QGLContext - Encapsulates an OpenGL rendering context
QGLContext ( const QGLFormat & format, QPaintDevice * device )
virtual ~QGLContext ()
virtual bool create ( const QGLContext * shareContext = 0 )
bool isValid () const
bool isSharing () const
virtual void reset ()
QGLFormat format () const
QGLFormat requestedFormat () const
virtual void setFormat ( const QGLFormat & format )
virtual void makeCurrent ()
virtual void swapBuffers () const
QPaintDevice * device () const
QColor overlayTransparentColor () const
Static Public Members
const QGLContext * currentContext ()
virtual bool chooseContext ( const QGLContext * shareContext = 0 )
virtual void doneCurrent ()
virtual int choosePixelFormat ( void * dummyPfd, HDC pdc )
virtual void * chooseMacVisual ( GDHandle )
bool deviceIsPixmap () const
bool windowCreated () const
void setWindowCreated ( bool on )
bool initialized () const
void setInitialized ( bool on )
void generateFontDisplayLists ( const QFont & font, int listBase )
The QGLContext class encapsulates an OpenGL rendering context.
An OpenGL<sup>*</sup> rendering context is a complete set of OpenGL state variables.
The context's format is set in the constructor or later with setFormat(). The format
options that are actually set are returned by format(); the options you asked for are
returned by requestedFormat(). Note that after a QGLContext object has been constructed,
the actual OpenGL context must be created by explicitly calling the create() function. The
makeCurrent() function makes this context the current rendering context. You can make no
context current using doneCurrent(). The reset() function will reset the context and make
You can examine properties of the context with, e.g. isValid(), isSharing(),
initialized(), windowCreated() and overlayTransparentColor().
If you're using double buffering you can swap the screen contents with the off-screen
buffer using swapBuffers().
Please note that QGLContext is not thread safe.
<sup>*</sup> OpenGL is a trademark of Silicon Graphics, Inc. in the United States and
See also Graphics Classes and Image Processing Classes.
MEMBER FUNCTION DOCUMENTATION
QGLContext::QGLContext ( const QGLFormat & format, QPaintDevice * device )
Constructs an OpenGL context for the paint device device, which can be a widget or a
pixmap. The format specifies several display options for the context.
If the underlying OpenGL/Window system cannot satisfy all the features requested in
format, the nearest subset of features will be used. After creation, the format() method
will return the actual format obtained.
Note that after a QGLContext object has been constructed, create() must be called
explicitly to create the actual OpenGL context. The context will be invalid if it was not
possible to obtain a GL context at all.
See also format() and isValid().
QGLContext::~QGLContext () [virtual]
Destroys the OpenGL context and frees its resources.
bool QGLContext::chooseContext ( const QGLContext * shareContext = 0 ) [virtual protected]
This semi-internal function is called by create(). It creates a system-dependent OpenGL
handle that matches the format() of shareContext as closely as possible.
On Windows, it calls the virtual function choosePixelFormat(), which finds a matching
pixel format identifier. On X11, it calls the virtual function chooseVisual() which finds
an appropriate X visual. On other platforms it may work differently.
int QGLContext::choosePixelFormat ( void * dummyPfd, HDC pdc ) [virtual protected]
Win32 only This virtual function chooses a pixel format that matches the OpenGL format.
Reimplement this function in a subclass if you need a custom context.
Warning: The dummyPfd pointer and pdc are used as a PIXELFORMATDESCRIPTOR*. We use void to
avoid using Windows-specific types in our header files.
See also chooseContext().
bool QGLContext::create ( const QGLContext * shareContext = 0 ) [virtual]
Creates the GL context. Returns TRUE if it was successful in creating a valid GL rendering
context on the paint device specified in the constructor; otherwise returns FALSE (i.e.
the context is invalid).
After successful creation, format() returns the set of features of the created GL
If shareContext points to a valid QGLContext, this method will try to establish OpenGL
display list sharing between this context and the shareContext. Note that this may fail if
the two contexts have different formats. Use isSharing() to see if sharing succeeded.
Warning: Implementation note: initialization of C++ class members usually takes place in
the class constructor. QGLContext is an exception because it must be simple to customize.
The virtual functions chooseContext() (and chooseVisual() for X11) can be reimplemented in
a subclass to select a particular context. The problem is that virtual functions are not
properly called during construction (even though this is correct C++) because C++
constructs class hierarchies from the bottom up. For this reason we need a create()
See also chooseContext(), format(), and isValid().
const QGLContext * QGLContext::currentContext () [static]
Returns the current context, i.e. the context to which any OpenGL commands will currently
be directed. Returns 0 if no context is current.
See also makeCurrent().
QPaintDevice * QGLContext::device () const
Returns the paint device set for this context.
See also QGLContext::QGLContext().
bool QGLContext::deviceIsPixmap () const [protected]
Returns TRUE if the paint device of this context is a pixmap; otherwise returns FALSE.
void QGLContext::doneCurrent () [virtual protected]
Makes no GL context the current context. Normally, you do not need to call this function;
QGLContext calls it as necessary.
QGLFormat QGLContext::format () const
Returns the frame buffer format that was obtained (this may be a subset of what was
See also requestedFormat().
void QGLContext::generateFontDisplayLists ( const QFont & font, int listBase ) [protected]
Generates a set of 256 display lists for the 256 first characters in the font font. The
first list will start at index listBase.
See also QGLWidget::renderText().
bool QGLContext::initialized () const [protected]
Returns TRUE if this context has been initialized, i.e. if QGLWidget::initializeGL() has
been performed on it; otherwise returns FALSE.
See also setInitialized().
bool QGLContext::isSharing () const
Returns TRUE if display list sharing with another context was requested in the create()
call and the GL system was able to fulfill this request; otherwise returns FALSE. Note
that display list sharing might not be supported between contexts with different formats.
bool QGLContext::isValid () const
Returns TRUE if a GL rendering context has been successfully created; otherwise returns
void QGLContext::makeCurrent () [virtual]
Makes this context the current OpenGL rendering context. All GL functions you call operate
on this context until another context is made current.
In some very rare cases the underlying call may fail. If this occurs an error message is
output to stderr.
QColor QGLContext::overlayTransparentColor () const
If this context is a valid context in an overlay plane, returns the plane's transparent
color. Otherwise returns an invalid color.
The returned color's pixel value is the index of the transparent color in the colormap of
the overlay plane. (Naturally, the color's RGB values are meaningless.)
The returned QColor object will generally work as expected only when passed as the
argument to QGLWidget::qglColor() or QGLWidget::qglClearColor(). Under certain
circumstances it can also be used to draw transparent graphics with a QPainter. See the
examples/opengl/overlay_x11 example for details.
QGLFormat QGLContext::requestedFormat () const
Returns the frame buffer format that was originally requested in the constructor or
See also format().
void QGLContext::reset () [virtual]
Resets the context and makes it invalid.
See also create() and isValid().
void QGLContext::setFormat ( const QGLFormat & format ) [virtual]
Sets a format for this context. The context is reset.
Call create() to create a new GL context that tries to match the new format.
f.setStereo( TRUE );
cx->setFormat( f );
if ( !cx->create() )
exit(); // no OpenGL support, or cannot render on the specified paintdevice
if ( !cx->format().stereo() )
exit(); // could not create stereo context
See also format(), reset(), and create().
void QGLContext::setInitialized ( bool on ) [protected]
If on is TRUE the context has been initialized, i.e. QGLContext::setInitialized() has been
called on it. If on is FALSE the context has not been initialized.
See also initialized().
void QGLContext::setWindowCreated ( bool on ) [protected]
If on is TRUE the context has had a window created for it. If on is FALSE no window has
been created for the context.
See also windowCreated().
void QGLContext::swapBuffers () const [virtual]
Swaps the screen contents with an off-screen buffer. Only works if the context is in
double buffer mode.
See also QGLFormat::setDoubleBuffer().
bool QGLContext::windowCreated () const [protected]
Returns TRUE if a window has been created for this context; otherwise returns FALSE.
See also setWindowCreated().
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Trolltech AS 9 December 2002 QGLContext(3qt)