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Next: <a href="Tile-Maps.html#Tile-Maps" accesskey="n" rel="next">Tile Maps</a>, Previous: <a href="Textures.html#Textures" accesskey="p" rel="prev">Textures</a>, Up: <a href="Graphics.html#Graphics" accesskey="u" rel="up">Graphics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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<span id="Sprites-1"></span><h4 class="subsection">2.3.3 Sprites</h4>
<p>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.
</p>
<p>In Chickadee, the <code>(chickadee render sprite)</code> module provides the
interface for working with sprites. Bitmaps are stored in textures
(see <a href="Textures.html#Textures">Textures</a>) and can be used to draw sprites via the
<code>draw-sprite</code> procedure.
</p>
<dl>
<dt id="index-draw_002dsprite">Procedure: <strong>draw-sprite</strong> <em>texture position [#:tint white] [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] [#:rect]</em></dt>
<dd>
<p>Draw <var>texture</var> at <var>position</var>.
</p>
<p>Optionally, other transformations may be applied to the sprite.
<var>rotation</var> specifies the angle to rotate the sprite, in radians.
<var>scale</var> specifies the scaling factor as a 2D vector. All
transformations are applied relative to <var>origin</var>, a 2D vector,
which defaults to the lower-left corner.
</p>
<p><var>tint</var> specifies the color to multiply against all the sprite’s
pixels. By default white is used, which does no tinting at all.
</p>
<p>Alpha blending is used by default but the blending method can be
changed by specifying <var>blend-mode</var>.
</p>
<p>The area drawn to is as big as the texture, by default. To draw to an
arbitrary section of the screen, specify <var>rect</var>.
</p></dd></dl>
<p>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 <code>draw-sprite</code> on its own
will involve at least one GPU call <em>per sprite</em>. 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 of 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 <a href="Textures.html#Textures">Textures</a>) to access the sub-images within.
</p>
<dl>
<dt id="index-make_002dsprite_002dbatch">Procedure: <strong>make-sprite-batch</strong> <em>texture [#:capacity 256]</em></dt>
<dd><p>Create a new sprite batch for <var>texture</var> with initial space for
<var>capacity</var> sprites. Sprite batches automatically resize when they
are full to accomodate as many sprites as necessary.
</p></dd></dl>
<dl>
<dt id="index-sprite_002dbatch_003f">Procedure: <strong>sprite-batch?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is a sprite batch.
</p></dd></dl>
<dl>
<dt id="index-sprite_002dbatch_002dtexture">Procedure: <strong>sprite-batch-texture</strong> <em>batch</em></dt>
<dd><p>Return the texture for <var>batch</var>.
</p></dd></dl>
<dl>
<dt id="index-set_002dsprite_002dbatch_002dtexture_0021">Procedure: <strong>set-sprite-batch-texture!</strong> <em>batch texture</em></dt>
<dd><p>Set texture for <var>batch</var> to <var>texture</var>.
</p></dd></dl>
<dl>
<dt id="index-sprite_002dbatch_002dadd_0021">Procedure: <strong>sprite-batch-add!</strong> <em>batch position [#:origin] [#:scale] [:rotation] [#:tint <code>white</code>] [#:texture-region]</em></dt>
<dd>
<p>Add sprite located at <var>position</var> to <var>batch</var>.
</p>
<p>To render a subsection of the batch’s texture, a texture object whose
parent is the batch texture may be specified as <var>texture-region</var>.
</p>
<p>See <code>draw-sprite</code> for information about the other arguments.
</p></dd></dl>
<dl>
<dt id="index-sprite_002dbatch_002dclear_0021">Procedure: <strong>sprite-batch-clear!</strong> <em>batch</em></dt>
<dd><p>Reset size of <var>batch</var> to 0.
</p></dd></dl>
<dl>
<dt id="index-draw_002dsprite_002dbatch">Procedure: <strong>draw-sprite-batch</strong> <em>batch [#:blend-mode <code>alpha</code>]</em></dt>
<dd><p>Render <var>batch</var> using <var>blend-mode</var>. Alpha blending is used by
default.
</p></dd></dl>
<p>With a basic sprite abstraction in place, it’s possible to build other
abstractions on top of it. One such example is the “nine patch”. A
nine patch is a sprite that can be rendered at various sizes without
becoming distorted. This is achieved by dividing up the sprite into
nine regions:
</p>
<ul>
<li> the center, which can be scaled horizontally and vertically
</li><li> the four corners, which can never be scaled
</li><li> the left and right sides, which can be scaled vertically
</li><li> the top and bottom sides, which can be scaled horizontally
</li></ul>
<p>The one caveat is that the bitmap regions must be designed in such a
way so that they are not distorted when stretched along the affected
axes. For example, that means that the top and bottom sides could
have varying colored pixels vertically, but not horizontally.
</p>
<p>The most common application of this technique is for graphical user
interface widgets like buttons and dialog boxes. By using a nine
patch, they can be rendered at any size without unappealing scaling
artifacts.
</p>
<dl>
<dt id="index-draw_002dnine_002dpatch">Procedure: <strong>draw-nine-patch</strong> <em>texture rect [#:margin 0] [#:top-margin margin] [#:bottom-margin margin] [#:left-margin margin] [#:right-margin margin] [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] [#:tint white]</em></dt>
<dd>
<p>Draw a nine patch sprite. A nine patch sprite renders <var>texture</var>
as a <var>width</var> x <var>height</var> rectangle whose stretchable areas are
defined by the given margin measurements <var>top-margin</var>,
<var>bottom-margin</var>, <var>left-margin</var>, and <var>right-margin</var>. The
<var>margin</var> argument may be used to configure all four margins at
once.
</p>
<p>Refer to <code>draw-sprite</code> (see <a href="#Sprites">Sprites</a>) for information about
the other arguments.
</p></dd></dl>
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