<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
<!-- This component and the accompanying materials are made available under the terms of the License 
"Eclipse Public License v1.0" which accompanies this distribution, 
and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
<!-- Initial Contributors:
    Nokia Corporation - initial contribution.
Contributors: 
-->
<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-41802B91-26B3-5F3C-AE04-B6954F3804B7" xml:lang="en"><title>Scene
Elements</title><shortdesc>This topic provides an introduction to the concept of <b>scene
elements</b> within the context of graphics composition. Scene elements are
sometimes referred to as <b>layers</b>. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p> <b>Variant</b>: ScreenPlay. <b>Target audience</b>: Device creators. </p>
<p>Within a composition scene, elements map surfaces to screen rectangles.
Depth ordering (z-order) is used to resolve how the elements overlap each
other. The elements support some basic pixel rectangle transformations such
as scaling and rotation in 90 degree increments. </p>
<p>The following diagram shows the three elements in a video telephony use
case. The uppermost element (with a red border) contains the UI content (called
the UI surface). This is semi-transparent and is owned by the Window Server.
The next element down is the camera viewfinder (green border). The surface
associated with this element is owned by the camera. The final element (blue
border) shows a video of the caller. The surface associated with this element
is owned by the video renderer. </p>
<fig id="GUID-C2FEB262-DA13-5172-A432-EAC03F0FC80A">
<title>           The elements in a video telephony use case          </title>
<image href="GUID-E1E25E68-7FE3-53A7-980E-CCF25090715F_d0e281300_href.png" placement="inline"/>
</fig>
<p>A similar principle applies when a game is running on the phone. The semi-transparent
UI surface would overlay the element containing the EGL-owned surface. When
the game or a video is in full screen mode, the composition engine can enter <b>fast
path mode</b>. This indicates to the LCD controller that there is no UI content
to blend with the background element. </p>
<fig id="GUID-A52A1E62-E929-5CC3-A756-A7E332951155">
<title>           Fast path mode          </title>
<image href="GUID-F5BFE35F-ED90-50C5-8F63-7A780A97689B_d0e281314_href.png" placement="inline"/>
</fig>
<p> </p>
<p>The z-order of an element is determined by its position in the list of
elements. Several additional pieces of positional information are stored as
element metadata. The element maps the surface to the device screen. More
specifically it maps a part of the surface, the <b>source rectangle</b>, to
a <b>destination rectangle</b> on the screen. The destination rectangle defines
the position and size of the element itself. </p>
<fig id="GUID-FCF3894F-F8D5-511F-881A-169501AA27B5">
<title>           Element terminology          </title>
<image href="GUID-FBF2ED6F-D1C5-5368-96C2-FCF38234BF43_d0e281334_href.png" placement="inline"/>
</fig>
<p>The source rectangle corresponds to the <b>viewport</b> in the Window Server
documentation and the destination rectangle corresponds to the <b>extent</b>. </p>
<p>The source and destination rectangles do not have to be the same size—the
composition engine can clip or scale the data to fit. </p>
<p>The following concepts are related to elements: </p>
<ul>
<li id="GUID-53CC7FB9-1BB9-50B1-8840-D83439BE4DBF"><p> <b>Scene structure</b>  </p> <p>A
set of elements, all with a specific size, visible region and z-order, which
when composed form an entire scene. </p> </li>
<li id="GUID-AC3AD2BA-EE44-54B7-AA37-73CC8D0CF158"><p> <b>Scene content</b>  </p> <p>A
set of surfaces bound with elements that contain the pixels to be composed
according to the structure described by the scene structure. </p> </li>
</ul>
<p>This distinction between the scene structure and scene content is important.
The architecture is split so that the Window Server deals only with the scene
structure and UI content and not with the scene content. This allows optimal
data and control paths. For example, it has the advantage that the Window
Server does not need to take any action when the content of a video surface
changes but the UI does not change. </p>
<p>The components in the Graphics Composition collection are divided between
those that manage the scene structure and those that manage scene content: </p>
<ul>
<li id="GUID-DB21268B-F180-5985-BEAE-ECC4E1DC2420"><p>The composition engine
manages scene structure. This means that the composition engine mainly deals
in elements and dictates how the scene is composed. </p> </li>
<li id="GUID-F32404D9-5DD8-53A8-95D2-664BC3E25A79"><p>The Surface Manager
and the Surface Update Server manage scene content. This means that they mainly
deal in surfaces. </p> </li>
</ul>

</conbody><related-links>
<link href="GUID-783086B9-D6FE-539C-90F1-D745253A640D.dita"><linktext>Composition
Concepts</linktext></link>
<link href="GUID-0DC3E5AA-1706-5255-ACD6-E7AB732E1095.dita"><linktext>Graphics
Composition Collection Overview</linktext></link>
</related-links></concept>