Detailed Description
The QGLContext class encapsulates an OpenGL rendering context.
An OpenGL* 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 it invalid.
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.
* OpenGL is a trademark of Silicon Graphics, Inc. in the
United States and other countries.
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.
void * QGLContext::chooseMacVisual ( GDHandle device ) [virtual protected]
Mac only: This virtual function tries to find a
visual that matches the format using the handle device, reducing
the demands if the original request cannot be met.
The algorithm for reducing the demands of the format is quite
simple-minded, so override this method in your subclass if your
application has specific requirements on visual selection.
See also chooseContext().
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().
void * QGLContext::chooseVisual () [virtual protected]
X11 only: This virtual function tries to find a
visual that matches the format, reducing the demands if the original
request cannot be met.
The algorithm for reducing the demands of the format is quite
simple-minded, so override this method in your subclass if your
application has spcific requirements on visual selection.
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 rendering context.
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() function.
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 requested).
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 FALSE.
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 setFormat().
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.
QGLContext *cx;
// ...
QGLFormat f;
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().
This file is part of the Qt toolkit.
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