QGLFramebufferObject Class | Qt OpenGL

Qt 5.13 | Qt OpenGL | C++ Classes | QGLFramebufferObject

Qt 5.13.2 Reference Documentation

# QGLFramebufferObject Class

The QGLFramebufferObject class encapsulates an OpenGL framebuffer object. More...

Header: | #include <QGLFramebufferObject>
qmake: | QT += opengl
Since: | Qt 4.2
Inherits: | QPaintDevice

This class is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

This class was introduced in Qt 4.2.

- List of all members, including inherited members

## Public Types

enum | Attachment { NoAttachment, CombinedDepthStencil, Depth }

## Public Functions

| QGLFramebufferObject(int width, int height, const QGLFramebufferObjectFormat &format)
| QGLFramebufferObject(const QSize &size, const QGLFramebufferObjectFormat &format)
| QGLFramebufferObject(int width, int height, QGLFramebufferObject::Attachment attachment, GLenum target = GL_TEXTURE_2D, GLenum internal_format = 0)
| QGLFramebufferObject(const QSize &size, QGLFramebufferObject::Attachment attachment, GLenum target = GL_TEXTURE_2D, GLenum internal_format = 0)
| QGLFramebufferObject(int width, int height, GLenum target = GL_TEXTURE_2D)
| QGLFramebufferObject(const QSize &size, GLenum target = GL_TEXTURE_2D)
virtual | ~QGLFramebufferObject()
QGLFramebufferObject::Attachment | attachment() const
bool | bind()
void | drawTexture(const QRectF &target, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D)
void | drawTexture(const QPointF &point, GLuint textureId, GLenum textureTarget = GL_TEXTURE_2D)
QGLFramebufferObjectFormat | format() const
GLuint | handle() const
bool | isBound() const
bool | isValid() const
bool | release()
QSize | size() const
GLuint | texture() const
QImage | toImage() const

## Reimplemented Public Functions

virtual QPaintEngine * | paintEngine() const override

## Static Public Members

bool | bindDefault()
void | blitFramebuffer(QGLFramebufferObject *target, const QRect &targetRect, QGLFramebufferObject *source, const QRect &sourceRect, GLbitfield buffers = GL_COLOR_BUFFER_BIT, GLenum filter = GL_NEAREST)
bool | hasOpenGLFramebufferBlit()
bool | hasOpenGLFramebufferObjects()

## Reimplemented Protected Functions

virtual int | metric(QPaintDevice::PaintDeviceMetric metric) const override

## Detailed Description

The QGLFramebufferObject class encapsulates an OpenGL framebuffer object, defined by the `GL_EXT_framebuffer_object` extension. In addition it provides a rendering surface that can be painted on with a QPainter, rendered to using native GL calls, or both. This surface can be bound and used as a regular texture in your own GL drawing code. By default, the QGLFramebufferObject class generates a 2D GL texture (using the `GL_TEXTURE_2D` target), which is used as the internal rendering target.

It is important to have a current GL context when creating a QGLFramebufferObject, otherwise initialization will fail.

OpenGL framebuffer objects and pbuffers (see QGLPixelBuffer) can both be used to render to offscreen surfaces, but there are a number of advantages with using framebuffer objects instead of pbuffers:

1. A framebuffer object does not require a separate rendering context, so no context switching will occur when switching rendering targets. There is an overhead involved in switching targets, but in general it is cheaper than a context switch to a pbuffer.
2. Rendering to dynamic textures (i.e. render-to-texture functionality) works on all platforms. No need to do explicit copy calls from a render buffer into a texture, as was necessary on systems that did not support the `render_texture` extension.
3. It is possible to attach several rendering buffers (or texture objects) to the same framebuffer object, and render to all of them without doing a context switch.
4. The OpenGL framebuffer extension is a pure GL extension with no system dependant WGL, CGL, or GLX parts. This makes using framebuffer objects more portable.

When using a QPainter to paint to a QGLFramebufferObject you should take care that the QGLFramebufferObject is created with the CombinedDepthStencil attachment for QPainter to be able to render correctly. Note that you need to create a QGLFramebufferObject with more than one sample per pixel for primitives to be antialiased when drawing using a QPainter. To create a multisample framebuffer object you should use one of the constructors that take a QGLFramebufferObjectFormat parameter, and set the QGLFramebufferObjectFormat::samples() property to a non-zero value.

When painting to a QGLFramebufferObject using QPainter, the state of the current GL context will be altered by the paint engine to reflect its needs. Applications should not rely upon the GL state being reset to its original conditions, particularly the current shader program, GL viewport, texture units, and drawing modes.

For multisample framebuffer objects a color render buffer is created, otherwise a texture with the specified texture target is created. The color render buffer or texture will have the specified internal format, and will be bound to the `GL_COLOR_ATTACHMENT0` attachment in the framebuffer object.

If you want to use a framebuffer object with multisampling enabled as a texture, you first need to copy from it to a regular framebuffer object using QGLContext::blitFramebuffer().

### Threading

As of Qt 4.8, it's possible to draw into a QGLFramebufferObject using a QPainter in a separate thread. Note that OpenGL 2.0 or OpenGL ES 2.0 is required for this to work.

Note: This class has been deprecated in favor of QOpenGLFramebufferObject.

## Member Type Documentation

### enum QGLFramebufferObject::Attachment

This enum type is used to configure the depth and stencil buffers attached to the framebuffer object when it is created.

Constant | Value | Description
--- | --- | ---
`QGLFramebufferObject::NoAttachment` | `0` | No attachment is added to the framebuffer object. Note that the OpenGL depth and stencil tests won't work when rendering to a framebuffer object without any depth or stencil buffers. This is the default value.
`QGLFramebufferObject::CombinedDepthStencil` | `1` | If the `GL_EXT_packed_depth_stencil` extension is present, a combined depth and stencil buffer is attached. If the extension is not present, only a depth buffer is attached.
`QGLFramebufferObject::Depth` | `2` | A depth buffer is attached to the framebuffer object.

This enum was introduced or modified in Qt 4.3.

See also attachment().

## Member Function Documentation

### QGLFramebufferObject::QGLFramebufferObject(int width, int height, const QGLFramebufferObjectFormat &format)

This is an overloaded function.

Constructs an OpenGL framebuffer object of the given width and height based on the supplied format.

### QGLFramebufferObject::QGLFramebufferObject(const QSize &size, const QGLFramebufferObjectFormat &format)

This is an overloaded function.

Constructs an OpenGL framebuffer object of the given size based on the supplied format.

### QGLFramebufferObject::QGLFramebufferObject(int width, int height, QGLFramebufferObject::Attachment attachment, GLenum target = GL_TEXTURE_2D, GLenum internal_format = 0)

This is an overloaded function.

Constructs an OpenGL framebuffer object and binds a texture to the buffer of the given width and height.

The attachment parameter describes the depth/stencil buffer configuration, target the texture target and internal_format the internal texture format. The default texture target is `GL_TEXTURE_2D`, while the default internal format is `GL_RGBA8` for desktop OpenGL and `GL_RGBA` for OpenGL/ES.

See also size(), texture(), and attachment().

### QGLFramebufferObject::QGLFramebufferObject(const QSize &size, QGLFramebufferObject::Attachment attachment, GLenum target = GL_TEXTURE_2D, GLenum internal_format = 0)

This is an overloaded function.

Constructs an OpenGL framebuffer object and binds a texture to the buffer of the given size.

The attachment parameter describes the depth/stencil buffer configuration, target the texture target and internal_format the internal texture format. The default texture target is `GL_TEXTURE_2D`, while the default internal format is `GL_RGBA8` for desktop OpenGL and `GL_RGBA` for OpenGL/ES.

See also size(), texture(), and attachment().

### QGLFramebufferObject::QGLFramebufferObject(int width, int height, GLenum target = GL_TEXTURE_2D)

This is an overloaded function.

Constructs an OpenGL framebuffer object and binds a 2D GL texture to the buffer of the given width and height.

See also size() and texture().

### QGLFramebufferObject::QGLFramebufferObject(const QSize &size, GLenum target = GL_TEXTURE_2D)

Constructs an OpenGL framebuffer object and binds a 2D GL texture to the buffer of the size size. The texture is bound to the `GL_COLOR_ATTACHMENT0` target in the framebuffer object.

The target parameter is used to specify the GL texture target. The default target is `GL_TEXTURE_2D`. Keep in mind that `GL_TEXTURE_2D` textures must have a power of 2 width and height (e.g. 256x512), unless you are using OpenGL 2.0 or higher.

By default, no depth and stencil buffers are attached. This behavior can be toggled using one of the overloaded constructors.

The default internal texture format is `GL_RGBA8` for desktop OpenGL, and `GL_RGBA` for OpenGL/ES.

It is important that you have a current GL context set when creating the QGLFramebufferObject, otherwise the initialization will fail.

See also size(), texture(), and attachment().

### `[virtual]` QGLFramebufferObject::~QGLFramebufferObject()

Destroys the framebuffer object and frees any allocated resources.

### QGLFramebufferObject::Attachment QGLFramebufferObject::attachment() const

Returns the status of the depth and stencil buffers attached to this framebuffer object.

### bool QGLFramebufferObject::bind()

Switches rendering from the default, windowing system provided framebuffer to this framebuffer object. Returns `true` upon success, false otherwise.

See also release().

### `[static]` bool QGLFramebufferObject::bindDefault()

Switches rendering back to the default, windowing system provided framebuffer. Returns `true` upon success, false otherwise.

See also bind() and release().

### `[static]` void QGLFramebufferObject::blitFramebuffer(QGLFramebufferObject *target, const QRect &targetRect, QGLFramebufferObject *source, const QRect &sourceRect, GLbitfield buffers = GL_COLOR_BUFFER_BIT, GLenum filter = GL_NEAREST)

Blits from the sourceRect rectangle in the source framebuffer object to the targetRect rectangle in the target framebuffer object.

If source or target is 0, the default framebuffer will be used instead of a framebuffer object as source or target respectively.

The buffers parameter should be a mask consisting of any combination of `GL_COLOR_BUFFER_BIT`, `GL_DEPTH_BUFFER_BIT`, and `GL_STENCIL_BUFFER_BIT`. Any buffer type that is not present both in the source and target buffers is ignored.

The sourceRect and targetRect rectangles may have different sizes; in this case buffers should not contain `GL_DEPTH_BUFFER_BIT` or `GL_STENCIL_BUFFER_BIT`. The filter parameter should be set to `GL_LINEAR` or `GL_NEAREST`, and specifies whether linear or nearest interpolation should be used when scaling is performed.

If source equals target a copy is performed within the same buffer. Results are undefined if the source and target rectangles overlap and have different sizes. The sizes must also be the same if any of the framebuffer objects are multisample framebuffers.

Note that the scissor test will restrict the blit area if enabled.

This function will have no effect unless hasOpenGLFramebufferBlit() returns true.

This function was introduced in Qt 4.6.

See also hasOpenGLFramebufferBlit().

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

Draws the given texture, textureId, to the given target rectangle, target, in OpenGL model space. The textureTarget should be a 2D texture target.

The framebuffer object should be bound when calling this function.

Equivalent to the corresponding QGLContext::drawTexture().

This function was introduced in Qt 4.4.

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

Draws the given texture, textureId, at the given point in OpenGL model space. The textureTarget should be a 2D texture target.

The framebuffer object should be bound when calling this function.

Equivalent to the corresponding QGLContext::drawTexture().

This function was introduced in Qt 4.4.

### QGLFramebufferObjectFormat QGLFramebufferObject::format() const

Returns the format of this framebuffer object.

### GLuint QGLFramebufferObject::handle() const

Returns the GL framebuffer object handle for this framebuffer object (returned by the `glGenFrameBuffersEXT()` function). This handle can be used to attach new images or buffers to the framebuffer. The user is responsible for cleaning up and destroying these objects.

### `[static]` bool QGLFramebufferObject::hasOpenGLFramebufferBlit()

Returns `true` if the OpenGL `GL_EXT_framebuffer_blit` extension is present on this system; otherwise returns `false`.

This function was introduced in Qt 4.6.

See also blitFramebuffer().

### `[static]` bool QGLFramebufferObject::hasOpenGLFramebufferObjects()

Returns `true` if the OpenGL `GL_EXT_framebuffer_object` extension is present on this system; otherwise returns `false`.

### bool QGLFramebufferObject::isBound() const

Returns `true` if the framebuffer object is currently bound to a context, otherwise false is returned.

This function was introduced in Qt 4.5.

### bool QGLFramebufferObject::isValid() const

Returns `true` if the framebuffer object is valid.

The framebuffer can become invalid if the initialization process fails, the user attaches an invalid buffer to the framebuffer object, or a non-power of two width/height is specified as the texture size if the texture target is `GL_TEXTURE_2D`. The non-power of two limitation does not apply if the OpenGL version is 2.0 or higher, or if the GL_ARB_texture_non_power_of_two extension is present.

The framebuffer can also become invalid if the QGLContext that the framebuffer was created within is destroyed and there are no other shared contexts that can take over ownership of the framebuffer.

### `[override virtual protected]` int QGLFramebufferObject::metric(QPaintDevice::PaintDeviceMetric metric) const

Reimplements: QPaintDevice::metric(QPaintDevice::PaintDeviceMetric metric) const.

### `[override virtual]` QPaintEngine *QGLFramebufferObject::paintEngine() const

Reimplements: QPaintDevice::paintEngine() const.

### bool QGLFramebufferObject::release()

Switches rendering back to the default, windowing system provided framebuffer. Returns `true` upon success, false otherwise.

See also bind().

### QSize QGLFramebufferObject::size() const

Returns the size of the texture attached to this framebuffer object.

### GLuint QGLFramebufferObject::texture() const

Returns the texture id for the texture attached as the default rendering target in this framebuffer object. This texture id can be bound as a normal texture in your own GL code.

If a multisample framebuffer object is used then the value returned from this function will be invalid.

### QImage QGLFramebufferObject::toImage() const

Returns the contents of this framebuffer object as a QImage.

The returned image has a format of premultiplied ARGB32 or RGB32. The latter is used only when internalTextureFormat() is set to `GL_RGB`.

If the rendering in the framebuffer was not done with premultiplied alpha in mind, create a wrapper QImage with a non-premultiplied format. This is necessary before performing operations like QImage::save() because otherwise the image data would get unpremultiplied, even though it was not premultiplied in the first place. To create such a wrapper without performing a copy of the pixel data, do the following:

      QImage fboImage(fbo.toImage());
      QImage image(fboImage.constBits(), fboImage.width(), fboImage.height(), QImage::Format_ARGB32);

On QNX the back buffer is not preserved when a buffer swap occures. So this function might return old content.