[Qt-interest] Intercepting Phonon VideoWidget frames as they are pushed

[Trenton Tuggle]

Ok...

So in my continuing saga to snag video frames, I'm now simply trying  
to render() a VideoWidget to a QImage and it's not working...  I can't  
seem to trace into the call, but it's certainly not drawing anything  
into the image!

Does anyone know whether or not this *should* succeed?

Thanks in advance!

Trent Tuggle <Trent at Tuggle.Org>

On Mar 24, 2009, at 5:38 PM, Trenton Tuggle wrote:

> On Mar 24, 2009, at 4:02 PM, Kaleb Pederson wrote:
>
>> On Tuesday 24 March 2009 11:50:56 am Trenton Tuggle wrote:
>>> Interesting update...
>>>
>>> I've been unable to "capture" VideoWidget paint events at all -- it
>>> appears as if it somehow gets drawn outside of a PaintEvent -- but
>>> how
>>> could it then be captured and redirected onto a
>>> QGraphicsViewProxyWidget?
>>
>> Wouldn't you really need a proxy QPaintDevice?  The QPaintDevice
>> could then render onto whatever it wanted.
>>
>> Please let us know how it goes.
>
>
> Kaleb, thanks for the thoughts!  I'm trying to use a QImage as my  
> proxy.
>
> What I've tried to create is a QWidget subclass which draws to a
> QImage, then emits a signal signifying the update.
>
> The primary methods I'm trying are capturing PaintEvents, and
> overloading the paintEngine().  Neither technique seems to be working
> -- I know this because my image never gets updated and PaintEvents  
> never get seen.
>
> Note that my only test case is trying to capture a
> Phonon::VideoWidget, so I suppose a possibility is that I'm doing the
> right things, but VideoWidget somehow bypasses normal stuff for
> performance purposes.  I've been looking through the Phonon backend
> (Quicktime) code, but it's not apparent how drawing is performed.
>
> Here's my code:
>
> class WidgetImager
>    : public QWidget
> {
>    Q_OBJECT
>
> public:
>    WidgetImager(QWidget *parent = 0, Qt::WindowFlags f = 0);
>    virtual ~WidgetImager();
>
>    QImage *image();
>
> signals:
>    void updated();
>
> protected:
>    virtual QPaintEngine * paintEngine () const;
>    virtual bool event(QEvent *event);
>    virtual void paintEvent(QPaintEvent *event);
>    virtual void resizeEvent(QResizeEvent *event);
>
> private:
>    QImage* image_;
> };
>
>
> WidgetImager::WidgetImager(QWidget *parent, Qt::WindowFlags f)
> : QWidget(parent, f)
> {
>    setAttribute(Qt::WA_PaintOnScreen);
>    setAttribute(Qt::WA_NoSystemBackground);
>    setAttribute(Qt::WA_OpaquePaintEvent, true);
>
>    image_ = new QImage(size(), QImage::Format_RGB32);
> }
>
> WidgetImager::~WidgetImager()
> {
>    delete image_;
> }
>
> QImage *WidgetImager::image()
> {
>    return image_;
> }
>
> bool WidgetImager::event(QEvent *event)
> {
>    if (event->type() == QEvent::Paint)
>    {
>        //Make sure all paint operations redirect here
>        QPainter::setRedirected(this, image_);
>        bool accept = QWidget::event(event);
>
>        // Notify clients of the update
>        emit updated();
>
>        return accept;
>    }
>
>    return QWidget::event(event);
> }
>
> void WidgetImager::paintEvent(QPaintEvent *event)
> {
>    Q_UNUSED(event);
> }
>
> void WidgetImager::resizeEvent(QResizeEvent *event)
> {
>    QWidget::resizeEvent(event);
>
>    delete image_;
>    image_ = new QImage(event->size(), QImage::Format_RGB32);
> }
>
> QPaintEngine * WidgetImager::paintEngine () const
> {
>    // Try using the image's painter to capture updates
>    return image_->paintEngine();
> }
>
>
>> --Kaleb
>>
>>> What's the mechanism that QGraphicsViewProxyWidget uses to "capture"
>>> the paint events of its children and "redirect" them to be drawn on
>>> the appropriate Graphics View/Scene?
>>>
>>> Thanks in advance for any pointers...
>>>
>>> -Trent Tuggle <Trent at Tuggle.Org>
>>>
>>>
>>> On Mar 21, 2009, at 11:39 AM, Trenton Tuggle wrote:
>>>
>>>> I have an interesting challenge I've been working on:
>>>>
>>>> I'm working on video processing, and am investigating using Phonon
>>>> for
>>>> platform-independent back-ends.
>>>>
>>>> My interest is not the usual, though -- I have little interest in
>>>> showing the video on-screen (although that would be nice, also) but
>>>> primarily, I'm interested in obtaining the video frames.
>>>>
>>>> Now I know it's easy to render() a widget (such as VideoWidget)
>>>> onto a
>>>> pixmap or other, but I'm really interested in obtaining the frames
>>>> in
>>>> a more automated way -- such as getting a signal when the next  
>>>> frame
>>>> is ready, etc.  My thought is that since VideoWidget can be  
>>>> embedded
>>>> in a graphics scene, it must be possible to intercept its painting
>>>> events and redirect it to a qimage or something: but so far I've
>>>> failed.
>>>>
>>>> So that's my question: how can I embed a VideoWidget in something
>>>> that
>>>> can intercept paint events, redirect them to a QImage, then  
>>>> signal a
>>>> processor that a frame is ready for processing?
>>>>
>>>> I've tried stuff along these lines:
>>>>
>>>> http://www.kdedevelopers.org/node/3644
>>>>
>>>> The thought was that I'll create a Phonon path to play on a
>>>> VideoWidget embedded in something like that which would capture the
>>>> frame, then signal my processor....
>>>>
>>>> But so far, I've been unable to get the widget I create to receive
>>>> any
>>>> paint events, or intercept anything at all -- it doesn't even get
>>>> resized properly when a video is loaded!
>>>>
>>>> If anyone has any advice or input on this, I'd really appreciate  
>>>> it!
>>>>
>>>> Thanks in advance!
>>>>
>>>> Trent Tuggle <Trent at Tuggle.Org>
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>>>
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