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# Drawing with Z-Index Ordering

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Prioritized Drawing

We already have the ability to perform prioritized “z-index” drawing of a sprite image with the SpriteBatch.Draw() method, and we have been using the z-index parameter all along, just set to a value of zero. This effectively gave every sprite the same priority. When that is the case, priority will be based entirely on the order at which sprites are drawn (in gameplay code). SpriteBatch.Draw() has the capability to automatically prioritize the drawing of some sprites over the top of other sprites using this z-index buffer.

What does “z-index” mean, you may be wondering? When doing 2D sprite programming, we deal only with the X and Y coordinates on the screen. The Z coordinate, then, is the position of the sprite in relation to other sprites that overlap each other. The range for the z-index goes from 0.0 to 1.0. So, a sprite with a z-index priority of 0.6 will draw over a sprite with a z-index priority of 0.3. Note that 1.0 is the highest, and 0.0 is the lowest.

Sprite Class Changes

Adding Z-Buffering to the Sprite Class

A very minor change is required to enable z-index drawing in our Sprite class. We’ll go over those changes here.

1. Add a zindex variable to the class:
[code]
public float zindex;
[/code]
2. In the Sprite class constructor, initialize the new variable:
[code]
zindex = 0.0f;
[/code]
3. Replace the hard-coded zero with the zindex variable in the SpriteBatch.Draw() calls inside Sprite.Draw() (there are two of them):
[code]
public void Draw()
{
if (!visible) return;
if (totalFrames > 1)
{
Rectangle source = new Rectangle();
source.X = (frame % columns) * (int)size.X;
source.Y = (frame / columns) * (int)size.Y;
source.Width = (int)size.X;
source.Height = (int)size.Y;
p_spriteBatch.Draw(image, position, source, color,
rotation, origin, scale, SpriteEffects.None, zindex);
}
else
{
p_spriteBatch.Draw(image, position, null, color, rotation,
origin, scaleV, SpriteEffects.None, zindex);
}
}
[/code]

To use a z-buffer for prioritized drawing, a change must be made to the call to SpriteBatch.Begin(), which gets drawing started in the main program code. No change is made to SpriteBatch.End(). There are actually five overloads of SpriteBatch.Begin()! The version we want requires just two parameters: SpriteSortMode and BlendState. Table 16.1 shows the SpriteSortMode enumeration values.

The default sorting mode is Deferred, which is what SpriteBatch uses when the default Begin() is called. For z-index ordering, we will want to use either BackToFront or FrontToBack. There is very little difference between these two except the weight direction of each sprite’s z-index.

When using BackToFront, smaller z-index values have higher priority, with 0.0 being drawn over other sprites within the range up to 1.0.

When using FrontToBack, the opposite is true: Larger z-index values (such as 1.0) are treated with higher priority than lower values (such as 0.0).

It works best if you just choose one and stick with it to avoid confusion. If you think of a z-index value of 1.0 being “higher” in the screen depth, use FrontToBack. If 0.0 seems to have a higher priority, use BackToFront.

In our example, we will use FrontToBack, with larger values for the z-index having higher priority.

Z-Index Demo

To demonstrate the effect z-index ordering has on a scene, I have prepared an example that draws a screen full of sprites and then moves a larger sprite across the screen. Based on the z-index value of each sprite, the larger sprite will appear either over or under the other sprites. In this example, the screen is filled with animated asteroid sprites, and the larger sprite is an image of a shuttle. Figure 16.1 shows the demo with the shuttle sprite drawn on top of the first half of the asteroids.

Figure 16.2 shows the shuttle sprite farther across the screen, where it appears under the second half of the rows of asteroid sprites (which have a higher z-index than the shuttle sprite).

Wrapping Around the Screen Edge

To assist with this demo, a new Animation subclass has been written to automatically wrap a sprite around the edges of the screen. This completely eliminates any gameplay code that would otherwise have to be added to the Update() method. This is called synergy—when a combination of simple things produces awesome results! We’re starting to see that happen with our sprite and animation code now. Wrapping around the edges of the screen might be considered more of a behavior than an animation.

[code]
public class WrapBoundary : Animation
{
Rectangle boundary;
public WrapBoundary(Rectangle boundary)
: base()
{
animating = true;
this.boundary = boundary;
}
public override Vector2 ModifyPosition(Vector2 original)
{
Vector2 pos = original;
if (pos.X < boundary.Left)
pos.X = boundary.Right;
else if (pos.X > boundary.Right)
pos.X = boundary.Left;
if (pos.Y < boundary.Top)
pos.Y = boundary.Bottom;
else if (pos.Y > boundary.Bottom)
pos.Y = boundary.Top;
return pos;
}
}
[/code]

Z-Index Demo Source Code

Listing 16.1 contains the source code for the Z-Index Demo. Note the code highlighted in bold—these lines are relevant to our discussion of z-buffering.

LISTING 16.1 Source Code for the Z-Index Demo

[code]
public class Game1 : Microsoft.Xna.Framework.Game
{
GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;
TouchLocation oldTouch;
Random rand;
SpriteFont font;
List<Sprite> objects;
Texture2D asteroidImage;
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = “Content”;
TargetElapsedTime = TimeSpan.FromTicks(333333);
oldTouch = new TouchLocation();
}
protected override void Initialize()
{
base.Initialize();
}
{
rand = new Random();
spriteBatch = new SpriteBatch(GraphicsDevice);
//create object list
objects = new List<Sprite>();
//create shuttle sprite
Sprite shuttle = new Sprite(Content, spriteBatch);
shuttle.scale = 0.4f;
shuttle.position = new Vector2(0, 240);
shuttle.velocityLinear = new Vector2(4, 0);
Rectangle bounds = new Rectangle(-80, 0, 800 + 180, 480);
shuttle.zindex = 0.5f;
//create asteroid sprites with increasing z-index
for (int row = 0; row < 13; row++)
{
float zindex = row / 12.0f;
for (int col = 0; col < 8; col++)
{
Sprite spr = new Sprite(Content, spriteBatch);
spr.image = asteroidImage;
spr.columns = 8;
spr.totalFrames = 64;
spr.position = new Vector2(30 + 60 * row, 30 + col * 60);
spr.size = new Vector2(60, 60);
spr.zindex = zindex;
}
}
}
protected override void Update(GameTime gameTime)
{
ButtonState.Pressed)
this.Exit();
foreach (Sprite spr in objects)
{
spr.Rotate();
spr.Move();
}
base.Update(gameTime);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin(SpriteSortMode.FrontToBack, BlendState.AlphaBlend);
foreach (Sprite spr in objects)
{
spr.Animate();
spr.Draw();
}
spriteBatch.End();
base.Draw(gameTime);
}
}
[/code]

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