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.
• The plane of an overlay format.
• Multisample buffers.

You can also specify preferred bit depths for the depth buffer, alpha buffer, accumulation buffer and the stencil buffer with the functions: 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

enum QGLFormat::OpenGLVersionFlag
flags QGLFormat::OpenGLVersionFlags

This enum describes the various OpenGL versions that are recognized by Qt. Use the QGLFormat::openGLVersionFlags() function to identify which versions that are supported at runtime.

See also http://www.opengl.org for more information about the different revisions of OpenGL.

This enum was introduced in Qt 4.2.

The OpenGLVersionFlags type is a typedef for QFlags<OpenGLVersionFlag>. It stores an OR combination of OpenGLVersionFlag values.

See also openGLVersionFlags().

Member Function Documentation

QGLFormat::QGLFormat ()

Constructs a QGLFormat object with the factory default settings:

• 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).
• Multisample buffers: Disabled.

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

Creates a QGLFormat object that is a copy of the current application default format.

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)
     : 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");
     ...
 }

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 implementations supports overlay/underlay rendering planes.

See also defaultFormat() and setOption().

QGLFormat::QGLFormat ( const QGLFormat & other )

Constructs a copy of other.

QGLFormat::~QGLFormat ()

Destroys the QGLFormat.

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() and setAccumBufferSize().

int QGLFormat::accumBufferSize () const

Returns the accumulation buffer size.

See also setAccumBufferSize(), accum(), and setAccum().

bool QGLFormat::alpha () const

Returns true if the alpha buffer in the framebuffer is enabled; otherwise returns false. The alpha buffer is disabled by default.

See also setAlpha() and setAlphaBufferSize().

int QGLFormat::alphaBufferSize () const

Returns the alpha buffer size.

See also alpha(), setAlpha(), and setAlphaBufferSize().

int QGLFormat::blueBufferSize () const

Returns the blue buffer size.

This function was introduced in Qt 4.2.

See also setBlueBufferSize().

QGLFormat QGLFormat::defaultFormat ()   [static]

Returns the default QGLFormat for the application. All QGLWidgets that are created use this format unless another format is specified, e.g. when they are constructed.

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]

Returns the default QGLFormat for overlay contexts.

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 setDefaultOverlayFormat() and 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() and setDepthBufferSize().

int QGLFormat::depthBufferSize () const

Returns the depth buffer size.

See also depth(), setDepth(), and setDepthBufferSize().

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().

int QGLFormat::greenBufferSize () const

Returns the green buffer size.

This function was introduced in Qt 4.2.

See also setGreenBufferSize().

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().

OpenGLVersionFlags QGLFormat::openGLVersionFlags ()   [static]

Identifies, at runtime, which OpenGL versions that are supported by the current platform.

Note that if OpenGL version 1.5 is supported, its predecessors (i.e., version 1.4 and lower) are also supported. To identify the support of a particular feature, like multi texturing, test for the version in which the feature was first introduced (i.e., version 1.3 in the case of multi texturing) to adapt to the largest possible group of runtime platforms.

This function needs a valid current OpenGL context to work; otherwise it will return OpenGL_Version_None.

This function was introduced in Qt 4.2.

See also hasOpenGL() and hasOpenGLOverlays().

int QGLFormat::plane () const

Returns the plane of this format. The default for normal formats is 0, which means the normal plane. The default for overlay formats is 1, which is the first overlay plane.

See also setPlane().

int QGLFormat::redBufferSize () const

Returns the red buffer size.

This function was introduced in Qt 4.2.

See also setRedBufferSize().

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().

bool QGLFormat::sampleBuffers () const

Returns true if multisample buffer support is enabled; otherwise returns false.

The multisample buffer is disabled by default.

See also setSampleBuffers().

int QGLFormat::samples () const

Returns the number of samples per pixel when multisampling is enabled. By default, the highest number of samples that is available is used.

See also setSampleBuffers(), sampleBuffers(), and setSamples().

void QGLFormat::setAccum ( bool enable )

If enable is true enables the accumulation buffer; otherwise disables the accumulation buffer.

The accumulation buffer is disabled by default.

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

See also accum() and setAccumBufferSize().

void QGLFormat::setAccumBufferSize ( int size )

Set the preferred accumulation buffer size, where size is the bit depth for each RGBA component.

See also accum(), setAccum(), and accumBufferSize().

void QGLFormat::setAlpha ( bool enable )

If enable is true enables the alpha buffer; otherwise disables the alpha buffer.

The alpha buffer is disabled by default.

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

See also alpha() and setAlphaBufferSize().

void QGLFormat::setAlphaBufferSize ( int size )

Set the preferred alpha buffer size to size. This function implicitly enables the alpha channel.

See also setRedBufferSize(), setGreenBufferSize(), and alphaBufferSize().

void QGLFormat::setBlueBufferSize ( int size )

Set the preferred blue buffer size to size.

This function was introduced in Qt 4.2.

See also blueBufferSize(), setRedBufferSize(), setGreenBufferSize(), and setAlphaBufferSize().

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

Sets a new default QGLFormat for the application to f. For example, to set single buffering as the default instead of double buffering, your main() might contain code like this:

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

See also defaultFormat().

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

Sets a new default QGLFormat for overlay contexts to f. This format is used whenever a QGLWidget is created with a format that hasOverlay() enabled.

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 )

If enable is true enables the depth buffer; otherwise disables the depth buffer.

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() and setDepthBufferSize().

void QGLFormat::setDepthBufferSize ( int size )

Set the minimum depth buffer size to size.

See also depthBufferSize(), setDepth(), and depth().

void QGLFormat::setDirectRendering ( bool enable )

If enable is true enables direct rendering; otherwise disables direct rendering.

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 )

If enable is true sets double buffering; otherwise sets single buffering.

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::setGreenBufferSize ( int size )

Set the preferred green buffer size to size.

This function was introduced in Qt 4.2.

See also greenBufferSize(), setRedBufferSize(), setBlueBufferSize(), and setAlphaBufferSize().

void QGLFormat::setOption ( QGL::FormatOptions opt )

Sets the format option to opt.

See also testOption().

void QGLFormat::setOverlay ( bool enable )

If enable is true enables an overlay plane; otherwise disables the overlay plane.

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 )

Sets the requested plane to plane. 0 is the normal plane, 1 is the first overlay plane, 2 is the second overlay plane, etc.; -1, -2, etc. are underlay planes.

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::setRedBufferSize ( int size )

Set the preferred red buffer size to size.

This function was introduced in Qt 4.2.

See also redBufferSize(), setGreenBufferSize(), setBlueBufferSize(), and setAlphaBufferSize().

void QGLFormat::setRgba ( bool enable )

If enable is true sets RGBA mode. If enable is false sets color index mode.

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::setSampleBuffers ( bool enable )

If enable is true, a GL context with multisample buffer support is picked; otherwise ignored.

See also sampleBuffers(), setSamples(), and samples().

void QGLFormat::setSamples ( int numSamples )

Set the preferred number of samples per pixel when multisampling is enabled to numSamples. By default, the highest number of samples available is used.

See also setSampleBuffers(), sampleBuffers(), and samples().

void QGLFormat::setStencil ( bool enable )

If enable is true enables the stencil buffer; otherwise disables the stencil buffer.

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() and setStencilBufferSize().

void QGLFormat::setStencilBufferSize ( int size )

Set the preferred stencil buffer size to size.

See also stencilBufferSize(), setStencil(), and stencil().

void QGLFormat::setStereo ( bool enable )

If enable is true enables stereo buffering; otherwise disables stereo buffering.

Stereo buffering is disabled by default.

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

See also stereo().

void QGLFormat::setSwapInterval ( int interval )

Set the preferred swap interval. This can be used to sync the GL drawing into a system window to the vertical refresh of the screen. Setting an interval value of 0 will turn the vertical refresh syncing off, any value higher than 0 will turn the vertical syncing on.

Under Windows and under X11, where the WGL_EXT_swap_control and GLX_SGI_video_sync extensions are used, the interval parameter can be used to set the minimum number of video frames that are displayed before a buffer swap will occur. In effect, setting the interval to 10, means there will be 10 vertical retraces between every buffer swap.

Under Windows the WGL_EXT_swap_control extension has to be present, and under X11 the GLX_SGI_video_sync extension has to be present.

This function was introduced in Qt 4.2.

See also swapInterval().

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() and setStencilBufferSize().

int QGLFormat::stencilBufferSize () const

Returns the stencil buffer size.

See also stencil(), setStencil(), and setStencilBufferSize().

bool QGLFormat::stereo () const

Returns true if stereo buffering is enabled; otherwise returns false. Stereo buffering is disabled by default.

See also setStereo().

int QGLFormat::swapInterval () const

Returns the currently set swap interval. -1 is returned if setting the swap interval isn't supported in the system GL implementation.

This function was introduced in Qt 4.2.

See also setSwapInterval().

bool QGLFormat::testOption ( QGL::FormatOptions opt ) const

Returns true if format option opt is set; otherwise returns false.

See also setOption().

QGLFormat & QGLFormat::operator= ( const QGLFormat & other )

Assigns other to this object.