src/devices/graphics.jd

page.title=Graphics
@jd:body

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<p>The Android framework offers a variety of graphics rendering APIs for 2D and
3D that interact with manufacturer implementations of graphics drivers, so it
is important to have a good understanding of how those APIs work at a higher
level. This page introduces the graphics hardware abstraction layer (HAL) upon
which those drivers are built.</p>

<p>Application developers draw images to the screen in two ways: with Canvas or
OpenGL. See <a
href="{@docRoot}devices/graphics/architecture.html">System-level graphics
architecture</a> for a detailed description of Android graphics
components.</p>

<p><a
href="http://developer.android.com/reference/android/graphics/Canvas.html">android.graphics.Canvas</a>
is a 2D graphics API and is the most popular graphics API among developers.
Canvas operations draw all the stock and custom <a
href="http://developer.android.com/reference/android/view/View.html">android.view.View</a>s
in Android. In Android, hardware acceleration for Canvas APIs is accomplished
with a drawing library called OpenGLRenderer that translates Canvas operations
to OpenGL operations so they can execute on the GPU.</p>

<p>Beginning in Android 4.0, hardware-accelerated Canvas is enabled by default.
Consequently, a hardware GPU that supports OpenGL ES 2.0 is mandatory for
Android 4.0 and later devices. See the <a
href="https://developer.android.com/guide/topics/graphics/hardware-accel.html">Hardware
Acceleration guide</a> for an explanation of how the hardware-accelerated
drawing path works and the differences in its behavior from that of the
software drawing path.</p>

<p>In addition to Canvas, the other main way that developers render graphics is
by using OpenGL ES to directly render to a surface. Android provides OpenGL ES
interfaces in the <a
href="http://developer.android.com/reference/android/opengl/package-summary.html">android.opengl</a>
package that developers can use to call into their GL implementations with the
SDK or with native APIs provided in the <a
href="https://developer.android.com/tools/sdk/ndk/index.html">Android
NDK</a>.</p>

<h2 id=android_graphics_components>Android graphics components</h2>

<p>No matter what rendering API developers use, everything is rendered onto a
"surface." The surface represents the producer side of a buffer queue that is
often consumed by SurfaceFlinger. Every window that is created on the Android
platform is backed by a surface. All of the visible surfaces rendered are
composited onto the display by SurfaceFlinger.</p>

<p>The following diagram shows how the key components work together:</p>

<img src="graphics/images/graphics_surface.png" alt="image-rendering components">

<p class="img-caption"><strong>Figure 1.</strong> How surfaces are rendered</p>

<p>The main components are described below:</p>

<h3 id=image_stream_producers>Image Stream Producers</h3>

<p>An image stream producer can be anything that produces graphic buffers for
consumption. Examples include OpenGL ES, Canvas 2D, and mediaserver video
decoders.</p>

<h3 id=image_stream_consumers>Image Stream Consumers</h3>

<p>The most common consumer of image streams is SurfaceFlinger, the system
service that consumes the currently visible surfaces and composites them onto
the display using information provided by the Window Manager. SurfaceFlinger is
the only service that can modify the content of the display. SurfaceFlinger
uses OpenGL and the Hardware Composer to compose a group of surfaces.</p>

<p>Other OpenGL ES apps can consume image streams as well, such as the camera
app consuming a camera preview image stream. Non-GL applications can be
consumers too, for example the ImageReader class.</p>

<h3 id=window_manager>Window Manager</h3>

<p>The Android system service that controls a window, which is a container for
views. A window is always backed by a surface. This service oversees
lifecycles, input and focus events, screen orientation, transitions,
animations, position, transforms, z-order, and many other aspects of a window.
The Window Manager sends all of the window metadata to SurfaceFlinger so
SurfaceFlinger can use that data to composite surfaces on the display.</p>

<h3 id=hardware_composer>Hardware Composer</h3>

<p>The hardware abstraction for the display subsystem. SurfaceFlinger can
delegate certain composition work to the Hardware Composer to offload work from
OpenGL and the GPU. SurfaceFlinger acts as just another OpenGL ES client. So
when SurfaceFlinger is actively compositing one buffer or two into a third, for
instance, it is using OpenGL ES. This makes compositing lower power than having
the GPU conduct all computation.</p>

<p>The Hardware Composer HAL conducts the other half of the work. This HAL is
the central point for all Android graphics rendering. Hardware Composer must
support events, one of which is VSYNC. Another is hotplug for plug-and-play
HDMI support.</p>

<p>See the <a href="{@docRoot}devices/graphics.html#hardware_composer_hal">Hardware Composer
HAL</a> section for more information.</p>

<h3 id=gralloc>Gralloc</h3>

<p>The graphics memory allocator is needed to allocate memory that is requested
by image producers. See the <a
href="{@docRoot}devices/graphics.html#gralloc">Gralloc HAL</a> section for more
information.</p>

<h2 id=data_flow>Data flow</h2>

<p>See the following diagram for a depiction of the Android graphics
pipeline:</p>

<img src="graphics/images/graphics_pipeline.png" alt="graphics data flow">

<p class="img-caption"><strong>Figure 2.</strong> How graphic data flow through
Android</p>

<p>The objects on the left are renderers producing graphics buffers, such as
the home screen, status bar, and system UI. SurfaceFlinger is the compositor
and Hardware Composer is the composer.</p>

<h3 id=bufferqueue>BufferQueue</h3>

<p>BufferQueues provide the glue between the Android graphics components. These
are a pair of queues that mediate the constant cycle of buffers from the
producer to the consumer. Once the producers hand off their buffers,
SurfaceFlinger is responsible for compositing everything onto the display.</p>

<p>See the following diagram for the BufferQueue communication process.</p>

<img src="graphics/images/bufferqueue.png" alt="BufferQueue communication process">

<p class="img-caption"><strong>Figure 3.</strong> BufferQueue communication
process</p>

<p>BufferQueue contains the logic that ties image stream producers and image
stream consumers together. Some examples of image producers are the camera
previews produced by the camera HAL or OpenGL ES games. Some examples of image
consumers are SurfaceFlinger or another app that displays an OpenGL ES stream,
such as the camera app displaying the camera viewfinder.</p>

<p>BufferQueue is a data structure that combines a buffer pool with a queue and
uses Binder IPC to pass buffers between processes. The producer interface, or
what you pass to somebody who wants to generate graphic buffers, is
IGraphicBufferProducer (part of <a
href="http://developer.android.com/reference/android/graphics/SurfaceTexture.html">SurfaceTexture</a>).
BufferQueue is often used to render to a Surface and consume with a GL
Consumer, among other tasks.

BufferQueue can operate in three different modes:</p>

<p><em>Synchronous-like mode</em> - BufferQueue by default operates in a
synchronous-like mode, in which every buffer that comes in from the producer
goes out at the consumer. No buffer is ever discarded in this mode. And if the
producer is too fast and creates buffers faster than they are being drained, it
will block and wait for free buffers.</p>

<p><em>Non-blocking mode</em> - BufferQueue can also operate in a non-blocking
mode where it generates an error rather than waiting for a buffer in those
cases. No buffer is ever discarded in this mode either. This is useful for
avoiding potential deadlocks in application software that may not understand
the complex dependencies of the graphics framework.</p>

<p><em>Discard mode</em> - Finally, BufferQueue may be configured to discard
old buffers rather than generate errors or wait. For instance, if conducting GL
rendering to a texture view and drawing as quickly as possible, buffers must be
dropped.</p>

<p>To conduct most of this work, SurfaceFlinger acts as just another OpenGL ES
client. So when SurfaceFlinger is actively compositing one buffer or two into a
third, for instance, it is using OpenGL ES.</p>

<p>The Hardware Composer HAL conducts the other half of the work. This HAL acts
as the central point for all Android graphics rendering.</p>

<h3 id=synchronization_framework>Synchronization framework</h3>

<p>Since Android graphics offer no explicit parallelism, vendors have long
implemented their own implicit synchronization within their own drivers. This
is no longer required with the Android graphics synchronization framework. See
the <a href="#explicit_synchronization">Explicit synchronization</a> section
for implementation instructions.</p>

<p>The synchronization framework explicitly describes dependencies between
different asynchronous operations in the system. The framework provides a
simple API that lets components signal when buffers are released. It also
allows synchronization primitives to be passed between drivers from the kernel
to userspace and between userspace processes themselves.</p>

<p>For example, an application may queue up work to be carried out in the GPU.
The GPU then starts drawing that image. Although the image hasnt been drawn
into memory yet, the buffer pointer can still be passed to the window
compositor along with a fence that indicates when the GPU work will be
finished. The window compositor may then start processing ahead of time and
hand off the work to the display controller. In this manner, the CPU work can
be done ahead of time. Once the GPU finishes, the display controller can
immediately display the image.</p>

<p>The synchronization framework also allows implementers to leverage
synchronization resources in their own hardware components. Finally, the
framework provides visibility into the graphics pipeline to aid in
debugging.</p>