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<html devsite>
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<title>Surface and SurfaceHolder</title>
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<p>The
<a href="http://developer.android.com/reference/android/view/Surface.html">Surface</a>
class has been part of the public API since 1.0. Its description simply says,
"Handle onto a raw buffer that is being managed by the screen compositor." The
statement was accurate when initially written but falls well short of the mark
on a modern system.</p>
<p>The Surface represents the producer side of a buffer queue that is often (but
not always!) consumed by SurfaceFlinger. When you render onto a Surface, the
result ends up in a buffer that gets shipped to the consumer. A Surface is not
simply a raw chunk of memory you can scribble on.</p>
<p>The BufferQueue for a display Surface is typically configured for
triple-buffering; but buffers are allocated on demand. So if the producer
generates buffers slowly enough -- maybe it's animating at 30fps on a 60fps
display -- there might only be two allocated buffers in the queue. This helps
minimize memory consumption. You can see a summary of the buffers associated
with every layer in the <code>dumpsys SurfaceFlinger</code> output.</p>
<h2 id="canvas">Canvas Rendering</h2>
<p>Once upon a time, all rendering was done in software, and you can still do this
today. The low-level implementation is provided by the Skia graphics library.
If you want to draw a rectangle, you make a library call, and it sets bytes in a
buffer appropriately. To ensure that a buffer isn't updated by two clients at
once, or written to while being displayed, you have to lock the buffer to access
it. <code>lockCanvas()</code> locks the buffer and returns a Canvas to use for drawing,
and <code>unlockCanvasAndPost()</code> unlocks the buffer and sends it to the compositor.</p>
<p>As time went on, and devices with general-purpose 3D engines appeared, Android
reoriented itself around OpenGL ES. However, it was important to keep the old
API working, for apps as well as app framework code, so an effort was made to
hardware-accelerate the Canvas API. As you can see from the charts on the
<a href="http://developer.android.com/guide/topics/graphics/hardware-accel.html">Hardware
Acceleration</a>
page, this was a bit of a bumpy ride. Note in particular that while the Canvas
provided to a View's <code>onDraw()</code> method may be hardware-accelerated, the Canvas
obtained when an app locks a Surface directly with <code>lockCanvas()</code> never is.
As of API 23, a hardware-accelerated Canvas can be obtained from a Surface using
<code>lockHardwareCanvas()</code> instead.</p>
<p>When you lock a Surface for Canvas access, the "CPU renderer" connects to the
producer side of the BufferQueue and does not disconnect until the Surface is
destroyed. Most other producers (like GLES) can be disconnected and reconnected
to a Surface, but the Canvas-based "CPU renderer" cannot. This means you can't
draw on a surface with GLES or send it frames from a video decoder if you've
ever locked it for a Canvas.</p>
<p>The first time the producer requests a buffer from a BufferQueue, it is
allocated and initialized to zeroes. Initialization is necessary to avoid
inadvertently sharing data between processes. When you re-use a buffer,
however, the previous contents will still be present. If you repeatedly call
<code>lockCanvas()</code> and <code>unlockCanvasAndPost()</code> without
drawing anything, you'll cycle between previously-rendered frames.</p>
<p>The Surface lock/unlock code keeps a reference to the previously-rendered
buffer. If you specify a dirty region when locking the Surface, it will copy
the non-dirty pixels from the previous buffer. There's a fair chance the buffer
will be handled by SurfaceFlinger or HWC; but since we need to only read from
it, there's no need to wait for exclusive access.</p>
<p>The main non-Canvas way for an application to draw directly on a Surface is
through OpenGL ES. That's described in the <a href="#eglsurface">EGLSurface and
OpenGL ES</a> section.</p>
<h2 id="surfaceholder">SurfaceHolder</h2>
<p>Some things that work with Surfaces want a SurfaceHolder, notably SurfaceView.
The original idea was that Surface represented the raw compositor-managed
buffer, while SurfaceHolder was managed by the app and kept track of
higher-level information like the dimensions and format. The Java-language
definition mirrors the underlying native implementation. It's arguably no
longer useful to split it this way, but it has long been part of the public API.</p>
<p>Generally speaking, anything having to do with a View will involve a
SurfaceHolder. Some other APIs, such as MediaCodec, will operate on the Surface
itself. You can easily get the Surface from the SurfaceHolder, so hang on to
the latter when you have it.</p>
<p>APIs to get and set Surface parameters, such as the size and format, are
implemented through SurfaceHolder.</p>
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