Transparent +Windows

Note: Semi-transparent UI content over content provided by Direct +Screen Access (DSA) has never been supported and ScreenPlay does not change +this.

Variant: ScreenPlay and non-ScreenPlay. Target audience: +Application developers.

Alpha compositing

In alpha compositing, an alpha +channel is used to store opacity information along with the color values for +each pixel. For example, if the color is represented as RGB values, the pixel +values are typically stored as RGBA, where the final value is the alpha value. +This represents the percentage opacity. Like the RGB values, the alpha value +is frequently expressed in the range of 0 (completely transparent) to 255 +(fully opaque). For example, (0, 0, 0, 0) represents fully transparent black +and (255, 255, 255, 127) represents white that has 50% opacity.

Each +of the RGB values are multiplied by the alpha percentage to give the opacity +of the pixel. In order to reduce the need for multiplication during the compositing +process, the RGB values are sometimes multiplied by the alpha value before +storage. This is known as premultiplied alpha format. For example, +a premultiplied alpha representation of white that has 50% opacity is (127, +127, 127, 127).

Enabling transparency

To enable transparency, call +the RWindow::SetTransparencyAlphaChannel() method +on the window (which must be an RWindow type) +before it is activated. The transparency is then determined by the alpha values +of the content rendered to the window. Typically you set the background to +be fully or partially transparent, as shown in the following example:

// Set transparency mode on. +iWindow.SetTransparencyAlphaChannel(); + +// Set the background to be fully transparent black. +iWindow.SetBackgroundColor(TRgb(0,0,0,0)); + +// Activate the window. +iWindow.Activate(); + +// Begin drawing to the window. +...

Note

In earlier versions of the Symbian platform, +it was possible to create transparent windows using a mask and the RWindow class's SetTransparencyFactor(), SetTransparencyBitmap() and SetTransparencyWsBitmap() methods. This +mechanism is now deprecated and these methods are no longer functional.

A +new application and UI framework is planned for Symbian^4 (S^4). In order +for AVKON applications (and clients that draw using CWindowGc) +to work correctly on this new framework, the main view must be opaque. See How will the New Application +Architecture Affect my Applications? for more information.