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<concept id="GUID-859CAA08-59C9-5FD3-98DE-6BDD0D6ED50B" xml:lang="en"><title>Graphics

Composition</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>Composition is the process of putting together the output elements from

various different sources to create the screen display that the end user sees

on the device. </p>

<p> <b>Variant</b>: ScreenPlay. </p>

<p>In a multi-tasking device many of the activities taking place simultaneously

generate output for display on the screen. Their output can include words,

pictures, video, games and the screen furniture (scroll bars, buttons, icons,

borders, tabs, menus, title bars) familiar to every computer user. </p>

<p>Many of these output elements can appear at the same time, either next

to each other or overlapping each other. They can be opaque, such that they

obscure anything behind, or semi-transparent such that the elements underneath

are partially visible. </p>

<p>The diagram below illustrates how the display that the viewer sees (looking

down from the top) is a two-dimensional representation composed from a series

of layers. </p>

<fig id="GUID-19D41F69-1264-5726-9CDD-C4DD0F231BE3">

<title>           The display is an orthogonal view of a series of layers.

         </title>

</fig>

<p>Composition requires: </p>

<ul>

<li id="GUID-7C593A87-268F-59E1-B295-9DE2A0DF3092"><p>Calculations based on

the size, position, visibility, transparency and ordering of the layers to

determine what will be displayed. This is a <b>logic</b> exercise referred

to as UI Composition. </p> </li>

<li id="GUID-B10758D4-8C30-5446-958D-0B81530B69BF"><p>Handling of image content,

which is a <b>data processing</b> exercise referred to as Image Composition. </p> </li>

</ul>

<p>For this reason the two are handled separately. </p>

<p><b>UI Composition </b> </p>

<p>UI Composition is performed by the Window Server. Each application has

its own window group containing all of its child windows. The Window Server

keeps track of the windows' positions, sizes, visibilities, transparencies

and z-order and is able to establish which windows, and which bits of each

window, are visible. It ensures that each visible bit of window is kept up-to-date

by calling its application when necessary. </p>

<p><b>Image composition </b> </p>

<p>Prior to the introduction of ScreenPlay the Window Server did rudimentary

composition in its main thread and rendered composited output to the screen

buffer using the GDI (Graphics Device Interface). To achieve high frame rates,

applications typically bypassed the Window Server using Direct Screen Access

(DSA) and animation DLLs. </p>

<p>In <xref href="GUID-D93978BE-11A3-5CE3-B110-1DEAA5AD566C.dita">ScreenPlay</xref> sources

that generate complex graphical output render directly to graphics surfaces.

The Window Server delegates the composition of surfaces to a composition engine

that device creators can adapt to take advantage of graphics processing hardware. </p>

<p>Although the composition engine runs in the Window Server's process, it

is largely transparent to application developers and works quite differently.

Instead of using windows, it uses surfaces to store pixel data and elements

to manipulate size, position, z-order, visibility and transparency. </p>

<p>While an element is a simple lightweight object, and easy to manipulate,

a surface stores a large amount of data and its handling requires more consideration. </p>

<p>The diagram below is a simplistic representation of how applications create

output which is rendered, composited and displayed. </p>

<fig id="GUID-9DF4C86A-C06D-5C5A-9AB6-E9991CC1937A">

<title>           Graphics Composition          </title>

</fig>

<p>In a device with graphics acceleration hardware (a Graphics Processing

Unit or GPU) there might, in addition to the memory managed by the CPU, be

additional memory managed by the GPU. Image data may therefore be considered

to have been <b>software rendered</b> (onto a surface in CPU memory) or <b>hardware

rendered</b> (onto a surface in hardware accelerated GPU memory). The following

diagram shows applications and other graphical data sources rendering to surfaces

in software and hardware. </p>

<fig id="GUID-2717F861-0045-5598-A3EC-7CF678BEFA70">

<title>           Hardware rendered graphics - the wrong way!          </title>

</fig>

<p>The diagram, however, represents a system with several problem areas that

would render it unsuitable for any practical implementation </p>

<ul>

<li id="GUID-AF13B0A1-3DC0-576D-81B1-6DF66AA9FA77"><p>In practice it is likely

that once data has been rendered to hardware-managed memory it is, to all

intents and purposes, unavailable to software: the CPU is unable to access

it sufficiently quickly. The dotted paths on the diagram above must therefore

be avoided. </p> </li>

<li id="GUID-0B109999-94FE-5A5C-B0D0-D3F2157B5CC6"><p>Furthermore, GDI rendered

data (the UI) is typically 'stored' as redrawing instructions rather than

bitmapped pixel data—so rendering it to bitmaps before composition is likely

to use a lot of memory unnecessarily. </p> </li>

</ul>

<p>In ScreenPlay, the UI is therefore composed and rendered onto a single

surface before being composited with any other surfaces. This surface is termed

the <b>UI surface</b> and is displayed on a layer placed in front of all of

the others. In fact, the UI surface is created for the Window Server during

system start up and is then passed to the composition components as a 'special

case' surface for composition. </p>

<section id="GUID-6E274D1A-E4CB-4980-B351-B396FEC48DB8"><title>Combining Window Server and Composition </title> <p>The compositing

of surfaces according to their origin means that the physical composition

process behaves differently from the logical composition process that is based

on what the user and the UI are doing. Logically the windows in the UI and

memory rendered surfaces may be on layers that are interleaved yet the memory

rendered surfaces are physically composed behind. </p> <p>ScreenPlay addresses

this issue by associating 'external' surfaces with windows in the Window Server

using the <xref href="GUID-1460DD8F-9AA1-3B99-8FFD-F309959CCA34.dita#GUID-1460DD8F-9AA1-3B99-8FFD-F309959CCA34/GUID-4EBFFA14-418A-3A2A-B147-39BFC96CE45F"><apiname>RWindowBase::SetBackgroundSurface()</apiname></xref> method. This

means that the Window Server is able to include them in its logical composition

and make provision for them during data composition. A window with its background

set to a surface in this way becomes transparent in the UI rendered UI surface. </p> <p>In

most cases this is not apparent to the viewer. Surfaces that are physically

composed behind the UI appear in the correct position on the two-dimensional

display. The diagram below illustrates how Window Server-rendered UI content

and external surfaces are composited using the UI surface. </p> <fig id="GUID-3448FD6B-9A39-58CD-8819-39B0B6CC4E13">

<title>              Hardware composition and the Flattened UI            </title>

</fig> <p>Although this method of composition is flexible and powerful, it

does have some limitations, particularly with respect to semi-transparent

hardware-accelerated surfaces. </p> <p>It is not possible, for example, to

include a semi-transparent surface in front of the UI surface. In such cases

the composition engine determines how the display is composed. </p> <p>Here

is a second version of the diagram at the top of the page showing how the

same composition might be achieved in practice. The UI menus, windows and

dialogs are composited by the Window Server onto a single surface. The light

green layer displays a hardware rendered surface so it is actually behind

the layer on which it appears. </p> <fig id="GUID-1FB1194D-FCAE-534C-94DC-AF4CF00E3A1C">

<title>              The UI surface            </title>

</fig> </section>

<section id="GUID-41E9CA54-759F-5651-95F8-9F39808BE740"><title>Composition

Examples</title> <p>This illustrations below illustrate the use of hardware

accelerated surfaces and the UI surface </p> <fig id="GUID-8178DBA8-E10C-56F3-A828-80746CE6A993">

<title>              Video rendered to a hardware accelerated surface mapped

to a layer              behind the UI surface            </title>

</fig> <fig id="GUID-51411B10-6DC3-5B74-BCEF-11EC1D0FBCA8">

<title>              As above with a semi-transparent dialog on the UI surface

           </title>

</fig> </section>

</conbody><related-links>

<link href="GUID-D93978BE-11A3-5CE3-B110-1DEAA5AD566C.dita"><linktext>The ScreenPlay

Architecture</linktext></link>

<link href="GUID-EF62BF88-3687-505D-8BD7-EEDF36246E56.dita"><linktext>Hardware

Acceleration</linktext></link>

<link href="GUID-63CB6C7E-44EC-5D0B-A37D-FE78F7D76592.dita">

<linktext>Graphics Composition Collection</linktext></link>

</related-links></concept>