xref: /frameworks/base/docs/html/guide/topics/graphics/index.jd

page.title=Graphics
@jd:body

<div id="qv-wrapper">
  <div id="qv">
  <h2>In this document</h2>
  <ol>
    <li><a href="#options">Consider your Options</a></li>
    <li><a href="#draw-to-view">Simple Graphics Inside a View</a></li>
    <li><a href="#draw-with-canvas">Draw with a Canvas</a>
    <ol>
      <li><a href="#on-view">On a View</a></li>
      <li><a href="#on-surfaceview">On a SurfaceView</a></li>
    </ol>
    </li>
  </ol>
  </div>
</div>
<p>Android graphics are powered by a custom 2D graphics library and OpenGL ES 1.0
for high performance 3D graphics. The most common 2D graphics APIs can be found in the 
{@link android.graphics.drawable drawable package}. OpenGL APIs are available
from the Khronos {@link javax.microedition.khronos.opengles OpenGL ES package},
plus some Android {@link android.opengl OpenGL utilities}.</p>

<p>When starting a project, it's important to consider exactly what your graphical demands will be. 
Varying graphical tasks are best accomplished with varying techniques. For example, graphics and animations
for a rather static application should be implemented much differently than graphics and animations
for an interactive game or 3D rendering.</p>

<p>Here, we'll discuss a few of the options you have for drawing graphics on Android, 
and which tasks they're best suited for.</p>

<p>If you're specifically looking for information on drawing 3D graphics, this page won't
help a lot. However, the information below about how to <a href="#draw-with-canvas">Draw with a
Canvas</a> (and the section on SurfaceView), 
will give you a quick idea of how you should draw to the View hierarchy. For more information
on Android's 3D graphic utilities (provided by the OpenGL ES API), 
read <a href="opengl.html">3D with OpenGL</a> and refer to other OpenGL documentation.</p>


<h2 id="options">Consider your Options</h2>

<p>When drawing 2D graphics, you'll typically do so in one of two ways:</p>
<ol type="a">
  <li>Draw your graphics or animations into a View object from your layout. In this manner, 
  the drawing (and any animation) of your graphics is handled by the system's 
  normal View hierarchy drawing process &mdash; you simply define the graphics to go inside the View.</li>
  <li>Draw your graphics directly to a Canvas. This way, you personally call the appropriate class's 
  <code>draw()</code> method (passing it your Canvas), or one of the Canvas <code>draw...()</code> methods (like 
  <code>{@link android.graphics.Canvas#drawPicture(Picture,Rect) drawPicture()}</code>). In doing so, you are also in
  control of any animation.</li>
</ol>

<p>Option "a," drawing to a View, is your best choice when you want to draw simple graphics that do not
need to change dynamically and are not part of a performance-intensive game. For example, you should
draw your graphics into a View when you want to display a static graphic or predefined animation, within 
an otherwise static application. Read <a href="#draw-to-view">Simple Graphics Inside a View</a>.</li>

<p>Option "b," drawing to a Canvas, is better when your application needs to regularly re-draw itself.
Basically, any video game should be drawing to the Canvas on its own. However, there's more than 
one way to do this: </p>
<ul>
  <li>In the same thread as your UI Activity, wherein you create a custom View component in
  your layout, call <code>{@link android.view.View#invalidate()}</code> and then handle the 
  <code>{@link android.view.View#onDraw(Canvas) onDraw()}</code> callback..</li>
  <li>Or, in a separate thread, wherein you manage a {@link android.view.SurfaceView} and 
  perform draws to the Canvas as fast as your thread is capable 
  (you do not need to request <code>invalidate()</code>).</li>
</ul>
<p>...Begin by reading <a href="#draw-with-canvas">Draw with a Canvas</a>.</p>

<h2 id="draw-to-view">Simple Graphics Inside a View</h2>

<p>If you'll be drawing some simple graphics (images, shapes, colors, pre-defined animations, etc.),
then you should probably just draw to the background of a View or
to the content of an {@link android.widget.ImageView} in your layout.
In this case, you can skip the rest of this document and learn how to
draw graphics and animations in the <a href="2d-graphics.html">2D Graphics</a> document.
</p>


<h2 id="draw-with-canvas">Draw with a Canvas</h2>

<p>When you're writing an application in which you would like to perform specialized drawing
and/or control the animation of graphics,
you should do so by drawing through a {@link android.graphics.Canvas}. A Canvas works for you as
a pretense, or interface, to the actual surface upon which your graphics will be drawn &mdash; it
holds all of your "draw" calls. Via the Canvas, your drawing is actually performed upon an 
underlying {@link android.graphics.Bitmap}, which is placed into the window.</p>

<p>In the event that you're drawing within the <code>{@link android.view.View#onDraw(Canvas) onDraw()}</code>
callback method, the Canvas is provided for you and you need only place your drawing calls upon it.
You can also acquire a Canvas from <code>{@link android.view.SurfaceHolder#lockCanvas() SurfaceHolder.lockCanvas()}</code>,
when dealing with a SurfaceView object. (Both of these scenarios are discussed in the following sections.)
However, if you need to create a new Canvas, then you must define the {@link android.graphics.Bitmap} 
upon which drawing will actually be performed. The Bitmap is always required for a Canvas. You can set up
a new Canvas like this:</p>
<pre>
Bitmap b = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888);
Canvas c = new Canvas(b);
</pre>

<p>Now your Canvas will draw onto the defined Bitmap. After drawing upon it with the Canvas, you can then carry your 
Bitmap to another Canvas with one of the <code>{@link android.graphics.Canvas#drawBitmap(Bitmap,Matrix,Paint)
Canvas.drawBitmap(Bitmap,...)}</code> methods. It's recommended that you ultimately draw your final
graphics through a Canvas offered to you
by <code>{@link android.view.View#onDraw(Canvas) View.onDraw()}</code> or 
<code>{@link android.view.SurfaceHolder#lockCanvas() SurfaceHolder.lockCanvas()}</code> (see the following sections).</p>

<p>The {@link android.graphics.Canvas} class has its own set of drawing methods that you can use, 
like <code>drawBitmap(...)</code>, <code>drawRect(...)</code>, <code>drawText(...)</code>, and many more.
Other classes that you might use also have <code>draw()</code> methods. For example, you'll probably
have some {@link android.graphics.drawable.Drawable} objects that you want to put on the Canvas. Drawable
has its own <code>{@link android.graphics.drawable.Drawable#draw(Canvas) draw()}</code> method 
that takes your Canvas as an argument.</p>


<h3 id="on-view">On a View</h3>

<p>If your application does not require a significant amount of processing or
frame-rate speed (perhaps for a chess game, a snake game, 
or another slowly-animated application), then you should consider creating a custom View component
and drawing with a Canvas in <code>{@link android.view.View#onDraw(Canvas) View.onDraw()}</code>. 
The most convenient aspect of doing so is that the Android framework will
provide you with a pre-defined Canvas to which you will place your drawing calls.</p>

<p>To start, extend the {@link android.view.View} class (or descendant thereof) and define
the <code>{@link android.view.View#onDraw(Canvas) onDraw()}</code> callback method. This method will be called by the Android 
framework to request that your View draw itself. This is where you will perform all your calls
to draw through the {@link android.graphics.Canvas}, which is passed to you through the <code>onDraw()</code> callback.</p>

<p>The Android framework will only call <code>onDraw()</code> as necessary. Each time that 
your application is prepared to be drawn, you must request your View be invalidated by calling
<code>{@link android.view.View#invalidate()}</code>. This indicates that you'd like your View to be drawn and
Android will then call your <code>onDraw()</code> method (though is not guaranteed that the callback will
be instantaneous). </p>

<p>Inside your View component's <code>onDraw()</code>, use the Canvas given to you for all your drawing,
using various <code>Canvas.draw...()</code> methods, or other class <code>draw()</code> methods that
take your Canvas as an argument. Once your <code>onDraw()</code> is complete, the Android framework will 
use your Canvas to draw a Bitmap handled by the system.</p>

<p class="note"><strong>Note: </strong> In order to request an invalidate from a thread other than your main
Activity's thread, you must call <code>{@link android.view.View#postInvalidate()}</code>.</p>

<p>Also read <a href="{@docRoot}guide/topics/ui/custom-components.html">Building Custom Components</a>
for a guide to extending a View class, and <a href="2d-graphics.html">2D Graphics: Drawables</a> for
information on using Drawable objects like images from your resources and other primitive shapes.</p>

<p>For a sample application, see the Snake game, in the SDK samples folder:
<code>&lt;your-sdk-directory>/samples/Snake/</code>.</p>

<h3 id="on-surfaceview">On a SurfaceView</h3>

<p>The {@link android.view.SurfaceView} is a special subclass of View that offers a dedicated
drawing surface within the View hierarchy. The aim is to offer this drawing surface to
an application's secondary thread, so that the application isn't required
to wait until the system's View hierarchy is ready to draw. Instead, a secondary thread
that has reference to a SurfaceView can draw to its own Canvas at its own pace.</p>

<p>To begin, you need to create a new class that extends {@link android.view.SurfaceView}. The class should also 
implement {@link android.view.SurfaceHolder.Callback}. This subclass is an interface that will notify you
with information about the underlying {@link android.view.Surface}, such as when it is created, changed, or destroyed. 
These events  are important so that you know when you can start drawing, whether you need 
to make adjustments based on new surface properties, and when to stop drawing and potentially 
kill some tasks. Inside your SurfaceView class is also a good place to define your secondary Thread class, which will
perform all the drawing procedures to your Canvas.</p>

<p>Instead of handling the Surface object directly, you should handle it via
a {@link android.view.SurfaceHolder}. So, when your SurfaceView is initialized, get the SurfaceHolder by calling 
<code>{@link android.view.SurfaceView#getHolder()}</code>. You should then notify the SurfaceHolder that you'd
like to receive SurfaceHolder callbacks (from {@link android.view.SurfaceHolder.Callback}) by calling 
{@link android.view.SurfaceHolder#addCallback(SurfaceHolder.Callback) addCallback()} 
(pass it <var>this</var>). Then override each of the 
{@link android.view.SurfaceHolder.Callback} methods inside your SurfaceView class.</p>

<p>In order to draw to the Surface Canvas from within your second thread, you must pass the thread your SurfaceHandler
and retrieve the Canvas with <code>{@link android.view.SurfaceHolder#lockCanvas() lockCanvas()}</code>. 
You can now take the Canvas given to you by the SurfaceHolder and do your necessary drawing upon it. 
Once you're done drawing with the Canvas, call 
<code>{@link android.view.SurfaceHolder#unlockCanvasAndPost(Canvas) unlockCanvasAndPost()}</code>, passing it
your Canvas object. The Surface will now draw the Canvas as you left it. Perform this sequence of locking and 
unlocking the canvas each time you want to redraw.</p>

<p class="note"><strong>Note:</strong> On each pass you retrieve the Canvas from the SurfaceHolder, 
the previous state of the Canvas will be retained. In order to properly animate your graphics, you must re-paint the 
entire surface. For example, you can clear the previous state of the Canvas by filling in a color
with <code>{@link android.graphics.Canvas#drawColor(int) drawColor()}</code> or setting a background image
with <code>{@link android.graphics.Canvas#drawBitmap(Bitmap,Rect,RectF,Paint) drawBitmap()}</code>. Otherwise,
you will see traces of the drawings you previously performed.</p>


<p>For a sample application, see the Lunar Lander game, in the SDK samples folder:
<code>&lt;your-sdk-directory>/samples/LunarLander/</code>. Or,
browse the source in the <a href="{@docRoot}guide/samples/index.html">Sample Code</a> section.</p>