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Update fbo toImage() docs regarding premultiplied alpha

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Task-number: QTBUG-42510
Task-number: QTBUG-37261
Change-Id: Ic11bec0a25e66df9d022f640621686be867e84d2
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
This commit is contained in:
Laszlo Agocs 2014-11-14 13:57:55 +01:00
parent f1aafb595b
commit 954552ceac
2 changed files with 31 additions and 3 deletions
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20
src/gui/opengl/qopenglframebufferobject.cpp
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@ -1203,9 +1203,23 @@ Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format,
If used together with QOpenGLPaintDevice, \a flipped should be the opposite of the value
of QOpenGLPaintDevice::paintFlipped().
Will try to return a premultiplied ARBG32 or RGB32 image. Since 5.2 it will fall back to
a premultiplied RGBA8888 or RGBx8888 image when reading to ARGB32 is not supported. Since 5.4 an
A2BGR30 image is returned if the internal format is RGB10_A2.
The returned image has a format of premultiplied ARGB32 or RGB32. The latter is used
only when internalTextureFormat() is set to \c 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:
\code
QImage fboImage(fbo.toImage());
QImage image(fboImage.constBits(), fboImage.width(), fboImage.height(), QImage::Format_ARGB32);
\endcode
Since Qt 5.2 the function will fall back to premultiplied RGBA8888 or RGBx8888 when
reading to (A)RGB32 is not supported. Since 5.4 an A2BGR30 image is returned if the
internal format is RGB10_A2.
For multisampled framebuffer objects the samples are resolved using the
\c{GL_EXT_framebuffer_blit} extension. If the extension is not available, the contents

14
src/opengl/qglframebufferobject.cpp
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@ -1111,6 +1111,20 @@ QGLFramebufferObjectFormat QGLFramebufferObject::format() 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 \c 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:
\code
QImage fboImage(fbo.toImage());
QImage image(fboImage.constBits(), fboImage.width(), fboImage.height(), QImage::Format_ARGB32);
\endcode
On QNX the back buffer is not preserved when a buffer swap occures. So this function
might return old content.
*/