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Hardware accelerated presentation—which includes video
scaling, color conversion, and blitting—enhances the performance
benefits of hardware accelerated decoding. On devices that offer
GPU (hardware) acceleration, you can use a flash.media.StageVideo
object to process video directly on the device hardware. Direct
processing frees the CPU to perform other tasks while the GPU handles
video. The legacy Video class, on the other hand, typically uses
software presentation. Software presentation occurs in the CPU and
can consume a significant share of system resources.
Currently, few devices provide full GPU acceleration. However,
stage video lets applications take maximum advantage of whatever
hardware acceleration is available.
The StageVideo class does not make the Video class obsolete.
Working together, these two classes provide the optimal video display
experience allowed by device resources at any given time. Your application
takes advantage of hardware acceleration by listening to the appropriate
events and switching between StageVideo and Video as necessary.
The StageVideo class imposes certain restrictions on video usage.
Before implementing StageVideo, review the guidelines and make sure
your application can accept them. If you accept the restrictions,
use the StageVideo class whenever Flash Player detects that hardware
accelerated presentation is available. See
Guidelines and limitations
.
Parallel planes: Stage video and the Flash display list
With the stage video model, Flash Player can separate video
from the display list. Flash Player divides the composite display
between two Z-ordered planes:
-
Stage video plane
-
The stage video plane sits in the background. It displays
only hardware accelerated video. Because of this design, this plane
is not available if hardware acceleration is not supported or not
available on the device. In ActionScript, StageVideo objects handle
videos played on the stage video plane.
-
Flash display list plane
-
Flash display list entities are composited on a plane in front
of the stage video plane. Display list entities include anything
that the runtime renders, including playback controls. When hardware
acceleration is not available, videos can be played only on this
plane, using the Video class object. Stage video always displays
behind Flash display list graphics.
The StageVideo object appears in a non-rotated, window-aligned
rectangular region of the screen. You cannot layer objects behind
the stage video plane. However, you can use the Flash display list
plane to layer other graphics on top of the stage video plane. Stage
video runs concurrently with the display list. Thus, you can use
the two mechanisms together to create a unified visual effect that uses
two discreet planes. For example, you can use the front plane for
playback controls that operate on the stage video running in the
background.
Stage video and H.264 codec
In Flash Player applications, implementing video hardware
acceleration involves two steps:
-
Encoding the video as H.264
-
Implementing the StageVideo API
For best results, perform both steps. The H.264 codec lets you
take maximum advantage of hardware acceleration, from video decoding
to presentation.
Stage video eliminates GPU-to-CPU read-back. In other words,
the GPU no longer sends decoded frames back to the CPU for compositing
with display list objects. Instead, the GPU blits decoded and rendered
frames directly to the screen, behind the display list objects.
This technique reduces CPU and memory usage and also provides better
pixel fidelity.
Guidelines and limitations
When video is running in full screen mode, stage video
is always available if the device supports hardware acceleration.
Flash Player, however, also runs within a browser. In the browser
context, the
wmode
setting affects stage video
availability. Try to use
wmode="direct"
at all
times if you want to use stage video. Stage video is not compatible
with other
wmode
settings when not in full screen mode.
This restriction means that, at run time, stage video can vacillate
unpredictably between being available and unavailable. For example,
if the user exits full screen mode while stage video is running,
the video context reverts to the browser. If the browser
wmode
parameter
is not set to
"direct"
, stage video can suddenly
become unavailable. Flash Player communicates playback context changes
to applications through a set of events. If you implement the StageVideo API,
maintain a Video object as a backup when stage video becomes unavailable.
Because of its direct relationship to hardware, stage video restricts
some video features. Stage video enforces the following constraints:
-
For
each SWF file, Flash Player limits the number of StageVideo objects
that can concurrently display videos to four. However, the actual
limit can be lower, depending on device hardware resources.
-
The video timing is not synchronized with the timing of content
that the runtime displays.
-
The video display area can only be a rectangle. You cannot
use more advanced display areas, such as elliptical or irregular
shapes.
-
You cannot rotate the video.
-
You cannot bitmap cache the video or use BitmapData object
to access it.
-
You cannot apply filters to the video.
-
You cannot apply color transforms to the video.
-
You cannot apply an alpha value to the video.
-
Blend modes that you apply to objects in the display list
plane do not apply to stage video.
-
You can place the video only on full pixel boundaries.
-
Though GPU rendering is the best available for the given
device hardware, it is not 100% “pixel identical” across devices.
Slight variations occur due to driver and platform differences.
-
A few devices do not support all required color spaces. For
example, some devices do not support BT.709, the H.264 standard.
In such cases, you can use BT.601 for fast display.
-
You cannot use stage video with WMODE settings such as normal,
opaque, or transparent. Stage video supports only
WMODE=direct
when
not in full screen mode. WMODE has no effect in Safari 4 or higher
and IE 9 or higher.
In most cases, these limitations do not affect video player applications.
If you can accept these limitations, use stage video whenever possible.
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