[JSR-184][3D编程指南]Part I: Quick jump into the world of Mobile Java 3D programming

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Introduction
To begin with I'd like you to know of a few links on the net that will be very helpful on your journey towards M3G land.

First of all, and probably most importantly, is the dedicated Mobile Java 3D web section on Sony Ericsson Developer World . Second, if you ever get stuck visit to the Sony Ericsson Mobile Java 3D forum . For everything else, use the Sony Ericsson Developer World portal , where you will find the answers to your questions and more.

Now that you know where to go if you get in trouble, let's proceed with the tutorial. The goal of this tutorial is to teach you how to set up your own 3D Canvas and make it render stuff on screen. To render models, I'll first show you how to load them and tell you about the tools that are available to create M3G models. Then we'll finish by manipulating the camera some so that we can walk around in our scene. I just want you to get warm in your seat and see how fast one can develop a 3D application with M3G, so this tutorial will be pretty fast and straight-forward with little in-depth explanation. The other parts of this series will explore the various M3G topics in detail.

Since the code is meant for educational purposes it isn't optimal nor does it cover all the errors the might occur. These are more advanced topics that will be addressed later on.

What you should know
Before you start reading this, you should know the basics of a MIDlet class and a Canvas class. This isn't a hard topic and if you feel lost, consult the source code (distributed with this tutorial) and check out the M3GMIDlet and M3GCanvas classes. It's also very good if you have some background in 3D programming/math, but it's not required.

The Canvas
When we develop in JSR 184 we will be using the MIDP 2.0 profile, which means we get a few great functions for free. Let's begin with setting up our Canvas. It is the same procedure as with a normal 2D Java game, you set up your MIDlet class, you start your Canvas and draw in your paint method. This is a fairly easy process and since you already should know this I'll just quickly skim through it. Let's first take a look at the header of the Canvas class, the import and variable declarations.

import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.game.GameCanvas;
import javax.microedition.M3G.Camera;
import javax.microedition.M3G.Graphics3D;
import javax.microedition.M3G.Light;
import javax.microedition.M3G.Loader;
import javax.microedition.M3G.Object3D;
import javax.microedition.M3G.Transform;
import javax.microedition.M3G.World;

/**
*
* @author Biovenger
* @version
*/
public class M3GCanvas
extends GameCanvas
implements Runnable {
    // Thread-control
    boolean running = false;
    boolean done = true;
   
    // If the game should end
    public static boolean gameOver = false;
   
    // Rendering hints
    public static final int STRONG_RENDERING_HINTS = Graphics3D.ANTIALIAS | Graphics3D.TRUE_COLOR | Graphics3D.DITHER;
    public static final int WEAK_RENDERING_HINTS = 0;
    public static int RENDERING_HINTS = STRONG_RENDERING_HINTS;
   
    // Key array
    boolean[] key = new boolean[5];
   
    // Key constants
    public static final int FIRE = 0;
    public static final int UP = FIRE + 1;
    public static final int DOWN = UP + 1;
    public static final int LEFT = DOWN + 1;
    public static final int RIGHT = LEFT + 1;

That was pretty basic stuff but let's quickly see what's going on. First of all we have a lot of imports, we're just importing all the classes that we're going to use in this tutorial and you can find their documentation in the general JSR 184 API javadoc. We also have some thread variables such as running and done, but those should be pretty self-explanatory.

Now, let's check out the rendering hints. These "hints" are ways of telling the mobile device just what kind of quality you want while rendering. However, since they are hints it's not guaranteed that the mobile device will act upon them. Here I define two different hints. Weak and Strong. As you see, the Strong rendering hints hold both anti-aliasing, true color and dithering. The weak holds no hints at all, which is basically the ugliest and fastest rendering you can get. As you see from the code, the hints can be combined by a simple logical OR. I'll talk more about hints in a later part of this tutorial series.

Next we have the key array, that's just a very simple array that holds which key was pressed. If you are curious as to how keys are processed, check out the source code of this example. Suffice to say: by querying if(key[UP]) you will find out if the UP key is pressed right now.

The M3G File Format
The JSR 184 standard has its own format, called M3G. This very versatile 3D format can hold tons of data such as models, lights, cameras, textures and even animation. Nifty! Not even is the format really good, it's also real easy to load into your application, more on that later though. Anyhow, I bet you're thinking "M3G? Never heard of it. How do I create an M3G file?" and even if you weren't thinking it, I'll explain. There are numerous ways to create M3G files:

1. First of all, the latest iteration of Discreet's 3D Studio Max has a built-in M3G exporter. Just hit the Export button and you can export your entire scene, animation, bones, materials and all, into an M3G file. However, many find Discreet's exporter a bit cumbersome and has some bugs, so for best results, use method 2.

2. HiCorp, who are also the providers of Sony Ericsson's JSR 184 implementation, have created very powerful exporters available to the 3 most popular 3D modeling programs. They're available for 3D Studio Max, LightWave and Maya. You can find them here>>

3. Blender, a powerful and free 3D modeling tool also has an M3G exporter available. However, it's still early in development and a bit buggy. Check out Blender here>>

So, how do we load these very powerful files into our program? Very easy. The JSR 184 contains a class called Loader, and it does exactly that, loads files. With one simple method call one can load all references from an M3G file. The method is called Loader.load and has two different argument lists. One takes an URL as a String and the other takes a raw byte array. Here's an example of how it is used.

Object3D[] objects = Loader.load("file.M3G");

Object3D[] objects2 = Loader.load(byteArray, offset);

The load method always returns an array of Object3Ds and there's a very good reason for it. The best one is that the Loader class can load much more than M3G files, it can basically deserialize any class that inherits from Object3D. However, you'll mostly use it for loading M3G files.

Now, I've created a simple M3G file, called map. M3G and I want to render it. To load this file we'll use the Loader. load method, however as you've just seen it returns an array of Object3D. We can't use the Object3D array for rendering as it is so we need to convert it to something that we can render to screen later on. In this tutorial we'll load the World node. The World node is the top node of the JSR 184 scene graph. It holds all kinds of information such as camera, lighting, background and a lot of meshes. I'll go through scene graphs and JSR 184's implementation of scene graphs in a later part of this series, for now you just need to know that the World class can hold a whole scene, and that's exactly what we want! Check out this method, it loads the World node from a M3G file.

/** Loads our world */
    private void loadWorld()
    {
        try
        {
            // Loading the world is very simple. Note that I like to use a
            // res-folder that I keep all files in. If you normally just put your
            // resources in the project root, then load it from the root.
            Object3D[] buffer = Loader.load("/res/map.M3G");
           
            // Find the world node, best to do it the "safe" way
            for(int i = 0; i < buffer.length; i++)
            {
                if(buffer[i] instanceof World)
                {
                    world = (World)buffer[i];
                    break;
                }
            }
           
            // Clean objects
            buffer = null;
        }
        catch(Exception e)
        {
            // ERROR!
            System.out.println("Loading error!");
            reportException(e);
        }
    }

As you can see, after we load the Object3D array with the Loader class, we just simply go through the entire array and look for the World note. This is the safest way of finding a World node. After we find our world node we just break out of the loop and clean our buffer (not that it's needed, as they'd automatically get cleared upon leaving the method. Good practice though.)

OK, now we have loaded our World node, which I've already told you is the top node of a scene graph and holds all scene information. Before I show you how easy it is to render, let's first extract a camera so that we can move around the world we just loaded.

Camera handling
We have our World node ready for rendering and now all we need is a camera, that we can move around in the world. If you remember, I've already told you that the World node holds Camera information, so we should be able to extract the camera from the World and manipulate it.

A camera in JSR 184 is described by the Camera class. This class makes it very easy to manipulate the camera in our 3D application with simple translation and orientation methods. The only two methods we'll be using in this example are the translate(float, float, float) and the setOrientation(float, float, float, float). The first one simply moves the camera in 3D space by an offset in x, y and z. So, for instance, if you wanted to move the camera 3 units on the X axis and 3 units on the Z axis, you'd do this:

Camera cam = new Camera(); // This is our camera

//Move camera   X Y Z
cam.translate(3.0f, 0.0f, 3.0f);

Piece of cake! Each method call translates the camera further, so two of the above calls would actually translate the camera 6 units along the x axis and 6 units along the z axis. Rotating is just as easy, but I have to explain the method first. It works like almost all 3D API rotation methods. You have four parameters, the first one is the actual rotation in degrees and the last three compose an orientation vector (xAxis, yAxis, zAxis) to rotate around. Orientation and orientation vectors will come later in the series, for now, just know this:

//Rotate camera 30 degrees around the X axis
cam.setOrientation(30.0f, 1.0f, 0.0f, 0.0f);

//Rotate camera 30 degrees around the Y axis
cam.setOrientation(30.0f, 0.0f, 1.0f, 0.0f);

//Rotate camera 30 degrees around the Z axis
cam.setOrientation(30.0f, 0.0f, 0.0f, 1.0f);

Note that the method is named setOrientation, which means that it actually clears any previous rotation you might have done. I'll assume that you already know what rotation around an axis means and won't go into detail on that topic here.

Now that you know everything to make the camera move and rotate as you wish I'll show you how to extract a Camera from a World.

/** Loads our camera */
    private void loadCamera()
    {
        // BAD!
        if(world == null)
            return;
       
        // Get the active camera from the world
        cam = world.getActiveCamera();
       
        // Create a light
        Light l = new Light();
       
        // Make sure it's AMBIENT
        l.setMode(Light.AMBIENT);
       
        // We want a little higher intensity
        l.setIntensity(3.0f);
       
        // Add it to our world
        world.addChild(l);
    }

It's that simple? Yes, it's that simple. We just extract the active camera from the World by using the getActiveCamera. This gives us the camera that the world was exported with. With the method above we have a camera that we can move around as much as we want to. However, the method is doing something else as well, it adds a light! We will delve deeper into lights in later parts, but here you see how easy it is to add a light to your world. I create an Ambient light (for you who don't know, an ambient light lights all surfaces from all directions) and add it to the world. This way we get a very-well lit world. As I told you earlier, the World node can hold all kinds of information, including light, so we only need to add the light once to our world and JSR 184 will do the rest for us. Isn't that handy? Before we get to the last part; rendering, let's make the camera move. I've already told you that the array of booleans, key, holds our key information so all we have to do is query the array and make our camera behave. First of all, we'll need some variables to make our camera obey.

// Camera rotation
float camRot = 0.0f;
double camSine = 0.0f;
double camCosine = 0.0f;
   
// Head bobbing
float headDeg = 0.0f;

The variables above will help us keep track of the camera's rotation, trigonometry and the head bobbing. The trigonometry is used for movement later on. Head bobbing is really quite simple, it's just a cheap effect we'll insert to make the camera bob up and down as we walk around the world, for a more natural feeling. All right, all we need to do is move the camera. This is done in the following method:

private void moveCamera() {
        // Check controls
        if(key[LEFT])
        {
            camRot += 5.0f;
        }
        else if(key[RIGHT])
        {
            camRot -= 5.0f;
        }
       
        // Set rotation
        cam.setOrientation(camRot, 0.0f, 1.0f, 0.0f);
       
        // Calculate trigonometry for camera movement
        double rads = Math.toRadians(camRot);
        camSine = Math.sin(rads);
        camCosine = Math.cos(rads);

As you can see this half of the method is pretty simple. First we check if the user has pressed the LEFT or the RIGHT joystick key and if he has we just increase or decrease the rotation of the camera. That's simple enough. The next few lines are interesting though. We want the head to turn as the user presses right or left, so we'll rotate around the Y axis, which means an orientation vector of 0.0f, 1.0f, 0.0f. After we rotate the camera we calculate the new Sine and Cosine of the angle, which is used later for movement calculation. Now on to the next half of the method:

if(key[UP])
        {
            // Move forward
            cam.translate(-0.1f * (float)camSine, 0.0f, -0.1f * (float)camCosine);
           
            // Bob head
            headDeg += 0.5f;
           
            // A simple way to "bob" the camera as the user moves
            cam.translate(0.0f, (float)Math.sin(headDeg) / 40.0f, 0.0f);
        }
        else if(key[DOWN])
        {
            // Move backward
            cam.translate(0.1f * (float)camSine, 0.0f, 0.1f * (float)camCosine);
           
            // Bob head
            headDeg -= 0.5f;
           
            // A simple way to "bob" the camera as the user moves
            cam.translate(0.0f, (float)Math.sin(headDeg) / 40.0f, 0.0f);
        }

        // If the user presses the FIRE key, let's quit
        if(key[FIRE])
            M3GMidlet.die();
    }

Here we check for UP or DOWN. UP will move the camera forward and DOWN will move it backwards. These are really simple translations, but I'll explain them fast. The Camera always starts looking down the negative Z axis, so to move the camera forward, we only have to move it negative on the Z axis. However, if we rotate the camera we can't just move it along the Z axis anymore, it will look wrong. We have to move the camera on the X axis as well so that we get the movement we desire. This is attained by using trigonometrical functions. Since this isn't a tutorial on 3D math I won't go into more detail, after all you should already know this and if you think it's hard then look up a good 3D math tutorial on the internet.

After every translation we also move the head with my simple head-bobbing hack. I just supply the translate method with a sine function along the Y axis, so that it looks like the head is going up and down, that's why we either increase or decrease the headDeg variable each time the camera is moved. We also check for the FIRE key at the end, so that the user can quit the application whenever he wants. (He can also use the invisible EXIT command that I added when the canvas was created).
That's it! That's all our advanced camera movement, now all that's left is to render the World node!

Rendering
Before we jump into code, I have to tell you about immediate and retained mode rendering. The retained mode is what we are using in this tutorial and it basically is the mode you use when you render an entire World node with all its cameras, lights and meshes. This is the easiest mode to render in, but also the mode you have the least control over your world. Immediate mode is when you render a Group of meshes, a single mesh or vertex data directly. This gives you much more control as with each rendering you supply a transform matrix that transforms the object before it's rendered. You can render a World node in Immediate mode, by supplying a transform matrix to the render method call, but then you'd be ignoring all the nifty effects a World node has such as camera, background and others. I'll go more into detail about the two different rendering modes in later parts of this series. For now, let's see how we can render a world.

Graphics3D
All rendering in JSR 184 is done with the Graphics3D object. It can even hold camera and light information, if you are rendering in immediate mode. Let's not worry about that now though since that's an issue I'll address later on.

To render with a Graphics3D object, you first must bind it to a graphics context. A graphics context basically means any Graphics object that draws into something. It could be a Graphics object of an Image if you want to render to an Image, or it could be the main Graphics object obtained from the getGraphics() method. By using the main Graphics object you render directly to the screen, which is what we want to do here. To get a Graphics3D object is simple, you just call the Graphics3D.getInstance() method. You get only one Graphics3D object per MIDlet, which is why you only obtain it from the getInstance method. Binding is done with the bindTarget method, and there are a few ways of using it. Here are a few examples.

//Here is our Graphics3D object
Graphics3D g3d = Graphics3D.getInstance();

// Bind to an image
Image img = Image.createImage("myImage.png");
Graphics g = img.getGraphics();
g3d.bindTarget(g);

// Bind to the main Graphics object
g3d.bindTarget(getGraphics());

// We can also supply rendering hints. Remember those? I talked about them at the beginning.
// This is done by using the other form of the bindTarget method.
// It takes a Graphics object to begin with, as always, and then it needs a boolean
// and an integer mask of hints.
// The boolean simply tells the Graphics3D object if it should use a depth buffer
// and you'll probably always set it to 'true'. Here is how we'll use it to bind
// with our hints:
g3d.bindTarget(getGraphics(), true, RENDERING_HINTS);

Now that you know how to bind your target, you also must know that the target must be released each game loop. That means that when you've rendered everything, you must release the target. There are problems sometimes with releasing and binding, so most people just keep the whole game loop within a try/catch block and put the releaseTarget call in the finally clause. That's how we'll do it in this example. Now, let's take a look at the rendering method. To render something you can use a variety of rendering methods, but we'll only be interested in one today, the render(World) method. Simple huh? Yeah, you only need to supply your world node and it'll render it for you. Let's take a look at what our game loop will look like:

/** Draws to screen
     */
    private void draw(Graphics g)
    {
        // Envelop all in a try/catch block just in case
        try
        {
            // Move the camera around
            moveCamera();
           
            // Get the Graphics3D context
            g3d = Graphics3D.getInstance();
           
            // First bind the graphics object. We use our pre-defined rendering hints.
            g3d.bindTarget(g, true, RENDERING_HINTS);
        
            // Now, just render the world. Simple as pie!
            g3d.render(world);
        }
        catch(Exception e)
        {
            reportException(e);
        }
        finally
        {
            // Always remember to release!
            g3d.releaseTarget();
        }
    }

It also adds the rendering hints we defined at the start of our Canvas. After all this is done, it just calls the g3d.render(world) method which does all the work for us. It renders our entire scene, meshes, materials, lights and camera.

Conclusion

So to wrap it all up, here are a few screenshots of the code in action:

Doesn't look too shabby, does it?

So, here's the full source code of the MIDlet and Canvas classes. It's not that much code really, if you consider the amount that's rendered onscreen. Check out the code at the start of the tutorial and see how the Canvas works internally. Or you can download it and play with it at home. Below you'll also find the full JAR/JAD package of the application.

Wow, that's a really short game loop, let's see what it does! First is called the moveCamera method that moves and rotates our camera. We've seen it before. Then it gets the Graphics3D instance and binds it to the Graphics object supplied to the draw method. (Note: The draw method is called by the Thread's run method which passes the global Graphics object to it).

 

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