QGLFormat Class Reference
[OpenGL module]

Detailed Description

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.
• The plane of an overlay format.

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 f;
    f.setAlpha( TRUE );
    f.setStereo( TRUE );
    QGLFormat::setDefaultFormat( f );
    

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

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

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

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

^* OpenGL is a trademark of Silicon Graphics, Inc. in the United States and other countries.

See also QGLContext, QGLWidget, Graphics Classes, and Image Processing Classes.

Member Function Documentation

QGLFormat::QGLFormat ()

• Double buffer: Enabled.
• Depth buffer: Enabled.
• RGBA: Enabled (i.e., color index disabled).
• Alpha channel: Disabled.
• Accumulator buffer: Disabled.
• Stencil buffer: Disabled.
• Stereo: Disabled.
• Direct rendering: Enabled.
• Overlay: Disabled.
• Plane: 0 (i.e., normal plane).

QGLFormat::QGLFormat ( int options, int plane = 0 )

If options is not 0, this copy is modified by these format options. The options parameter should be FormatOption values OR'ed together.

This constructor makes it easy to specify a certain desired format in classes derived from QGLWidget, for example:

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

Note that there are FormatOption values to turn format settings both on and off, e.g. DepthBuffer and NoDepthBuffer, DirectRendering and IndirectRendering, etc.

The plane parameter defaults to 0 and is the plane which this format should be associated with. Not all OpenGL implmentations supports overlay/underlay rendering planes.

See also defaultFormat() and setOption().

bool QGLFormat::accum () const

Returns TRUE if the accumulation buffer is enabled; otherwise returns FALSE. The accumulation buffer is disabled by default.

See also setAccum().

bool QGLFormat::alpha () const

Returns TRUE if the alpha channel of the framebuffer is enabled; otherwise returns FALSE. The alpha channel is disabled by default.

See also setAlpha().

QGLFormat QGLFormat::defaultFormat () [static]

If no special default format has been set using setDefaultFormat(), the default format is the same as that created with QGLFormat().

See also setDefaultFormat().

QGLFormat QGLFormat::defaultOverlayFormat () [static]

The factory default overlay format is:

• Double buffer: Disabled.
• Depth buffer: Disabled.
• RGBA: Disabled (i.e., color index enabled).
• Alpha channel: Disabled.
• Accumulator buffer: Disabled.
• Stencil buffer: Disabled.
• Stereo: Disabled.
• Direct rendering: Enabled.
• Overlay: Disabled.
• Plane: 1 (i.e., first overlay plane).

See also setDefaultFormat().

bool QGLFormat::depth () const

Returns TRUE if the depth buffer is enabled; otherwise returns FALSE. The depth buffer is enabled by default.

See also setDepth().

bool QGLFormat::directRendering () const

Returns TRUE if direct rendering is enabled; otherwise returns FALSE.

Direct rendering is enabled by default.

See also setDirectRendering().

bool QGLFormat::doubleBuffer () const

Returns TRUE if double buffering is enabled; otherwise returns FALSE. Double buffering is enabled by default.

See also setDoubleBuffer().

bool QGLFormat::hasOpenGL () [static]

Returns TRUE if the window system has any OpenGL support; otherwise returns FALSE.

Warning: This function must not be called until the QApplication object has been created.

bool QGLFormat::hasOpenGLOverlays () [static]

Returns TRUE if the window system supports OpenGL overlays; otherwise returns FALSE.

Warning: This function must not be called until the QApplication object has been created.

bool QGLFormat::hasOverlay () const

Returns TRUE if overlay plane is enabled; otherwise returns FALSE.

Overlay is disabled by default.

See also setOverlay().

int QGLFormat::plane () const

See also setPlane().

bool QGLFormat::rgba () const

Returns TRUE if RGBA color mode is set. Returns FALSE if color index mode is set. The default color mode is RGBA.

See also setRgba().

void QGLFormat::setAccum ( bool enable )

The accumulation buffer is disabled by default.

The accumulation buffer is used to create blur effects and multiple exposures.

See also accum().

void QGLFormat::setAlpha ( bool enable )

The alpha buffer is disabled by default.

The alpha channel is typically used for implementing transparency or translucency. The A in RGBA specifies the transparency of a pixel.

See also alpha().

void QGLFormat::setDefaultFormat ( const QGLFormat & f ) [static]

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

See also defaultFormat().

void QGLFormat::setDefaultOverlayFormat ( const QGLFormat & f ) [static]

For example, to get a double buffered overlay context (if available), use code like this:

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

As usual, you can find out after widget creation whether the underlying OpenGL system was able to provide the requested specification:

    // ...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
    }
    

See also defaultOverlayFormat().

void QGLFormat::setDepth ( bool enable )

The depth buffer is enabled by default.

The purpose of a depth buffer (or Z-buffering) is to remove hidden surfaces. Pixels are assigned Z values based on the distance to the viewer. A pixel with a high Z value is closer to the viewer than a pixel with a low Z value. This information is used to decide whether to draw a pixel or not.

See also depth().

void QGLFormat::setDirectRendering ( bool enable )

Direct rendering is enabled by default.

Enabling this option will make OpenGL bypass the underlying window system and render directly from hardware to the screen, if this is supported by the system.

See also directRendering().

void QGLFormat::setDoubleBuffer ( bool enable )

Double buffering is enabled by default.

Double buffering is a technique where graphics are rendered on an off-screen buffer and not directly to the screen. When the drawing has been completed, the program calls a swapBuffers() function to exchange the screen contents with the buffer. The result is flicker-free drawing and often better performance.

See also doubleBuffer(), QGLContext::swapBuffers(), and QGLWidget::swapBuffers().

void QGLFormat::setOption ( FormatOption opt )

See also testOption().

void QGLFormat::setOverlay ( bool enable )

Enabling the overlay plane will cause QGLWidget to create an additional context in an overlay plane. See the QGLWidget documentation for further information.

See also hasOverlay().

void QGLFormat::setPlane ( int plane )

Note that in contrast to other format specifications, the plane specifications will be matched exactly. This means that if you specify a plane that the underlying OpenGL system cannot provide, an invalid QGLWidget will be created.

See also plane().

void QGLFormat::setRgba ( bool enable )

The default color mode is RGBA.

RGBA is the preferred mode for most OpenGL applications. In RGBA color mode you specify colors as red + green + blue + alpha quadruplets.

In color index mode you specify an index into a color lookup table.

See also rgba().

void QGLFormat::setStencil ( bool enable )

The stencil buffer is disabled by default.

The stencil buffer masks certain parts of the drawing area so that masked parts are not drawn on.

See also stencil().

void QGLFormat::setStereo ( bool enable )

Stereo buffering is disabled by default.

Stereo buffering provides extra color buffers to generate left-eye and right-eye images.

See also stereo().

bool QGLFormat::stencil () const

Returns TRUE if the stencil buffer is enabled; otherwise returns FALSE. The stencil buffer is disabled by default.

See also setStencil().

bool QGLFormat::stereo () const

Returns TRUE if stereo buffering is enabled; otherwise returns FALSE. Stereo buffering is disabled by default.

See also setStereo().

bool QGLFormat::testOption ( FormatOption opt ) const

See also setOption().

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