Qt Upgrade Repository..

[jiapei100]

Things become more weird now.
Please refer to this video http://www.visionopen.com/qgraphicsview_paintevent2.ogv

Now, after I changed my code a little bit to

this->m_QTGraphicsScene->invalidate();
this->repaint();

this->m_QTGraphicsScene->invalidate();
this->repaint();

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repaint() now is able to trigger paintEvent() as you can see from the above video file.
However, no matter it's

QPainter p(this);
or 
QPainter p(this->viewport());

QPainter p(this);
or 
QPainter p(this->viewport());

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in function paintEvent(),
I'm not able to display what has been drawn on the screen.

The processed image have been successfully stored, but it just didn't show up on the screen.

By the way, I guess why now paintEvent() was successfully triggered by

this->repaint() ;  // which was supposed to have the same functionality with this->update();

this->repaint() ;  // which was supposed to have the same functionality with this->update();

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is just because I overwrote all qt 4.6.2 related .so files, and reconstruct the symbolic links to qt 4.6.3 .so files.

We may neglect whether this is a qt 4.6.2 or qt 4.6.3 issue first.
But, as you may noticed from the video of this time,
paintEvent() is now successfully called whenever a new image is loaded (for processing),
but paintEvent() just didn't really draw the image on the screen.

Can you please help?

Cheers
JIA

[jiapei100]

Now, I'm even able to Save the QImage out, and I can see the saving is absolutely correct.
That is to say, I'm now very sure paintEvent causes this problem.

After staring at the GUI application for quite a while, I found something interesting.
It looks like paintEvent() did draw !!! But draw to a wrong position.
Not quite sure, but see the attached two .png pictures.

Images removed

Pay attention to the very top left corner, there is little rectangle which has exactly the same color of my loaded image's background.
Did you see?

What could be the problem??

Cheers
JIA

[wysota]

What could be the problem??

The problem is in your code. You can be chasing ghosts all the time or you can just correct your code and forget about the problem. Your code is clearly incorrect. You lack understanding of how Graphics View and Qt's painting works but you keep pushing pretending you do understand it. The paint event of the viewport is not called because the scene doesn't change and Qt takes the rendering of the viewport from the backing store. If you invalidate the whole scene then contents of the backing store is marked dirty and the viewport is repainted using your event because Qt thinks there are some changes in the scene.

repaint() and update() have different functionality - the first performs an immediate redraw, the other schedules one. A practical example:

repaint();
repaint();

repaint();
repaint();

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will cause the paint event to be executed twice and

update();
update();

update();
update();

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will cause the paint event to be executed once.

[jiapei100]

wysota:

You might be correct. I seriously have no idea how Qt's painting works. I dropped the question just to find the correct solution.
You gave so much explanation but only without a solution.
I'm wondering if you can afford me a direct solution, for example, affording an inherited class

myQGraphicsView :: public QGraphicsView

myQGraphicsView :: public QGraphicsView

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and the demo code, which is really able to successfully display an QImage .

By the way, I didn't pretend to do anything.
If I pretend to be like that, I will never drop questions.
The reason why I drop the question is just to let the whole world to know I'm not familiar with that,
and I beg a solution sincerely.

So, can you please help?
All kinds of blame is of no use, compared to giving out a solution.

Seriously hope you can give me a hand, by a solution to demonstrate you can do it.

Looking forward to your solution.

Best Regards
JIA

The problem is in your code. You can be chasing ghosts all the time or you can just correct your code and forget about the problem. Your code is clearly incorrect. You lack understanding of how Graphics View and Qt's painting works but you keep pushing pretending you do understand it. The paint event of the viewport is not called because the scene doesn't change and Qt takes the rendering of the viewport from the backing store. If you invalidate the whole scene then contents of the backing store is marked dirty and the viewport is repainted using your event because Qt thinks there are some changes in the scene.

repaint() and update() have different functionality - the first performs an immediate redraw, the other schedules one. A practical example:

repaint();
repaint();

repaint();
repaint();

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will cause the paint event to be executed twice and

update();
update();

update();
update();

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will cause the paint event to be executed once.

[wysota]

You gave so much explanation but only without a solution.

I have no idea what you are doing so I can't give you a solution.
If you want to display an image on Graphics View then either add it as an item to the scene or set it as the background brush of the scene (or reimplement drawBackground() if you need something more complex).

[jiapei100]

Hi, wysota:

I just want to show up an image using QGraphicsView widget.
As you can see from here
http://www.morethantechnical.com/200...ting-qwidgets/

This guy successfully display something out by using

this->update();

this->update();

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and

void FaceRecognizer::paintEvent(QPaintEvent* e) {
QPainter painter(this);
painter.drawImage(QPoint(ui.frame->x(),ui.frame->y()),m_i);
...
}

void FaceRecognizer::paintEvent(QPaintEvent* e) {
QPainter painter(this);
painter.drawImage(QPoint(ui.frame->x(),ui.frame->y()),m_i);
...
}

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He finally draw QImage m_i out at the position
"QPoint(ui.frame->x(),ui.frame->y()" .

Why he succeeded in doing this?

BTW, I promise this website is not owned by me and we are absolutely different developers.
So, according to your opinion, both of us succeeded in doing this but both of us are wrong, right?

I believe you can realize your correct solution and demonstrate it now, just based upon the full code at
http://www.morethantechnical.com/200...ting-qwidgets/

Seriously hope you can give me a hand. Could be useful for that guy as well.

Cheers
JIA

[wysota]

"This guy" is not using graphics view.

[jiapei100]

Hi, so, do you mean that
if I inherit my class from QWidget, namely,

revise

class CqtImageView : public QGraphicsView

class CqtImageView : public QGraphicsView

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to

class CqtImageView : public QWidget

class CqtImageView : public QWidget

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I should be able to make my code work, right??

Thanks for your suggestion.
And, could you please help to explain why QGraphicsView is not work?
Because QGraphicsView is just a type of QWidget.

Refer to the official documentation:

http://doc.qt.nokia.com/4.6/qgraphicsview.html
QGraphicsView<-QAbstractScrollArea<-QFrame<-QWidget.


Best Regards
JIA

[wysota]

I should be able to make my code work, right??

Yes.

And, could you please help to explain why QGraphicsView is not work?

Because it's much more complex than a simple QWidget and it works in a different fashion.

Because QGraphicsView is just a type of QWidget.

Refer to the official documentation:

http://doc.qt.nokia.com/4.6/qgraphicsview.html
QGraphicsView<-QAbstractScrollArea<-QFrame<-QWidget.

If you are so smart then why are you asking the questions and why is your code failing to work?

BTW. you fail to see the difference between "subclass of" and "type of".

Please just set the background brush for your scene as I told you if you want an immediate effect and then devote some time to study the documentation of graphics view architecture.

[jiapei100]

I love you man !! hehe...
Yes, you are right.
Have to change the scene, instead of the view !
Still not quite understand the whole theory at the back, but it works now.
I'm using

this->m_QTGraphicsScene->clear();
	this->m_QTGraphicsScene->addPixmap(QPixmap::fromImage(*this->m_QTImage));
	this->update();

this->m_QTGraphicsScene->clear();
	this->m_QTGraphicsScene->addPixmap(QPixmap::fromImage(*this->m_QTImage));
	this->update();

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after I reload a new image !

And it seems I don't need to overload paintEvent() now.

Not quite sure whether this is the right solution, but this works for me.
The reason why I didn't want to use "addPixmap"
is because it has to do a conversion from QImage to QPixmap,
which I had guessed would slow down the process speed.


Thanks wysota.

Cheers
JIA

Yes.



Because it's much more complex than a simple QWidget and it works in a different fashion.


If you are so smart then why are you asking the questions and why is your code failing to work?

BTW. you fail to see the difference between "subclass of" and "type of".

Please just set the background brush for your scene as I told you if you want an immediate effect and then devote some time to study the documentation of graphics view architecture.

[wysota]

If all you want is to show an image then why don't you just use QLabel? Graphics View is surely an overkill here.

QLabel *label = new QLabel;
label->setPixmap(QPixmap("myimage.png"));
label->show();

QLabel *label = new QLabel;
label->setPixmap(QPixmap("myimage.png"));
label->show();

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[jiapei100]

Revised to QWidget and tested already.
Same situation as using QGraphicsView.
I'm now upgrading to 4.6.3 using KUbuntu's lauchpad .
Let's see, how it goes.

Best Regards
JIA

"This guy" is not using graphics view.