What Is a Sprite?
Sprites are 2D bitmaps that are drawn directly to a render target without using the pipeline for transformations, lighting or effects. Sprites are commonly used to display information such as health bars, number of lives, or text such as scores. Some games, especially older games, are composed entirely of sprites.
- Sprite Origin
- Sprite Depth
- Sampling Textures
- Sprite Scaling
- Sprite Transformation Matrices
- Sprite Fonts
- Sprite Batching
Sprites are positioned on the screen by coordinates. The width and height of the screen is the same as the back buffer. The x-axis represents the screen width and the y-axis represents the screen height. The y-axis is measured from the top of the screen and increases as you move down the screen, and the x-axis is measured from left to right. For example, when the graphics back buffer is 800×600, 0,0 is the upper left of the screen, and 800,600 is the lower right of the screen (see Figure 1).
Figure 1. A sprite's location in screen coordinates (x-axis 400, y-axis 200)
To draw a sprite, create a SpriteBatch object, initialize it by calling Begin, and then call Draw for each sprite. The bitmap data for a sprite is taken from a Texture2D object. The texture may contain alpha channel information to make part of the texture transparent or semi-transparent. You can tint, rotate, or scale sprites by using Draw. This method also gives you the option of drawing only part of the texture on the screen. After you draw a sprite, call End before calling Present.
When you draw a sprite, the sprite origin is an important concept. The origin is a specific point on the sprite, which is by default the upper-left corner of the sprite, or (0,0). Draw draws the origin of the sprite at the screen location you specify. For example, if you draw a 50×50 pixel sprite at location (400,200) without specifying an origin, the upper left of the sprite will be on pixel (400,200). If you use the center of the 50×50 sprite as the origin (25,25), to draw the sprite in the same position you must add the origin coordinates to the position. In this case, the position is (425,225) and the origin is (25,25), as shown in Figure 2.
Figure 2. The blue dot indicates the center coordinate of the sprite
When rotating a sprite, the method uses the origin as the center of the rotation. In these cases, it is common to use the center of the sprite as the origin when calculating where to draw the sprite on the screen.
Sprites also have a concept of depth which is a floating-point number between 0 and 1. Sprites drawn at a depth of 0 are drawn in front of sprites which have a depth of greater than 0; sprites drawn at a depth of 1 are covered by sprites drawn at a depth less than 1.
A sprite is based on a Texture2D object—in other words, a bitmap. Use Draw to draw the entire texture or a portion of the texture. To draw a portion of the texture, use the sourceRectangle parameter to specify which texels, or texture pixel, to draw. A 32×32 texture has 1024 texels, specified as x and y values similar to how screen coordinates are specified. Specifying a sourceRectangle of (0, 0, 16, 16) would select the upper-left quadrant of a 32×32 texture.
Draw provides three options for scaling a sprite: using a uniform scaling parameter, a nonuniform scaling parameter, or a source and destination rectangle. The uniform scaling parameter is a floating-point number that multiplies the sprite size through both the x- and y-axes. This will shrink or expand the sprite along each axis equally, maintaining the original ratio between the sprite width and height.
To scale the x- and y-axes independently, Draw accepts a Vector2 value as a scalar. This Vector2 specifies nonuniform scaling: x- and y-axes are scaled independently according to the X and Y fields of the Vector2.
Draw also accepts a source and destination rectangle. The destination rectangle is specified in screen coordinates, while the source rectangle is specified in texels. Draw takes the pixels on the texture specified in sourceRectangle and scales them independently along the x- and y-axes until they fit the screen coordinates specified by destinationRectangle.
Sprite Transformation Matrices
You can also specify a transformation matrix that the batch can apply to each sprite before drawing. The transformation matrix can be any combination of translation, rotation, or scaling matrices multiplied together into a single matrix. This matrix is combined with the sprite position, rotation, scaling, and depth parameters supplied to Draw. Because the matrix also applies to depth, any z-coordinate transformation that makes the sprite depth greater than 1.0 or less than 0.0 will cause the sprite to disappear.
Use a SpriteBatch to draw text. The DrawString method will draw text on screen with position, color, rotation, origin, and scaling. DrawString also requires a special type of texture encapsulated by the SpriteFont class. A SpriteFont is created by the content pipeline when you add a Sprite Font file to your project. The sprite font file has information such as the name and point size of the font, and which Unicode characters to include in the SpriteFont texture. At run time, a SpriteFont is loaded with ContentManager.Load just like a Texture2D object.
See Sprite Font XML Schema Reference for a list of Sprite Font tags. You can use the content pipeline to determine your character regions automatically. For more information, see How to: Extend the Font Description Processor to Support Additional Characters.
In normal drawing, the SpriteBatch object does not change any render states or draw any sprites until you call End. This is known as Deferred mode. In Deferred mode, SpriteBatch saves the information from each Draw call until you call End.
If you call Begin, specifying SpriteSortMode.Immediate, it triggers Immediate mode. In Immediate mode, the SpriteBatch immediately changes the graphics device render states to begin drawing sprites. Thereafter, each call to Draw immediately draws the sprite using the current device settings.