QGLFormat Class Reference
[QtOpenGL module]

Detailed Description

The QGLFormat class specifies the display format of an OpenGL rendering context.

A display format has several characteristics:

• Double or single buffering.
• Depth buffer.
• RGBA or color index mode.
• Alpha channel.
• Accumulation buffer.
• Stencil buffer.
• Stereo buffers.
• Direct rendering.
• Presence of an overlay.
• Plane of an overlay.
• Multisample buffers.

You can also specify preferred bit depths for the color buffer, depth buffer, alpha buffer, accumulation buffer and the stencil buffer with the functions: setRedBufferSize(), setGreenBufferSize(), setBlueBufferSize(), setDepthBufferSize(), setAlphaBufferSize(), setAccumBufferSize() and setStencilBufferSize().

Note that even if you specify that you prefer a 32 bit depth buffer (e.g. with setDepthBufferSize(32)), the format that is chosen may not have a 32 bit depth buffer, even if there is a format available with a 32 bit depth buffer. The main reason for this is how the system dependant picking algorithms work on the different platforms, and some format options may have higher precedence than others.

You create and tell a QGLFormat object what rendering options you want from an OpenGL rendering context.

OpenGL drivers or accelerated hardware may or may not support advanced features such as alpha channel or stereographic viewing. If you request some features that the driver/hardware does not provide when you create a QGLWidget, you will get a rendering context with the nearest subset of features.

There are different ways to define the display characteristics of a rendering context. One is to create a QGLFormat and make it the default for the entire application:

 QGLFormat fmt;
 fmt.setAlpha(true);
 fmt.setStereo(true);
 QGLFormat::setDefaultFormat(fmt);

Or you can specify the desired format when creating an object of your QGLWidget subclass:

 QGLFormat fmt;
 fmt.setDoubleBuffer(false);                 // single buffer
 fmt.setDirectRendering(false);              // software rendering
 MyGLWidget* myWidget = new MyGLWidget(fmt, ...);

After the widget has been created, you can find out which of the requested features the system was able to provide:

 QGLFormat fmt;
 fmt.setOverlay(true);
 fmt.setStereo(true);
 MyGLWidget* myWidget = new MyGLWidget(fmt, ...);
 if (!myWidget->format().stereo()) {
     // ok, goggles off
     if (!myWidget->format().hasOverlay()) {
         qFatal("Cool hardware required");
     }
 }

See also QGLContext and QGLWidget.

Member Type Documentation

 // The rendering in MyGLWidget depends on using
 // stencil buffer and alpha channel
 MyGLWidget::MyGLWidget(QWidget* parent)
     : QGLWidget(QGLFormat(QGL::StencilBuffer | QGL::AlphaChannel), parent)
 {
     if (!format().stencil())
         qWarning("Could not get stencil buffer; results will be suboptimal");
     if (!format().alpha())
         qWarning("Could not get alpha channel; results will be suboptimal");
     ...
 }

 QApplication a(argc, argv);
 QGLFormat f;
 f.setDoubleBuffer(false);
 QGLFormat::setDefaultFormat(f);

 QGLFormat f = QGLFormat::defaultOverlayFormat();
 f.setDoubleBuffer(true);
 QGLFormat::setDefaultOverlayFormat(f);

 // ...continued from above
 MyGLWidget* myWidget = new MyGLWidget(QGLFormat(QGL::HasOverlay), ...);
 if (myWidget->format().hasOverlay()) {
     // Yes, we got an overlay, let's check _its_ format:
     QGLContext* olContext = myWidget->overlayContext();
     if (olContext->format().doubleBuffer())
         ; // yes, we got a double buffered overlay
     else
         ; // no, only single buffered overlays are available
 }