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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/platforms/emb-opengl.qdoc | 247 |
1 files changed, 125 insertions, 122 deletions
diff --git a/doc/src/platforms/emb-opengl.qdoc b/doc/src/platforms/emb-opengl.qdoc index fea09bb..2ed5d04 100644 --- a/doc/src/platforms/emb-opengl.qdoc +++ b/doc/src/platforms/emb-opengl.qdoc @@ -57,20 +57,20 @@ of the \l {http://www.opengl.org}{OpenGL} standard. Because it is meant for use in embedded systems, it has a smaller, more constrained API. -For reference, Nokia provides a plugin which integrates \l -{http://www.khronos.org/opengles}{OpenGL ES} with Qt for Embedded Linux, -but Qt for Embedded Linux can be adapted to a wide range of OpenGL -versions. +For reference, Nokia provides support for integrating \l +{http://www.khronos.org/opengles}{OpenGL ES} with Qt for Embedded Linux +for drawing into a QGLWidget. -There are three ways to use OpenGL with Qt for Embedded Linux: -\list - \o Perform OpenGL 3D graphics operations in applications; - \o Accelerate normal 2D painting operations; - \o Implement window compositing and special effects. -\endlist +The current implementation supports OpenGL and 2D painting within a +QGLWidget. Using OpenGL to accelerate regular widgets and compositing +top-level windows with OpenGL are not currently supported. These issues +will be addressed in future versions of Qt. -Qt for Embedded Linux is shipped with a reference integration example -that demonstrates all three uses. +It is recommended that Qt for Embedded Linux is configured with the +\c{-DQT_QWS_CLIENTBLIT} and \c{-DQT_NO_QWS_CURSOR} options for optimum +performance. OpenGL is rendered direct to the screen and these options +prevent Qt for Embedded Linux from trying to do its own non-OpenGL +compositing on the QGLWidget contents. \section2 Using OpenGL 3D Graphics in Applications @@ -82,146 +82,149 @@ To use OpenGL-enabled widgets in a Qt for Embedded Linux application, all that is required is to subclass the QGLWidget and draw into instances of the subclass with standard OpenGL functions. +Note that on most embedded hardware, the OpenGL implementation is +actually \l{http://www.khronos.org/opengles/1_X/}{OpenGL/ES 1.1} or +\l{http://www.khronos.org/opengles/2_X/}{OpenGL/ES 2.0}. When painting +within a QGLWidget::paintGL() override, it is necessary to limit the +application to only the features that are present in the OpenGL/ES +implementation. + \section2 Using OpenGL to Accelerate Normal 2D Painting -Qt provides QOpenGLPaintEngine, a subclass of QPaintEngine that -translates QPainter operations into OpenGL calls. This specialized -paint engine can be used to improve 2D rendering performance on -appropriate hardware. It can also overlay controls and decorations -onto 3D scenes drawn using OpenGL. +Qt provides a subclass of QPaintEngine that translates QPainter operations +into OpenGL calls (there are actually two subclasses, one for OpenGL/ES 1.1 +and another for OpenGL/ES 2.0). This specialized paint engine can be used +to improve 2D rendering performance on appropriate hardware. It can also +overlay controls and decorations onto 3D scenes drawn using OpenGL. + +As mentioned above, the OpenGL paint engine is not currently supported +in regular widgets. However, any application that uses QGraphicsView +can set a QGLWidget as the viewport and obtain access to the +OpenGL paint engine that way: + +\code +QGraphicsView view(&scene); +view.setViewport(new QGLWidget); +view.setViewportUpdateMode(QGraphicsView::FullViewportUpdate); +view.showFullScreen(); +\endcode + +It is recommended that the QGraphicsView::FullViewportUpdate flag +be set because the default double-buffered behavior of QGLWidget +does not support partial updates. It is also recommended that the +window be shown full-screen because that usually has the best +performance on current OpenGL/ES implementations. + +Once a QGraphicsView has been initialized as above, regular widgets +can be added to the canvas using QGraphicsProxyWidget if the +application requires them. \section2 Using OpenGL to Implement Window Compositing and Effects -Qt for Embedded Linux includes a complete windowing system, which implements -real transparency. The windowing system can be accelerated using -OpenGL to implement top level window compositing. This makes it easy -to add 3D effects to applications, for instance when windows are -minimized or maximized. - -\section1 Acceleration Architecture - -The diagram below shows the Qt for Embedded Linux painting architecture. - -\image qt-embedded-opengl3.png - -A client process widget uses a paint engine to draw into a window -surface. The server then combines the window surfaces and displays the -composition on the screen. This architecture lets you -control the steps of the painting process by subclassing. - -Subclassing QPaintEngine allows you to implement the QPainter API -using accelerated hardware. Subclassing QWindowSurface lets you -decide the properties of the space your widgets will draw themselves -into, as well as which paint engine they should use to draw themselves -into that space. Subclassing QScreen lets you control the creation of -window surfaces and lets you decide how to implement window -compositing. Using subclassing, your implementation work is minimized -since you can reuse base class functionality you don't need to change. +Compositing effects can be simulated by adjusting the opacity and +other parameters of the items within a QGraphicsView canvas on a +QGLWidget viewport. -The elements of an accelerated Qt for Embedded Linux system are shown in the -diagram below. +While Qt for Embedded Linux does include a complete windowing system, +using OpenGL to composite regular window surfaces can be quite difficult. +Most of Qt for Embedded Linux assumes that the window surface is a plain +raster memory buffer, with QGLWidget being the sole exception. +The need to constantly re-upload the raster memory buffers into OpenGL +textures for compositing can have a significant impact on performance, +which is why we do not recommend implementing that form of compositing. +We intend to address this problem in future versions of Qt. -\image qt-embedded-opengl1.png +\section1 Integrating OpenGL/ES into Qt for Embedded Linux -The applications, using the Qt API, do not depend on the presence of -the acceleration plugin. The plugin uses the graphics hardware to -accelerate painting primitives. Any operations not accelerated by the -plugin are done in software by the software paint engine. +\section2 Reference Integration -To integrate an OpenGL implementation into Qt for Embedded Linux for a -particular platform, you use the same mechanisms you would use for -writing any other accelerated driver. Base classes, e.g., QGLScreen -and QWSGLWindowSurface, are provided to minimize the need for -reimplementing common functionality. +The reference integration for OpenGL into Qt for Embedded Linux +is for the PowerVR chipset from \l{http://www.imgtec.com/}{Imagination +Technologies}. It consists of two components: \c{pvreglscreen} which +provides the Qt for Embedded Linux screen driver, and \c{QWSWSEGL} +which implements a plug-in to the PowerVR EGL implementation to +implement low-level OpenGL drawing surfaces. -\section1 The Reference Integration +\section2 Integrating Other Chipsets -The \l{OpenGL for Embedded Systems Example} is the reference implementation -for integrating OpenGL ES and \l{http://www.khronos.org/egl/}{EGL} with -the graphics acceleration architecture of Qt for Embedded Linux. -(\l{http://www.khronos.org/egl/}{EGL} is a library that binds OpenGL ES to -native windowing systems.) +In this section we discuss the essential features of the reference +integration that need to be provided for any other chipset integration. -The diagram below shows how OpenGL ES is used within the acceleration architecture: +The QtOpenGL module assumes that a QGLWidget can be represented +by a \c EGLNativeWindowType value in some underlying window system +implementation, and that \c{eglSwapBuffers()} is sufficient to copy +the contents of the native window to the screen when requested. -\image qt-embedded-opengl2.png +However, many EGL implementations do not have a pre-existing window system. +Usually only a single full-screen window is provided, and everything else +must be simulated some other way. This can be a problem because +of QtOpenGL's assumptions. We intend to address these assumptions in a +future version of Qt, but for now it is the responsibility of the integrator +to provide a rudimentary window system within the EGL implementation. +This is the purpose of \c{QWSWSEGL} in the reference integration. -The example implements a screen driver plugin that demonstrates all -three uses of OpenGL in Qt for Embedded Linux: 2D graphics acceleration, 3D -graphics operations using the \l {QtOpenGL module}, and top-level -window compositing and special effects. The applications still do -not talk directly to the accelerated plugin. +If it isn't possible for the EGL implementation to provide a rudimentary +window system, then full-screen windows using QGLWidget can be supported, +but very little else. -For 2D graphics, applications use the normal Qt painting API. The example accelerates 2D -painting by using the QOpenGLPaintEngine, which is included in the \l {QtOpenGL module}. +The screen driver needs to inherit from QGLScreen and perform the +following operations in its constructor: -For 3D graphics applications use the OpenGL API directly, together with the functionality -in the Qt OpenGL support classes. The example supports this by creating a -QWSGLWindowSurface whenever a QGLWidget is instantiated. +\snippet src/plugins/gfxdrivers/powervr/pvreglscreen/pvreglscreen.cpp 0 -All access to the display is done through OpenGL. The example subclasses -QWSGLWindowSurface implementation and uses the \l -{http://oss.sgi.com/projects/ogl-sample/registry/EXT/framebuffer_object.txt} -{OpenGL Framebuffer Object extension} to draw windows into an offscreen buffer. This -lets the example use OpenGL to implement top level window compositing of opaque and -semi-transparent windows, and to provide a 3D animated transition effect as each new -window is shown. +The \c{setSurfaceFunctions()} call supplies an object that takes care +of converting Qt paint devices such as widgets and pixmaps into +\c EGLNativeWindowType and \c EGLNativePixmapType values. Here we +only support native windows. Because OpenGL rendering is direct to +the screen, we also indicate that client blit is supported. -The specific OpenGL library being used by the example restricts all -OpenGL operations to occur in a single process. Hence the example -creates instances of QWSGLWindowSurface only in the server process. -Other processes then perform 2D graphics by creating instances -of the standard QWindowSurface classes for client processes. The -standard window surface performs software-based rendering into a -shared memory segment. The server then transfers the contents of this -shared memory into an OpenGL texture before they are drawn onto the -screen during window compositing. +Next, we override the \c{createSurface()} functions in QGLScreen: -\omit +\snippet src/plugins/gfxdrivers/powervr/pvreglscreen/pvreglscreen.cpp 1 -\section1 Future Directions +Even if Qt for Embedded Linux is used in single-process mode, it is +necessary to create both client-side and server-side versions of the +window surface. In our case, the server-side is just a stub because +the client side directly renders to the screen. -\section2 API Improvements +Note that we only create a \c{PvrEglWindowSurface} if the widget is a +QGLWidget. All other widgets use the normal raster processing. +It can be tempting to make \c{createSurface()} create an OpenGL +window surface for other widget types as well. This has not been +extensively tested and we do not recommend its use at this time. -Nokia is now working on enhancing the API for integrating OpenGL -with Qt for Embedded Linux. The current design plan includes the following -features: +The other main piece is the creation of the \c EGLNativeWindowType +value for the widget. This is done in the \c{createNativeWindow()} +override: -\list +\snippet src/plugins/gfxdrivers/powervr/pvreglscreen/pvreglscreen.cpp 2 - \o Provide convenience classes, e.g., QEGLScreen and - QWSEGLWindowSurface, which implement common uses of the EGL - API. These classes will simplify implementing an OpenGL ES - integration. +The details of what needs to be placed in this function will vary +from chipset to chipset. The simplest is to return the native window +handle corresponding to the "root" full-screen window: - \o Extend the screen driver API to provide more control over window - properties and animations, and provide a software-based integration - to enable testing on the desktop. +\code +*native = rootWindowHandle; +return true; +\endcode - \o Improve performance as opportunities arise. +The most common value for \c rootWindowHandle is zero, but this may +not always be the case. Consult the chipset documentation for the +actual value to use. The important thing is that whatever value is +returned must be suitable for passing to the \c{eglCreateWindowSurface()} +function of the chipset's EGL implementation. -\endlist +In the case of PowerVR, the rudimentary window system in \c{QWSWSEGL} +provides a \c PvrQwsDrawable object to represent the \c EGLNativeWindowType +value for the widget. -\section2 OpenVG Support +\section1 OpenVG Support \l {http://www.khronos.org/openvg} {OpenVG} is a dedicated API for 2D graphics on mobile devices. It is therefore more likely to be a better -alternative for 2D acceleration than OpenGL. Until recently, no -OpenVG-capable hardware has been available, so Nokia has not yet -included an OpenVG solution in Qt for Embedded Linux. - -However, Nokia has done a feasibility study, implementing an -OpenVG paint engine on top of a software OpenVG implementation. -Assuming availability of the appropriate hardware, this OpenVG paint -engine can easily be completed and integrated using the existing -acceleration architecture. Since OpenVG shares the same EGL layer as -OpenGL ES, the work already done on the OpenGL integration can be -reused. - -Related technologies included in the \l -{http://www.khronos.org/openkode} {OpenKODE} API set will also be -considered. - -\endomit +alternative for 2D acceleration than OpenGL/ES. Acceleration of +regular widgets is supported with OpenVG, unlike with OpenGL/ES. +See \l{OpenVG Rendering in Qt} for more information on the +OpenVG support in Qt. */ |