Qt Reference Documentation

QGLContext Class Reference

The QGLContext class encapsulates an OpenGL rendering context. More...

 #include <QGLContext>

Public Types

enum BindOption { NoBindOption, InvertedYBindOption, MipmapBindOption, PremultipliedAlphaBindOption, LinearFilteringBindOption, DefaultBindOption }
flags BindOptions

Public Functions

QGLContext ( const QGLFormat & format )
virtual ~QGLContext ()
GLuint bindTexture ( const QImage & image, GLenum target, GLint format, BindOptions options )
GLuint bindTexture ( const QString & fileName )
GLuint bindTexture ( const QImage & image, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA )
GLuint bindTexture ( const QPixmap & pixmap, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA )
GLuint bindTexture ( const QPixmap & pixmap, GLenum target, GLint format, BindOptions options )
virtual bool create ( const QGLContext * shareContext = 0 )
void deleteTexture ( GLuint id )
QPaintDevice * device () const
virtual void doneCurrent ()
void drawTexture ( const QRectF & target, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D )
void drawTexture ( const QPointF & point, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D )
QGLFormat format () const
void * getProcAddress ( const QString & proc ) const
bool isSharing () const
bool isValid () const
virtual void makeCurrent ()
QColor overlayTransparentColor () const
QGLFormat requestedFormat () const
void reset ()
void setFormat ( const QGLFormat & format )
virtual void swapBuffers () const

Static Public Members

bool areSharing ( const QGLContext * context1, const QGLContext * context2 )
const QGLContext * currentContext ()
void setTextureCacheLimit ( int size )
int textureCacheLimit ()

Protected Functions

virtual bool chooseContext ( const QGLContext * shareContext = 0 )
virtual void * chooseMacVisual ( GDHandle handle )
virtual int choosePixelFormat ( void * dummyPfd, HDC pdc )
virtual void * chooseVisual ()
bool deviceIsPixmap () const
bool initialized () const
void setInitialized ( bool on )
void setWindowCreated ( bool on )
bool windowCreated () const

Detailed Description

The QGLContext class encapsulates an OpenGL rendering context.

An OpenGL rendering context is a complete set of OpenGL state variables. The rendering context's format is set in the constructor, but it can also be set 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.

Member Type Documentation

enum QGLContext::BindOption
flags QGLContext::BindOptions

A set of options to decide how to bind a texture using bindTexture().

ConstantValueDescription
QGLContext::NoBindOption0x0000Don't do anything, pass the texture straight through.
QGLContext::InvertedYBindOption0x0001Specifies that the texture should be flipped over the X axis so that the texture coordinate 0,0 corresponds to the top left corner. Inverting the texture implies a deep copy prior to upload.
QGLContext::MipmapBindOption0x0002Specifies that bindTexture() should try to generate mipmaps. If the GL implementation supports the GL_SGIS_generate_mipmap extension, mipmaps will be automatically generated for the texture. Mipmap generation is only supported for the GL_TEXTURE_2D target.
QGLContext::PremultipliedAlphaBindOption0x0004Specifies that the image should be uploaded with premultiplied alpha and does a conversion accordingly.
QGLContext::LinearFilteringBindOption0x0008Specifies that the texture filtering should be set to GL_LINEAR. Default is GL_NEAREST. If mipmap is also enabled, filtering will be set to GL_LINEAR_MIPMAP_LINEAR.
QGLContext::DefaultBindOptionLinearFilteringBindOption | InvertedYBindOption | MipmapBindOptionIn Qt 4.5 and earlier, bindTexture() would mirror the image and automatically generate mipmaps. This option helps preserve this default behavior.

Used by x11 from pixmap to choose whether or not it can bind the pixmap upside down or not.

Used by paint engines to indicate that the pixmap should be memory managed along side with the pixmap/image that it stems from, e.g. installing destruction hooks in them.

This enum was introduced in Qt 4.6.

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

Member Function Documentation

QGLContext::QGLContext ( const QGLFormat & format )

Constructs an OpenGL context with the given format which 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::areSharing ( const QGLContext * context1, const QGLContext * context2 ) [static]

Returns true if context1 and context2 are sharing their GL resources such as textures, shader programs, etc; otherwise returns false.

This function was introduced in Qt 4.6.

GLuint QGLContext::bindTexture ( const QImage & image, GLenum target, GLint format, BindOptions options )

Generates and binds a 2D GL texture to the current context, based on image. The generated texture id is returned and can be used in later glBindTexture() calls.

The target parameter specifies the texture target. The default target is GL_TEXTURE_2D.

The format parameter sets the internal format for the texture. The default format is GL_RGBA.

The binding options are a set of options used to decide how to bind the texture to the context.

The texture that is generated is cached, so multiple calls to bindTexture() with the same QImage will return the same texture id.

Note that we assume default values for the glPixelStore() and glPixelTransfer() parameters.

This function was introduced in Qt 4.6.

See also deleteTexture().

GLuint QGLContext::bindTexture ( const QString & fileName )

This is an overloaded function.

Reads the compressed texture file fileName and generates a 2D GL texture from it.

This function can load DirectDrawSurface (DDS) textures in the DXT1, DXT3 and DXT5 DDS formats if the GL_ARB_texture_compression and GL_EXT_texture_compression_s3tc extensions are supported.

Since 4.6.1, textures in the ETC1 format can be loaded if the GL_OES_compressed_ETC1_RGB8_texture extension is supported and the ETC1 texture has been encapsulated in the PVR container format. Also, textures in the PVRTC2 and PVRTC4 formats can be loaded if the GL_IMG_texture_compression_pvrtc extension is supported.

See also deleteTexture().

GLuint QGLContext::bindTexture ( const QImage & image, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA )

Generates and binds a 2D GL texture to the current context, based on image. The generated texture id is returned and can be used in later glBindTexture() calls.

This is an overloaded function.

GLuint QGLContext::bindTexture ( const QPixmap & pixmap, GLenum target = GL_TEXTURE_2D, GLint format = GL_RGBA )

This is an overloaded function.

Generates and binds a 2D GL texture based on pixmap.

GLuint QGLContext::bindTexture ( const QPixmap & pixmap, GLenum target, GLint format, BindOptions options )

This is an overloaded function.

Generates and binds a 2D GL texture to the current context, based on pixmap.

This function was introduced in Qt 4.6.

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, returning true if successful or false if a suitable handle could not be found.

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 handle ) [virtual protected]

Mac OS X 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.

The handle argument is always zero and is not used

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 and texture object 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 is in effect.

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().

void QGLContext::deleteTexture ( GLuint id )

Removes the texture identified by id from the texture cache, and calls glDeleteTextures() to delete the texture from the context.

See also bindTexture().

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]

Makes no GL context the current context. Normally, you do not need to call this function; QGLContext calls it as necessary.

void QGLContext::drawTexture ( const QRectF & target, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D )

This function supports the following use cases:

  • On OpenGL and OpenGL ES 1.x it draws the given texture, textureId, to the given target rectangle, target, in OpenGL model space. The textureTarget should be a 2D texture target.
  • On OpenGL and OpenGL ES 2.x, if a painter is active, not inside a beginNativePainting / endNativePainting block, and uses the engine with type QPaintEngine::OpenGL2, the function will draw the given texture, textureId, to the given target rectangle, target, respecting the current painter state. This will let you draw a texture with the clip, transform, render hints, and composition mode set by the painter. Note that the texture target needs to be GL_TEXTURE_2D for this use case, and that this is the only supported use case under OpenGL ES 2.x.

This function was introduced in Qt 4.4.

void QGLContext::drawTexture ( const QPointF & point, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D )

This function supports the following use cases:

  • By default it draws the given texture, textureId, at the given point in OpenGL model space. The textureTarget should be a 2D texture target.
  • If a painter is active, not inside a beginNativePainting / endNativePainting block, and uses the engine with type QPaintEngine::OpenGL2, the function will draw the given texture, textureId, at the given point, respecting the current painter state. This will let you draw a texture with the clip, transform, render hints, and composition mode set by the painter. Note that the texture target needs to be GL_TEXTURE_2D for this use case.

Note: This function is not supported under any version of OpenGL ES.

This function was introduced in Qt 4.4.

QGLFormat QGLContext::format () const

Returns the frame buffer format that was obtained (this may be a subset of what was requested).

See also setFormat() and requestedFormat().

void * QGLContext::getProcAddress ( const QString & proc ) const

Returns a function pointer to the GL extension function passed in proc. 0 is returned if a pointer to the function could not be obtained.

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 this context is sharing its GL context with another QGLContext, otherwise false is returned. Note that context 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 ()

Resets the context and makes it invalid.

See also create() and isValid().

void QGLContext::setFormat ( const QGLFormat & format )

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::setTextureCacheLimit ( int size ) [static]

This function sets the limit for the texture cache to size, expressed in kilobytes.

By default, the cache limit is approximately 64 MB.

See also textureCacheLimit().

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().

int QGLContext::textureCacheLimit () [static]

Returns the current texture cache limit in kilobytes.

See also setTextureCacheLimit().

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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