Treating Bitmaps as Sprites

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Bringing Bitmaps to Life

A sprite is a bitmap with benefits. XNA provides the SpriteBatch class to draw bitmaps with the SpriteBatch.Draw() method, of which there are several overloaded variants. But despite the name, SpriteBatch does not give us “sprite” capabilities from the gameplay perspective. SpriteBatch is a rendering class, used solely for drawing, not “managing” game entities traditionally known as “sprites.” The difference might seem a subtle one, but it’s actually quite a distinction. SpriteBatch might have been a somewhat incorrect name for the class. The “Batch” part of the name refers to the way in which “bitmaps” (not sprites) are drawn—in a batch. That is, all bitmap drawing via SpriteBatch.Draw() is put into a queue and then all the drawing is done quickly when SpriteBatch.End() is called. It’s faster to perform many draw calls at once in this manner, since the video card is going to be switching state only once. Every time SpriteBatch.Begin() and SpriteBatch.End() are called, that involves a state change (which is very slow in terms of rendering). The fewer state changes that happen, the better!

SpriteBatch.Draw() can handle animation, rotation, scaling, and translation (movement). But, without properties, we have to write custom code to do all of these things with just global variables. It can get tedious! We will see how tedious by writing an example using all global variables. Then, for comparison, we’ll write a simple Sprite class and convert the program. This is not just an illustration of how useful object-oriented programming can be (which is true) but to show why we need a Sprite class for gameplay.

SpriteBatch.Draw() works fine. We don’t need an alternative replacement because it can do anything we need. But the code to draw sprites is very specific. If we don’t want to manually draw every character, or vehicle, or avatar in the game, we have to automate the process in some manner. The key to making this work is via properties. A property is a trait or an attribute that partially describes something. In the case of a person, one property would be gender (male or female); other properties include race, height, weight, and age. No single property fully describes a person, but when all (or most) of the properties are considered, it gives you a pretty good idea of what that person looks like.

The simplest and most common property for a sprite is its position on the screen. In the previous hour, we used a variable called Vector2 shipPos to represent the position of the bitmap, and a variable called Texture2D shipImage to represent the image. These two variables were properties for a game object—a spaceship to be used in a sci-fi game. Wouldn’t it be easier to manage both of these properties inside a Sprite class? Before we do that, let’s see whether it’s really that big of a deal to keep track of properties with global variables.

Drawing Lots of Bitmaps

Let’s create a short example. Now, working with just one bitmap is a piece of cake, because there’s only one call to SpriteBatch.Draw(), only one position variable, and only one image variable. When things get messy is when about five or more bitmaps need to be manipulated and drawn. Up to that point, managing variables for four or so images isn’t so bad, but as the number climbs, the amount of manual code grows and becomes unwieldy (like a giant two-handed sword).

[code]
Vector2 position1, position2, position3, position4, position5;
Texture2D image1, image2, image3, image4, image5;
[/code]

It’s not just declaring and using the variables that can be a problem. It’s the ability to use any more than this practically in a game’s sources. But let’s give it a try anyway for the sake of the argument. Figure 6.1 shows the output for this short program, which is a prototype for a solar system simulation we’ll be creating in this hour. The source code is found in Listing 6.1.

The Many Bitmaps Demo program.
FIGURE 6.1 The Many Bitmaps Demo program.

LISTING 6.1 Source code for the Many Bitmaps Demo program, a precursor to a larger project.

[code]
public class Game1 : Microsoft.Xna.Framework.Game
{
GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;
Vector2 position1, position2, position3, position4, position5;
Texture2D image1, image2, image3, image4, image5;
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = “Content”;
TargetElapsedTime = TimeSpan.FromTicks(333333);
}
protected override void Initialize()
{
base.Initialize();
}
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
image1 = Content.Load<Texture2D>(“sun”);
image2 = Content.Load<Texture2D>(“planet1”);
image3 = Content.Load<Texture2D>(“planet3”);
image4 = Content.Load<Texture2D>(“planet2”);
image5 = Content.Load<Texture2D>(“planet4”);
position1 = new Vector2(100, 240-64);
position2 = new Vector2(300, 240-32);
position3 = new Vector2(400, 240-32);
position4 = new Vector2(500, 240-16);
position5 = new Vector2(600, 240-16);
}
protected override void Update(GameTime gameTime)
{
if (GamePad.GetState(PlayerIndex.One).Buttons.Back ==
ButtonState.Pressed)
this.Exit();
base.Update(gameTime);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin();
spriteBatch.Draw(image1, position1, Color.White);
spriteBatch.Draw(image2, position2, Color.White);
spriteBatch.Draw(image3, position3, Color.White);
spriteBatch.Draw(image4, position4, Color.White);
spriteBatch.Draw(image5, position5, Color.White);
spriteBatch.End();
base.Draw(gameTime);
}
}
[/code]

Running into Limits with Global Variables

The Many Bitmaps Demo program wasn’t too difficult to deal with, was it? I mean, there were only five bitmaps to draw, so we needed five position variables. But what if there were 20, or 50, or 100? With so many game objects, it would be impossible to manage them all with global variables. Furthermore, that’s bad programming style when there are better ways to do it. Obviously, I’m talking about arrays and collections. But not only is it a quantity issue with regard to the global variables, but if we want to add another property to each object, we’re talking about adding another 20, 50, or 100 variables for that new property!

Let’s rewrite the program using an array. Later in the hour, we’ll work with a list, which is a container class, but for this next step, an array is a little easier to follow. Here is a new version using arrays. The new source code is found in Listing 6.2.

LISTING 6.2 New source code for the program rewritten to more efficiently store the planet bitmaps and vectors in arrays.

[code]
public class Game1 : Microsoft.Xna.Framework.Game
{
GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;
Texture2D[] images;
Vector2[] positions;
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = “Content”;
TargetElapsedTime = TimeSpan.FromTicks(333333);
}
protected override void Initialize()
{
base.Initialize();
}
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
images = new Texture2D[5];
images[0] = Content.Load<Texture2D>(“sun”);
images[1] = Content.Load<Texture2D>(“planet1”);
images[2] = Content.Load<Texture2D>(“planet3”);
images[3] = Content.Load<Texture2D>(“planet2”);
images[4] = Content.Load<Texture2D>(“planet4”);
positions = new Vector2[5];
positions[0] = new Vector2(100, 240-64);
positions[1] = new Vector2(300, 240-32);
positions[2] = new Vector2(400, 240-32);
positions[3] = new Vector2(500, 240-16);
positions[4] = new Vector2(600, 240-16);
}
protected override void Update(GameTime gameTime)
{
if (GamePad.GetState(PlayerIndex.One).Buttons.Back ==
ButtonState.Pressed)
this.Exit();
base.Update(gameTime);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin();
for (int n = 0; n < 5; n++)
{
spriteBatch.Draw(images[n], positions[n], Color.White);
}
spriteBatch.End();
base.Draw(gameTime);
}
}
[/code]

As you examine the code in this version of the program, what stands out? I notice that in ContentLoad(), the initialization code is actually a bit more complicated than it was previously, but the code in Draw() is shorter. We’re not counting lines of code, but in Draw(), the for loop could accommodate 100 or 1,000 objects with the same amount of code. This is the most significant difference between this and the previous program—we now can handle any arbitrary number of objects.

We could shorten the code in LoadContent() even further by building the filename for each planet. The key is to make the asset names consistent. So, if we rename ”sun” to ”planet0”, then loading the assets becomes a simple for loop. Here is an improvement:

[code]
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
images = new Texture2D[5];
positions = new Vector2[5];
for (int n = 0; n < 5; n++)
{
string filename = “planet” + n.ToString();
images[n] = Content.Load<Texture2D>(filename);
}
positions[0] = new Vector2(100, 240 – 64);
positions[1] = new Vector2(300, 240 – 32);
positions[2] = new Vector2(400, 240 – 32);
positions[3] = new Vector2(500, 240 – 16);
positions[4] = new Vector2(600, 240 – 16);
}
[/code]

The positions array must still be set manually due to the differences in the sizes of the planet images. But I think we could automate that as well by looking at the width and height of each image. The point is not just to make the code shorter, but to find ways to improve the code, make it more versatile, more reusable, and easier to modify. Using a consistent naming convention for asset files will go a long way toward that end.

Creating a Simple Sprite Class

Now let’s experiment with some code that actually does something interesting besides drawing fixed images. We’ll start by creating a simple class to encapsulate a sprite, and then add some features to make the Sprite class useful. This will be a hands-on section where we build the Sprite class in stages. If you are an experienced programmer, you may skip this section.

Creating the Sprite Class

Let’s begin with a new project. The project type will be, as usual, Windows Phone (4.0), and the name is Sprite Demo. In the Game1.cs file, which is the main source code file for the game, we’re going to add the new Sprite class to the top of the file, above the Game1 class. After a while, the Sprite class can be moved into its own file called Sprite.cs. I find this more convenient while working on a new class. See Listing 6.3 for the complete source code.

Classes do not need to be stored in unique source code files; that’s a practice to keep a large project tidy and easier to maintain. But it is acceptable (and often practical) to define a new class inside an existing file. This is especially true when several classes are closely related and you want to better organize the project. The best practice, though, for large classes, is to define each class in its own file.

LISTING 6.3 Source code for the new project with included Sprite class.

[code]
public class Sprite
{
public Texture2D image;
public Vector2 position;
public Color color;
}
public class Game1 : Microsoft.Xna.Framework.Game
{
GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;
Viewport viewport;
Sprite sun;
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = “Content”;
TargetElapsedTime = TimeSpan.FromTicks(333333);
}protected override void Initialize()
{
base.Initialize();
}
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
//get screen dimensions
viewport = GraphicsDevice.Viewport;
//create sun sprite
sun = new Sprite();
sun.image = Content.Load<Texture2D>(“sun”);
//center sun sprite on screen
float x = (viewport.Width – sun.image.Width) / 2;
float y = (viewport.Height – sun.image.Height) / 2;
sun.position = new Vector2(x,y);
//set color
sun.color = Color.White;
}
protected override void Update(GameTime gameTime)
{
if (GamePad.GetState(PlayerIndex.One).Buttons.Back ==
ButtonState.Pressed)
this.Exit();
base.Update(gameTime);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin();
//draw the sun
spriteBatch.Draw(sun.image, sun.position, sun.color);
spriteBatch.End();
base.Draw(gameTime);
}
}
[/code]

The code listings in this book omit the using statements at the beginning of every XNA project, as well as the namespace line and surrounding brackets, to focus on just the functional part of a program, only when that code is generated by Visual Studio. You should assume that those lines are required to compile every example listed herein.

Scope and Clarity

This is how most classes begin life, with just some public properties that could have been equally well defined in a struct. In fact, if you just want to create a quick container for a few variables and don’t want to deal with a class, go ahead and do it. Structures (defined as struct) can even have constructor methods to initialize their properties, as well as normal methods. But in a struct, there is no scope; everything is public. In contrast, everything in a class is private by default. This would work, for example:

[code]
struct Sprite
{
Texture2D image;
Vector2 position;
Color color;
}
[/code]

But one limitation with a struct is room for growth; with a class, we can make changes to scope (which means changing whether individual properties and methods are visible outside of the class). At this simplistic level, there’s very little difference between a class and a struct. Some programmers differentiate between them by using structs only as property containers, with no methods, reserving methods only for defined classes. It’s ultimately up to you!

An OOP (object-oriented programming) “purist” would demand that our image, position, and color properties be defined with private scope, and accessed via property methods. The difference between property variables and property methods is fuzzy in C#, whereas they are very clear-cut in a more highly precise language such as C++. Let’s see what the class will look like when the three variables (image, position, and color) are converted into private properties with public accessor methods.

[code]
public class Sprite2 //”good” OOP version
{
Texture2D p_image;
Vector2 p_position;
Color p_color;
public Texture2D image
{
get { return p_image; }
set { p_image = value; }
}
public Vector2 position
{
get { return p_position; }
set { p_position = value; }
}
public Color color
{
get { return p_color; }
set { p_color = value; }
}
}
[/code]

In general, I prefer to not hide property variables (such as public Texture2D image in the Sprite class), because it just requires extra code to access the property later. This is, again, a matter of preference, and might be dependent on the coding standards of your team or employer. If it’s up to you, just focus on writing clean, tight code, and don’t worry about making your code “OOP safe” for others.

Initializing the Sprite Class with a Constructor

Compared to the original Sprite class defined in Game1.cs, what do you think of the “safe” version (renamed to Sprite2 to avoid confusion)? The three variable names have had “p_” added (to reflect that they are now private in scope), and now in their place are three “properly defined” properties. Each property now has an accessor method (get) and a mutator method (set). If you prefer this more highly structured form of object-oriented C#, I encourage you to continue doing what works best for you. But for the sake of clarity, I will use the original version of Sprite with the simpler public access property variables.

Let’s give the Sprite class more capabilities. Currently, it’s just a container for three variables. A constructor is a method that runs automatically when an object is created at runtime with the new operator, for example:

[code]
Sprite sun = new Sprite();
[/code]

The term Sprite(), with the parentheses, denotes a method—the default method since it requires no parameters. Here is ours:

[code]
public class Sprite
{
public Texture2D image;
public Vector2 position;
public Color color;
public Sprite()
{
image = null;
position = Vector2.Zero;
color = Color.White;
}
}
[/code]

Here, we have a new constructor that initializes the class’s properties to some initial values. This is meant to avoid problems later if one forgets to initialize them manually. In our program now, since color is automatically set to Color.White, we no longer need to manually set it, which cleans up the code in LoadContent() a bit.

[code]
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
//get screen dimensions
viewport = GraphicsDevice.Viewport;
//create sun sprite
sun = new Sprite();
sun.image = Content.Load<Texture2D>(“sun”);
//center sun sprite on screen
float x = (viewport.Width – sun.image.Width) / 2;
float y = (viewport.Height – sun.image.Height) / 2;
sun.position = new Vector2(x,y);
}
[/code]

Writing Reusable Code with Abstraction

A usual goal for an important base game class like Sprite is to abstract the XNA code, at least somewhat, to make the class stand on its own as much as possible. This becomes a priority when you find yourself writing games on several platforms. Within the XNA family, we have Windows, Xbox 360, and Windows Phone. But on a larger scale, it’s fairly common to port games to other systems. After you have rewritten your Sprite class a few times for different platforms (and even languages, believe it or not!), you begin to see similarities among the different systems, and begin to take those similarities into account when writing game classes.

There are two aspects that I want to abstract in the Sprite class. First, there’s loading the image. This occurs in LoadContent() when we simply expose the image property to Content.Load(). Second, there’s drawing the sprite. This occurs in Draw(), also when we expose the image property. To properly abstract the class away from XNA, we need our own Load() and Draw() methods within Sprite itself. To do this, the Sprite class must have access to both ContentManager and SpriteBatch. We can do this by passing those necessary runtime objects to the Sprite class constructor. Listing 6.4 contains the new source code for the Sprite class.

LISTING 6.4 Source code for the expanded Sprite class.

[code]
public class Sprite
{
private ContentManager p_content;
private SpriteBatch p_spriteBatch;
public Texture2D image;
public Vector2 position;
public Color color;
public Sprite(ContentManager content, SpriteBatch spriteBatch)
{
p_content = content;
p_spriteBatch = spriteBatch;
image = null;
position = Vector2.Zero;
color = Color.White;
}
public bool Load(string assetName)
{
try
{
image = p_content.Load<Texture2D>(assetName);
}
catch (Exception) { return false; }
return true;
}
public void Draw()
{
p_spriteBatch.Draw(image, position, color);
}
}
[/code]

Putting our new changes into action (in Listing 6.5) reveals some very clean-looking code in LoadContent() and Draw(), with the output shown in Figure 6.2.

LISTING 6.5 Modifications to the project to support the new Sprite features.

[code]
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
viewport = GraphicsDevice.Viewport;
//create sun sprite
sun = new Sprite(Content, spriteBatch);
sun.Load(“sun”);
//center sun sprite on screen
float x = (viewport.Width – sun.image.Width) / 2;
float y = (viewport.Height – sun.image.Height) / 2;
sun.position = new Vector2(x, y);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin();
sun.Draw();
spriteBatch.End();
base.Draw(gameTime);
}
[/code]

Demonstrating the Sprite class.
FIGURE 6.2 Demonstrating the Sprite class.

Error Handling

The Sprite.Load() method has error handling built in via a try…catch block. Inside the try block is a call to Content.Load(). If the passed asset name is not found, XNA generates an exception error, as shown in Figure 6.3. We don’t want the end user to ever see an exception error, and in a very large game project, it is fairly common for asset files to be renamed and generate errors like this—it’s all part of the development process to track down and fix such common bugs.

To assist, the code in Sprite.Load() returns false if an asset is not found—rather than crashing with an exception error. The problem is, we can’t exit on an error condition from within LoadContent(); XNA is just not in a state that will allow the program to terminate at that point. What we need to do is set a flag and look for it after LoadContent() is finished running.

I have an idea. What if we add a feature to display an optional error message in a pre-shutdown process in the game loop? All it needs to do is check for this error state and then print whatever is in the global errorMessage variable. The user would then read the message and manually shut down the program by closing the window. Let’s just try it out; this won’t be a permanent fixture in future chapters, but you may continue to use it if you want to. First, we need some new variables.

 An exception error occurs when an asset cannot be found.
FIGURE 6.3 An exception error occurs when an asset cannot be found.

[code]
//experimental error handling variables
bool errorState;
string errorMessage;
SpriteFont ErrorFont;
Next, we initialize them.
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = “Content”;
TargetElapsedTime = TimeSpan.FromTicks(333333);
errorMessage = ““;
errorState = false;
}
[/code]

And in LoadContent(), we need to trap the exception error (note that ”sun” was temporarily renamed to ”sun1” to demonstrate the exception error).

[code]
//create sun sprite
sun = new Sprite(Content, spriteBatch);
if (!sun.Load(“sun1”))
{
errorState = true;
errorMessage = “Asset file ‘sun’ not found.”;
return;
}
[/code]

Draw() is where the error handling process comes into play.

[code]
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin();
//experimental error handler
if (errorState)
{
spriteBatch.DrawString(ErrorFont, “CRITICAL ERROR”,
Vector2.Zero, Color.Red);
spriteBatch.DrawString(ErrorFont, errorMessage,
new Vector2(0,100), Color.Red);
}
else
{
sun.Draw();
}
spriteBatch.End();
base.Draw(gameTime);
}
[/code]

When run again with the new error handling code in place, the previous exception error now becomes a nice in-game notification, as shown in Figure 6.4.

The exception error has been handled nicely.
FIGURE 6.4 The exception error has been handled nicely.

We’ve just scratched the surface of what will be possible with the new Sprite class in this hour. Over the next several chapters, the class will be enhanced significantly, making it possible—with properties and methods—to perform transformations