class QOpenGLFramebufferObject

The QOpenGLFramebufferObject class encapsulates an OpenGL framebuffer object. More…

Synopsis

Methods

Static functions

Note

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Detailed Description

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

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

Create the QOpenGLFrameBufferObject instance with the CombinedDepthStencil attachment if you want QPainter to render correctly. Note that you need to create a QOpenGLFramebufferObject 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 QOpenGLFramebufferObjectFormat parameter, and set the samples() property to a non-zero value.

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.

Multiple render targets are also supported, in case the OpenGL implementation supports this. Here there will be multiple textures (or, in case of multisampling, renderbuffers) present and each of them will get attached to GL_COLOR_ATTACHMENT0, 1, 2, …

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 QOpenGLContext::blitFramebuffer().

It is possible to draw into a QOpenGLFramebufferObject using QPainter and QOpenGLPaintDevice in a separate thread.

class Attachment

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

Constant

Description

QOpenGLFramebufferObject.NoAttachment

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.

QOpenGLFramebufferObject.CombinedDepthStencil

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.

QOpenGLFramebufferObject.Depth

A depth buffer is attached to the framebuffer object.

See also

attachment()

class FramebufferRestorePolicy

This enum type is used to configure the behavior related to restoring framebuffer bindings when calling blitFramebuffer().

Constant

Description

QOpenGLFramebufferObject.DontRestoreFramebufferBinding

Do not restore the previous framebuffer binding. The caller is responsible for tracking and setting the framebuffer binding as needed.

QOpenGLFramebufferObject.RestoreFramebufferBindingToDefault

After the blit operation, bind the default framebuffer.

QOpenGLFramebufferObject.RestoreFrameBufferBinding

Restore the previously bound framebuffer. This is potentially expensive because of the need to query the currently bound framebuffer.

See also

blitFramebuffer()

__init__(size, format)
Parameters:
__init__(size[, target=GL_TEXTURE_2D])
Parameters:

  • size – QSize

  • target – int

__init__(width, height, format)
Parameters:
__init__(width, height[, target=GL_TEXTURE_2D])
Parameters:

  • width – int

  • height – int

  • target – int

__init__(size, attachment[, target=GL_TEXTURE_2D[, internalFormat=0]])
Parameters:

  • size – QSize

  • attachment – Attachment

  • target – int

  • internalFormat – int

__init__(width, height, attachment[, target=GL_TEXTURE_2D[, internalFormat=0]])
Parameters:

  • width – int

  • height – int

  • attachment – Attachment

  • target – int

  • internalFormat – int

addColorAttachment(size[, internalFormat=0])
Parameters:

  • size – QSize

  • internalFormat – int

addColorAttachment(width, height[, internalFormat=0])
Parameters:

  • width – int

  • height – int

  • internalFormat – int

attachment()
Return type:

Attachment

bind()
Return type:

bool

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

Note

If takeTexture() was called, a new texture is created and associated with the framebuffer object. This is potentially expensive and changes the context state (the currently bound texture).

See also

release()

static bindDefault()
Return type:

bool

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

See also

bind() release()

static blitFramebuffer(target, source[, buffers=GL_COLOR_BUFFER_BIT[, filter=GL_NEAREST]])
Parameters:
static blitFramebuffer(target, targetRect, source, sourceRect[, buffers=GL_COLOR_BUFFER_BIT[, filter=GL_NEAREST]])
Parameters:
static blitFramebuffer(target, targetRect, source, sourceRect, buffers, filter, readColorAttachmentIndex, drawColorAttachmentIndex)
Parameters:
static blitFramebuffer(target, targetRect, source, sourceRect, buffers, filter, readColorAttachmentIndex, drawColorAttachmentIndex, restorePolicy)
Parameters:
format()
Return type:

QOpenGLFramebufferObjectFormat

handle()
Return type:

int

Returns the OpenGL 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 hasOpenGLFramebufferBlit()
Return type:

bool

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

static hasOpenGLFramebufferObjects()
Return type:

bool

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

height()
Return type:

int

Returns the height of the framebuffer object attachments.

isBound()
Return type:

bool

isValid()
Return type:

bool

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 QOpenGLContext that the framebuffer was created within is destroyed and there are no other shared contexts that can take over ownership of the framebuffer.

release()
Return type:

bool

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

See also

bind()

setAttachment(attachment)
Parameters:

attachment – Attachment

size()
Return type:

QSize

sizes()
Return type:

.list of QSize

takeTexture()
Return type:

int

Returns the texture id for the texture attached to this framebuffer object. The ownership of the texture is transferred to the caller.

If the framebuffer object is currently bound, an implicit release() will be done. During the next call to bind() a new texture will be created.

If a multisample framebuffer object is used, then there is no texture and the return value from this function will be invalid. Similarly, incomplete framebuffer objects will also return 0.

takeTexture(colorAttachmentIndex)
Parameters:

colorAttachmentIndex – int

Return type:

int

texture()
Return type:

int

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 OpenGL code.

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

When multiple textures are attached, the return value is the ID of the first one.

textures()
Return type:

.list of unsigned int

toImage([flipped=true])
Parameters:

flipped – bool

Return type:

QImage

Returns the contents of this framebuffer object as a QImage.

If flipped is true the image is flipped from OpenGL coordinates to raster coordinates. If used together with QOpenGLPaintDevice , flipped should be the opposite of the value of paintFlipped() .

The returned image has a format of premultiplied ARGB32 or RGB32. The latter is used only when internalTextureFormat() is set to GL_RGB. Since Qt 5.2 the function will fall back to premultiplied RGBA8888 or RGBx8888 when reading to (A)RGB32 is not supported, and this includes OpenGL ES. Since Qt 5.4 an A2BGR30 image is returned if the internal format is RGB10_A2, and since Qt 5.12 a RGBA64 image is return if the internal format is RGBA16.

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);

For multisampled framebuffer objects the samples are resolved using the GL_EXT_framebuffer_blit extension. If the extension is not available, the contents of the returned image is undefined.

For singlesampled framebuffers the contents is retrieved via glReadPixels. This is a potentially expensive and inefficient operation. Therefore it is recommended that this function is used as seldom as possible.

See also

paintFlipped()

toImage(flipped, colorAttachmentIndex)
Parameters:

  • flipped – bool

  • colorAttachmentIndex – int

Return type:

QImage

width()
Return type:

int

Returns the width of the framebuffer object attachments.