Sprites (The Chickadee Game Toolkit)

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5.3.3 Sprites

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For those who are new to this game, a sprite is a 2D rectangular -bitmap that is rendered to the screen. For 2D games, sprites are the -most essential graphical abstraction. They are used for drawing maps, -players, NPCs, items, particles, text, etc. -

In Chickadee, the (chickadee graphics sprite) module provides the -interface for working with sprites. Bitmaps are stored in textures -(see Textures) and can be used to draw sprites via the -draw-sprite procedure. -

Procedure: draw-sprite texture position [#:blend-mode] [#:origin] [#:rect] [#:rotation] [#:scale] [#:shear] [#:tint white]

Draw texture at position. -

Optionally, other transformations may be applied to the sprite. -rotation specifies the angle to rotate the sprite, in radians. -scale specifies the scaling factor as a 2D vector. shear -specifies the shearing factor as a 2D vector. All transformations are -applied relative to origin, a 2D vector, which defaults to the -lower-left corner. -

tint specifies the color to multiply against all the sprite’s -pixels. By default white is used, which does no tinting at all. -

Alpha blending is used by default but the blending method can be -changed by specifying blend-mode. -

The area drawn to is as big as the texture, by default. To draw to an -arbitrary section of the screen, specify rect. -

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5.3.3.1 Sprite Batches

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It’s not uncommon to need to draw hundreds or thousands of sprites -each frame. However, GPUs (graphics processing units) are tricky -beasts that prefer to be sent few, large chunks of data to render -rather than many, small chunks. Using draw-sprite on its own -will involve at least one GPU call per sprite. This is fine -for rendering a few dozen sprites, but will become a serious -bottleneck when rendering hundreds or thousands of sprites. To deal -with this, a technique known as “sprite batching” is used. Instead -of drawing each sprite immediately, the sprite batch will build up a -large buffer of sprites to draw and send them to the GPU all at once. -There is one caveat, however. Batching only works if the sprites -being drawn share a common texture. A good strategy for reducing the -number of different textures is to stuff many bitmaps into a single -image file and create a “texture atlas” (see Textures) to access -the sub-images within. -

Procedure: make-sprite-batch texture [#:capacity 256]: Create a new sprite batch for texture with initial space for -capacity sprites. Sprite batches automatically resize when they -are full to accomodate as many sprites as necessary. -

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Procedure: sprite-batch? obj: Return #t if obj is a sprite batch. -

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Procedure: sprite-batch-texture batch: Return the texture for batch. -

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Procedure: set-sprite-batch-texture! batch texture: Set texture for batch to texture. -

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Procedure: sprite-batch-add! batch position [#:origin] [:rotation] [#:scale] [#:shear] [#:texture-region] [#:tint white]

Add sprite located at position to batch. -

To render a subsection of the batch’s texture, a texture object whose -parent is the batch texture may be specified as texture-region. -

See draw-sprite for information about the other arguments. -

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Procedure: sprite-batch-clear! batch: Reset size of batch to 0. -

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Procedure: draw-sprite-batch batch [#:blend-mode]: Render batch using blend-mode. Alpha blending is used by -default. -

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5.3.3.2 9-Patches

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A 9-patch is a method of rendering a texture so that it can be -stretched to cover an area of any size without becoming distorted. -This is achieved by dividing up the sprite into nine regions: -

the center, which can be stretched or tiled horizontally and vertically -
the four corners, which are never stretched or tiled -
the left and right sides, which can be stretched or tiled vertically -
the top and bottom sides, which can be stretched or tiled horizontally -

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The most common application of this technique is for graphical user -interface widgets like buttons and dialog boxes which are often -dynamically resizable. By using a 9-patch, they can be rendered at -any size without scaling artifacts. The (chickadee graphics -9-patch) module provides this functionality. -

Procedure: draw-9-patch texture rect [#:margin 0] [#:top-margin margin] [#:bottom-margin margin] [#:left-margin margin] [#:right-margin margin] [#:mode stretch] [#:origin] [#:scale] [#:rotation] [#:blend-mode] [#:tint white]

Draw a 9-patch over the area rect using texture whose -stretchable/tileable patches are defined by the given margin -measurements. The corners are never stretched/tiled, the left and -right edges will be stretched/tiled vertically, the top and bottom -edges may be stretched/tiled horizontally, and the center may be -stretched/tiled in both directions. -

mode may be either stretch (the default) or tile. -

margin specifies the margin size for all sides of the 9-patch. -To make margins of differing sizes, the top-margin, -bottom-margin, left-margin, and right-margin -arguments may be used. -

Refer to draw-sprite for information about the other arguments -as they are the same. -

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