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<a name="Sprites-1"></a>
<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. In Chickadee, 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><a name="index-draw_002dsprite"></a>Procedure: <strong>draw-sprite</strong> <em><var>texture</var> <var>position</var> [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] [#:rect] [#:shader]</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>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>
<p>Finally, advanced users may specify <var>shader</var> to change the way the
sprite is rendered entirely.
</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>, which will
quickly lead to poor performance. To deal with this, a technique
known as “sprite batching” can be used. Instead of drawing each
sprite immediately, the sprite batch will build up a large of buffer
of sprites to draw and defer rendering until the last possible moment.
Batching isn’t a panacea, though. Batching only works if the sprites
being drawn share as much in common as possible. Every time you draw
a sprite with a different texture or blend mode, the batch will be
sent off to the GPU. Therefore, batching is most useful if you
minimize such changes. A good strategy for reducing texture changes
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>
<p>Taking advantage of sprite batching in Chickadee is easy, just wrap
the code that is calling <code>draw-sprite</code> a lot in the
<code>with-batched-sprites</code> form.
</p>
<dl>
<dt><a name="index-with_002dbatched_002dsprites"></a>Syntax: <strong>with-batched-sprites</strong> <em><var>body</var> …</em></dt>
<dd><p>Use batched rendering for all <code>draw-sprite</code> calls within
<var>body</var>.
</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 diving 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><a name="index-draw_002dnine_002dpatch"></a>Procedure: <strong>draw-nine-patch</strong> <em><var>texture</var> <var>rect</var> [#:margin 0] [#:top-margin margin] [#:bottom-margin margin] [#:left-margin margin] [#:right-margin margin] [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] [#:shader]</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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