[Qt-interest] Paint QWebView to a QImage

[Richard Hosking]

Again, many thanks!  I think I am nearly there!

I am getting the paint events now, but I am
not seeing anything painted to the QImage when
I call render().

I noticed that qimage->paintEngine()->painter()
sometimes returns null which seemed strange, so
I have added a test to prevent a fault (line 20).

I have attached my test code.  After waiting for
3 seconds for the webpage to render, it saves
the qimage to 'output.png' and exits.

All I see in the output is the white background!
Have I missed anything?

Richard.

Richard Hosking wrote:
> Many thanks for the advice.  The repaintRequested approach seems to
> be the ideal method for me.
> 
> I have implemented this, but I am getting Painter errors:-
> 
> "A paint device can only be painted by one painter at a time"
> 
> ...when painting to the QImage upon each repaintRequested call.
> I suspect some concurrency issues here on my part!
> 
> I have attached my very stripped down test code which highlights
> my issue.  What do I need to do to make this work?  Do I need to
> 'queue' the paint events myself so they do not overlap?
> 
> Richard.
> 
> (Qt 4.5.1 on Ubuntu 9.04)
> 
> Girish Ramakrishnan wrote:
>> Hi,
>> Use a plain QWebPage and connect to it' repaintRequested(QRect). You can
>> then pass that QRect to QWebFrame's render() to update only the portion
>> that got changed.
>>
>> (Also, your approach on changing the paintEngine won't work. Just
>> reimplementing QWidget::paintEngine() doesn't actually switch the paint
>> engine - It's complicated)
>>
>> Girish
>>
>> Richard Hosking wrote:
>>> Hello, I am trying to render the contents of a QWebView
>>> to a QImage instance.
>>>
>>> I have tried the QWebFrame->render(QImage) method, but this
>>> is not the correct approach for me as I need to render
>>> at several frames a second.  I really only want to render
>>> when changes occur on the widget to keep CPU usage down.
>>>
>>> What I would ideally like is for QWebView to render directly
>>> to a QImage.
>>>
>>> I have tried subclassing QWebView's paintEngine() and
>>> returning my QImage->paintEngine() but the image remains
>>> unchanged.
>>>
>>> I have also tried hooking into QWebView's event() and
>>> listening for QEvent::Paint then calling
>>> QPainter::setRedirected(QWebView, QImage) but again nothing
>>> is produced in the image.
>>>
>>> Any pointers?
>>>
>>> (Qt 4.5.1 on Linux)
>>>
>>> Kind regards,
>>> Richard.
>>
>>
>>
> 
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