[Interest] How can i manipulate pixel data from a QWidget?

Harri Pasanen harri at mpaja.com
Sat Sep 22 21:36:27 CEST 2012

On 09/22/2012 09:25 PM, Mark wrote:
> On Sat, Sep 22, 2012 at 9:15 PM, Josiah Bryan <josiahbryan at gmail.com> wrote:
>> Subclass QWidget::paintEvent(QPaintEvent*)
>> (http://doc.qt.digia.com/4.6/qwidget.html#paintEvent) and do your own
>> painting of the widget. See the Analog Clock example
>> (http://doc.qt.digia.com/4.7-snapshot/widgets-analogclock.html) for an
>> example on how to draw the contents of the widget.
>> If you're wanting to do per-pixel work - the easiest (only?) way is via a
>> QImage - there is no direct per pixel access to video memory. The closest
>> thing you'll get to direct-pixel access of video memory is to use a
>> QGLWidget and implement pixel shaders.
>> On Sat, Sep 22, 2012 at 3:00 PM, Mark <markg85 at gmail.com> wrote:
>>> Hi,
>>> I know one can manipulate pixel data in a QImage using the bits()
>>> function, but how can i do the same for a QWidget?
>>> I want to play with a high performance line drawing algorithm [1] so i
>>> guess it's not very optimal to do the following:
>>> - draw in a QImage
>>> - put that image in the qwidget
>>> - further drawing in the QImage
>>> - put it on the QWidget again
>>> - ... and so on ...
>>> In this case i'm guessing that the QImage mapping to a QWidget would
>>> be the most costly one hence i'd like to prevent that altogether and
>>> "just" manipulate the QWidget pixel data directly.
>>> So how do i do that?
>>> Cheers,
>>> Mark
>>> [1] http://free.pages.at/easyfilter/bresenham.html
>>> _______________________________________________
>>> Interest mailing list
>>> Interest at qt-project.org
>>> http://lists.qt-project.org/mailman/listinfo/interest
> Thank you for your reply :)
> Though that is sadly not going to work. Remember, i want to test out
> another line drawing algorithm.. not the one from QPainter.
> QGLWidget and pixel shaders is also not an option. I first want to do
> this in software, not hardware/opengl.
> I could really use some way to get the widget pixel data...
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> Interest mailing list
> Interest at qt-project.org
> http://lists.qt-project.org/mailman/listinfo/interest

You draw to QImage in


and blit that to widget.


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