(50, 0, 0));

To this:

            SceneNode node2 = node.CreateChildSceneNode("RobotNode2", new Vector3(50, 0, 0));

Then we have made RobotNode2 a child of RobotNode. Moving node1 will move node2 along with it, but moving node2 will not affect node1. For example this code would move only RobotNode2:

            node2.Position += new Vector3(10, 0, 10);

The following code would move RobotNode, and since RobotNode2 is a child of RobotNode, RobotNode2 would be moved as well:

            node.Position += new Vector3(25, 0, 0);

If you are having trouble with this, the easiest thing to do is to start at the root SceneNode and go downwards. Let's say (as in this case), we started node1 and (0, 0, 0) and translated it by (25, 0, 0), thus node1's position is (25, 0, 0) relative to its parent. node2 started at (50, 0, 0) and we translated it by (10, 0, 10), so its new position is (60, 0, 10) relative to its parent.

Now let's figure out where these things really are. Start at the root SceneNode. Its position is always (0, 0, 0). Now, node1's position is (root + node1): (0, 0, 0) + (25, 0, 0) = (25, 0, 0). Not surprising. Now, node2 is a child of node1, so its position is (root + node1 + node2): (0, 0, 0) + (25, 0, 0) + (60, 0, 10) = (85, 0, 10). This is just an example to explain how to think about SceneNode position inheritance. You will rarely ever need to calculate the absolute position of your nodes.

Lastly, note that you can get both SceneNodes and Entities by their name by calling GetSceneNode and GetEntity methods of the SceneManager, so you don't have to keep a pointer to every SceneNode you create. You should hang on to the ones you use often though.

Things to Try

By now you should have a basic grasp of Entities, SceneNodes, and the SceneManager. I suggest starting with the code above and adding and removing Robots from the scene. Once you have done that try out some of the following:

Scale

You can scale the mesh by calling the scale method in SceneNode. Try changing the values in scale and see what you get (place this at the end of your CreateScene method):

            node.Scale(.5f, 1, 2);
            node2.Scale(1, 2, 1);

Rotations

You can rotate the object by using the Yaw, Pitch, and Roll methods using either Degree or Radian objects. Pitch is rotation around the x axis, yaw is around the y axis, and roll is around the z axis. Using your right hand as a guide: point your thumb to the axis, the other fingers points to the positive angles. For example, pitch(Degree(90)), point your thumb to right, the other fingers show the direction of the rotation. Imagine closing a box lid.

Try changing the Degree amount and combining multiple transforms:

            Entity ent3 = mgr.CreateEntity("Robot3", "robot.mesh");
            SceneNode node3 = mgr.RootSceneNode.CreateChildSceneNode("RobotNode3", new Vector3(100, 0, 0));
            node3.AttachObject(ent3);

            node.Pitch(new Degree(90));
            node2.Yaw(new Degree(90));
            node3.Roll(new Degree(90));

Conclusions

Final Note

By this point you should have a very basic grasp of the SceneManager, SceneNode, and Entity classes. You do not have to be familiar with all of the functions that I have given reference to. Since these are the most basic objects, we will be using them very often. You will get more familiar with them after working through the next few tutorials.

Source Code

You can see the final state of this tutorial here: Mogre Basic Tutorial 1 Source

Proceed to Mogre Basic Tutorial 2 Cameras, Lights, and Shadows