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Qt for Embedded Linux and DirectFBIntroductionDirectFB is an open source LGPL licensed project founded by Denis Oliver Kropp and generally chip vendors start out with the official version and implement their own plugins to optimize the operations their hardware supports. We recommend using Qt 4.6 or later with DirectFB. Support for DirectFB was introduced into Qt for Embedded Linux as a labs project for Qt 4.3 and folded into Qt as a screen driver for Qt 4.4, but not supported fully. In Qt 4.5, major changes were made to make it work with the optimized raster paint engine. These changes were further improved in Qt 4.6. Using DirectFB with QtDirectFB is centered around Surfaces which is the equivalent of a QPaintDevice. In the Qt/DirectFB plugin, DirectFB maps onto either a QPixmap or a QWindowSurface which essentially means that drawing onto QPixmap or a QWidget can be accelerated and drawing onto any other paint device (e.g. QImage) cannot. ConfigureWhen configuring Qt there are two options, from which you can choose: ./configure -plugin-gfx-directfb ./configure -qt-gfx-directfb With either mode, Qt will try the following to look for the DirectFB includes/libs.
Often the values returned from pkg-config/directfb-config indicates the locations of the libs/headers on the target rootfs, rather than their location on your host. The safest option is usually to explicitly populate these variables in your qmake.conf like this: QT_CFLAGS_DIRECTFB = /opt/toolchain/gcc4.3_mipsel_linux/usr/include/directfb -D_REENTRANT QT_LIBS_DIRECTFB = -L/opt/toolchain/gcc4.3_mipsel_linux/usr/lib/-ldirect -ldirectfb -lfusion Note: While DirectFB supports a multi-process setup through a kernel-extension called Fusion this setup is not well tested with Qt. Supported graphics operationsIDirectFBSurface supports blitting, filling, drawing lines rects etc, but it does not support everything Qt allows you to do. E.g. painter paths, polygons, complex transformations, antialiasing, gradients. Some of these things are handled in newer versions of DirectFB and could be supported by Qt. They are seemingly optional at the driver level, so you need to have fall back code paths for older drivers and drivers on which this is not implemented. The QDirectFBPaintEngine is a subclass of the QRasterPaintEngine, thus essentially supporting everything QRasterPaintEngine supports. This means that it supports all graphical operations that Qt supports, but certain operations will have to fall back to software rendering and that should be avoided due to performance issues. Instead, these operations should be rendered into a QPixmap once, and then reuse the pixmap. Note: Fallbacks to software rendering should be avoided. If unsupported operations are used, the paint engine must fallback to the QRasterPaintEngine engine. A good debugging tip is to make Qt warn you when such fall backs occur, and to disable the fall back and only return. Debugging options are listed below. DirectFB driverDirectFB also provides an abstraction for keyboard and mouse drivers. This simplifies the process of getting the target hardware up and running. It also brings us to a feature fragmentation issue between different versions of DirectFB. The Qt DirectFB driver currently supports DirectFB versions >= 0.9. Still, there are large differences in what each actual implementation handles correctly. It is relatively common not to properly support DirectFB windows, so Qt needs to handle this case with a different code path. In addition, certain drivers do not properly support DirectFB's cursor handling. This means Qt has to have a code path for rendering the cursor itself when this is the case. Some drivers do not let us create preallocated surfaces which means we have to have a conditional code path for that case. Optimize performance using define optionsQt/DirectFB comes with a number of defines that can be either uncommented in directfb.pri or added to the QT_DEFINES_DIRECTFB variable in your qmake.conf. Note: The defines have been moved from src/plugins/gfxdrivers/directfb/directfb.pro to src/gui/embedded/directfb.pri #DIRECTFB_DRAWINGOPERATIONS=DRAW_RECTS|DRAW_LINES|DRAW_IMAGE|DRAW_PIXMAP| DRAW_TILED_PIXMAP|STROKE_PATH|DRAW_PATH|DRAW_POINTS|DRAW_ELLIPSE|DRAW_POLYGON| DRAW_TEXT|FILL_PATH|FILL_RECT|DRAW_COLORSPANS|DRAW_ROUNDED_RECT #DEFINES += \"QT_DIRECTFB_WARN_ON_RASTERFALLBACKS=$$DIRECTFB_DRAWINGOPERATIONS\" #DEFINES += \"QT_DIRECTFB_DISABLE_RASTERFALLBACKS=$$DIRECTFB_DRAWINGOPERATIONS\" As demonstrated above, you need to tell Qt which drawing operations you want to warn/disable. Since there are varying implementations of DirectFB from manufacturer to manufacture, different operations will be optimized. This require you to define the operations you want to warn about or disable. These are listed above in the DIRECTFB_DRAWINGOPERATIONS variable. You can also customize this with environment variables. E.g. If you want to disable fallbacks for drawPixmap and fillRect and also get a warning printed on stderr when a fallback would have happened. $ export QT_DIRECTFB_WARN_ON_RASTERFALLBACKS="FILL_RECT|DRAW_PIXMAP" $ export QT_DIRECTFB_DISABLE_RASTERFALLBACKS="FILL_RECT|DRAW_PIXMAP" $ ./app -qws -display directfb Following is a table showing which options you have.
Unsupported graphics operationsThere are a number of unsupported operations causing fallbacks. DirectFB does not support the following functions.
Avoiding fallbacksTo avoid fallbacks make sure that the following points are true:
When painting imagesNote: You should use QPixmap instead of QImage. QImages are drawn by the QRasterPaintEngine. To get a warning for every fallback to the QRasterPaintEngine, use QT_DIRECTFB_WARN_ON_RASTERFALLBACKS. If QT_DIRECTFB_DISABLE_RASTERFALLBACKS is defined, DirectFB will only return instead of falling back to QRasterPaintEngine. Please note that these defines should only be used when optimizing the application. Top level transparencyNote: DirectFB supports partially or fully transparent top level windows, either through QWidget::setWindowOpacity or through setting a non-opaque background brush. Note that for the latter it is not supported to change an opaque window to be transparent at runtime. |
