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  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_d0e250255_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_d0e250269_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_d0e250289_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>