Window Surface Implementation

Symbian windows

Window surfaces are created by the eglCreateWindowSurface() function, to which is passed a handle to a window provided by the Symbian Window Server. Key Window Server classes include the following:

• RWindow, which is a handle to a standard window.
• RWsSession, which is the Window Server session class. This is required to create Window Server objects.
• CWsScreenDevice, which is a Window Server client-side object representing the screen.

Non-ScreenPlay

In the non-ScreenPlay variant there is no composition engine and both Symbian API drawing (for example, through CWindowGc) and EGL client API drawing are rendered to a single display. The EGL implementation can achieve this in a number of ways, including:

• Using Direct Screen Access (DSA) to render the EGL client API drawing directly to the screen. This is done in cooperation with the Window Server, which provides clipping instructions. This approach is particularly suitable when graphics acceleration hardware is available.

• Rendering the EGL client API drawing to an offscreen CFbsBitmap and then calling a Window Server API to blit the CFbsBitmap onto the screen, with any necessary clipping.

ScreenPlay

ScreenPlay has introduced the Graphics Composition Collection, which enables a number of surfaces to be arranged in layers and to be composed together to produce the final output on the display. In this section, these surfaces are called composition surfaces in order to distinguish them from EGL surfaces. Each composition surface is mapped to memory, which can be specialized graphics memory for faster performance.

All Window Server drawing (such as the application UI) is rendered to a composition surface called the UI surface, which is usually the topmost layer and can be semi-transparent.

When the EGL implementation creates an EGL window surface for a Symbian window (represented by RWindow), the EGL implementation must do the following:

• Create a new composition surface with the same width and height as the RWindow.

• Position the composition surface in a layer below the UI layer, directly below the RWindow.

• Make the RWindow transparent in order for the composition surface to show through.

All of the EGL client API drawing to the EGL window surface is then rendered to the composition surface and the application's UI appears as a semi-transparent layer above it.

When a window is resized, a new composition surface needs to be created with the new width and height, and positioned as a layer below the RWindow in place of the previous composition surface.

A composition surface can have more than one pixel buffer associated with it (each buffer has the same dimensions). For an EGL window surface, the composition surface should have two (or more) buffers for double-buffered drawing. This means, for example, that OpenVG commands can draw to the back buffer, while the front buffer is being composed to the screen.