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Qt/Embedded Virtual Framebuffer

The virtual framebuffer allows Qt/Embedded programs to be developed on your desktop machine, without switching between consoles and X11.

The virtual framebuffer is located in $QTDIR/tools/qvfb.

Using the Virtual Framebuffer

1. Make sure QT_NO_QWS_VFB in $QTDIR/src/tools/qconfig.h is not defined and compile the Qt/Embedded library.
2. Compile qvfb as a normal Qt/X11 application and run it. Do not compile it as a Qt/Embedded application.
3. Start a Qt/Embedded master application (i.e., construct QApplication with QApplication::GuiServer flag or use the -qws command line parameter). You will need to specify to the server that you wish to use the virtual framebuffer driver, e.g.:

       widgets -qws -display QVFb:0
       

4. You may prefer to set the QWS_DISPLAY environment variable to be QVFb:0.

qvfb supports the following command line options:

• -width width

  the width of the virtual framebuffer (default: 240).

• -height height

  the height of the virtual framebuffer (default: 320).

• -depth depth

  the depth of the virtual framebuffer (1, 8 or 32; default: 8).

• -nocursor

  do not display the X11 cursor in the framebuffer window.

• -qwsdisplay :id

  the Qt/Embedded display id to provide (default: 0).

Virtual Framebuffer Design

The virtual framebuffer emulates a framebuffer using a shared memory region (the virtual frame buffer) and a utility to display the framebuffer in a window (qvfb). The regions of the display that have changed are updated periodically, so you will see discrete snapshots of the framebuffer rather than each individual drawing operation. For this reason drawing problems such as flickering may not be apparent until the program is run using a real framebuffer.

The target refresh rate can be set via the View|Refresh Rate menu item. This will cause qvfb to check for updated regions more frequently. The rate is a target only. If little drawing is being done, the framebuffer will not show any updates between drawing events. If an application is displaying an animation the updates will be frequent, and the application and qvfb will compete for processor time.

Mouse and keyboard events are passed to the Qt/Embedded master process via named pipes.

The virtual framebuffer is a development tool only. No security issues have been considered in the virtual framebuffer design. It should be avoided in a production environment; QT_NO_QWS_VFB should always be defined in production libraries.