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# Qtopia Core Display Management

When rendering, Qtopia Core's default behavior is for each client to render its widgets into memory while the server is responsible for putting the contents of the memory onto the screen. The server uses the screen driver to copy the content of the memory to the display.

The screen driver is loaded by the server application when it starts running, using Qt's plugin system.

Contents:

- Available Drivers
- Specifying a Driver

## Available Drivers

Qtopia Core provides drivers for the Linux framebuffer, the virtual framebuffer, transformed screens, VNC servers and multi screens. Run the `configure` script to list the available drivers:

     ./configure -help

In the default Qt configuration, only an unaccelerated Linux framebuffer driver (`/dev/fb0`) is enabled. The various drivers can be enabled and disabled using the `configure` script. For example:

     ./configure -qt-gfx-transformed

Custom screen drivers can be implemented by subclassing the QScreen class and creating a screen driver plugin (derived from the QScreenDriverPlugin class). Qtopia Core's implementation of the QScreenDriverFactory class will automatically detect the plugin, loading the driver into the server application at runtime.

## Specifying a Driver

To specify which driver to use, set the QWS_DISPLAY environment variable. For example (if the current shell is bash, ksh, zsh or sh):

     export QWS_DISPLAY="<driver>[:<driver specific options>]...
              [:<display num>]"

The valid values for the `<driver>` argument are `LinuxFb`, `QVFb`, `VNC`, `Transformed`, `Multi` and keys identifying custom drivers, and the `<display num>` argument is used to separate screens that are using the same screen driver and to enable multiple displays (see the Running Qtopia Core Applications documentation for more details). The driver specific options are described in the table below.

Driver Specific Option | Available For | Description
--- | --- | ---
`tty=<device>` | LinuxFb | Passes the device file to the console the application is running on.
`nographicsmodeswitch` | LinuxFb | Ensures that the application is not in graphics mode.
`mmWidth=<value>` | LinuxFb, QVFb | The screen's physical width (used to calculate DPI).
`mmHeight=<value>` | LinuxFb, QVFb | The screen's physical height (used to calculate DPI).
`<device>` | LinuxFb |
`<subdriver>` | VNC, Transformed, Multi | Specifies a subdriver.
`<RotX>` | Transformed | Specifies the rotation of the screen. The valid values of `X` are 90, 180 and 270.
`offset=<x,y>` | Multi | Specifies the coordinates of a subscreens top-left corner (by default 0,0).

The QWS_DISPLAY environment variable can also be set using the `-display` option when running an application. For example:

     myApplication -display "<driver>[:<driver specific options>]...
              [:<display num>]"

Subdrivers
The VNC, Transformed and Multi screen drivers depend on subdrivers. The general syntax for specifying a driver is as follows:
     export QWS_DISPLAY="<driver>[:<subdriver>][:<subdriver options>]...
             [ :<display num>]"
In the case of subdrivers, it is important to add a space between each subdriver and before the display number to separate the various drivers and displays. Note that `Multi` screen drivers can have several subdrivers. For example:
     export QWS_DISPLAY="Multi: QVFb:0 QVFb:offset=640,0:1 :2"
Note also that the VNC screen driver defaults to a virtual screen driver if no subdriver is specified. In the case where a VNC driver acts as a `Multi` subdriver, but has no specified subdriver of its own, the VNC driver accepts a few additional (optional) arguments specifying the size and depth of the default virtual screen: `size=<width x height>` `depth=<value>` It also accepts the mmHeight and mmWidth arguments specifying the screens physical size.