Some of the reasons are right there in the documentation: QPainter::begin().
Generally speaking begin() returns false when the underlying resources are unavailable - another painter is active on that device, trying to paint to a null device or image, underlying paint engine is uninitialized or non existing etc. For end() it's similar - either the painter in not active on any device or the underlying paint engine failed to end the operation for whatever reason.

There is no API for getting the reason of failure of any of those as it would be hard to make one considering there are different paint engines and you can even make one yourself. It would have to be something really generic like a string, and having users parse that and basically expect every possible reason on every supported platform would not be a good API.

You won't get a paint event at all during drag'n'drop I would guess since the cursor is not directly painted by Qt when I'm correct.

kind of shooting in the dark here, but remember that the painter coordinate system can be scaled, rotated, and new origin set. Try explicitly defining the coordinate system...Also, if your coordinates are floating point then you have to deal with floating point roundoff error.

@Staslend if your issue is solved, please don't forget to mark your post as such! Thanks.

Ok solved

It appears that a GLOBAL surface setInterval overrides the setIntervals I did on my glWidget. Kinda weird that the global one overrides local one if local one was specified, is this a bug ?

format.setSwapInterval(0); QSurfaceFormat::setDefaultFormat(format);

overrides on widget >

QSurfaceFormat format; format.setSwapInterval(0); setFormat(format);

Hi
Well to get below the QWTPlot plot would
be QWTPlotptr->y() + QWTPlotptr->height()
However, a more solid approach could be to use a QLabel as
a canvas using a pixmap. That way, its very easy to position even if using layouts to make
GUI auto adjust to different screen sizes.

void PaintShapes() { // take size of label int h = ui->label->height(); int w = ui->label->width(); // make a pixmap of the wanted size QPixmap pix(w, h); // assign painter to it so we can paint on it QPainter paint(&pix); // fill it pix.fill( Qt::white ); // paint on canvas paint.setPen(QColor(0, 0, 0, 255)); paint.drawRect(0, 0, w, h); ..other paint operations-.. // set the pixmap to the label so its shown. ui->label->setPixmap(pix); }

@Niagarer
Good found. :)

You can make a "container widget" that will handle the showing of the label properly.

@Zhukov said in Рисование линии Qt:

Как передать точки x,y,x1,y1 в paintEvent()?

Как показали выше: сохранить координаты в полях класса, вызвать update() (repaint лучше не юзать без особых причин) и использовать эти поля в paintEvent()

Как вариант, можно использовать QPainterPath или даже QPicture, это хорошо подойдет если планируется рисовать много линий и добавлять/убирать их в процессе работы приложения. Еще можно нарисовать линии в QImage и затем рисовать получившуюся картинку в paintEvent(), хорошо подойдет для реализации растрового редактора типа MS Paint, но будет привязка к конкретным размерам картинки в пикселях.

Как использовать paintEvent() в любой момент?

Никак, paintEvent вызывается в моменты перерисовки окна графической системой. Если вызвать repaint(), то эта перерисвока произойдет практически сразу же, но лучше не делать код, зависящий от предположения мгновенной перерисовки, и использовать update

@mrjj Hey I implemanted the project the QGraphicsView way. But even though I can draw now. When I open a video I can only hear the audio but can't see the video. Can you help me with this now ?

mygraphicsview.h

#ifndef MYGRAPHICSVIEW_H #define MYGRAPHICSVIEW_H #include <QObject> #include <QWidget> #include <QGraphicsView> #include <QMouseEvent> #include <QPainter> #include <QRubberBand> #include <QLabel> class MyGraphicsView : public QGraphicsView { Q_OBJECT public: MyGraphicsView(QWidget *parent = nullptr); protected: void mouseMoveEvent(QMouseEvent *ev); void mousePressEvent(QMouseEvent *ev); void mouseReleaseEvent(QMouseEvent *ev); void paintEvent(QPaintEvent *ev); private: QRubberBand *rubberBand; QPoint origin; QRect rect; }; #endif // MYGRAPHICSVIEW_H

mygraphicsview.cpp

#include "mygraphicsview.h" MyGraphicsView::MyGraphicsView(QWidget *parent): QGraphicsView (parent), rubberBand(nullptr){} void MyGraphicsView::mousePressEvent(QMouseEvent *ev) { origin = ev->pos(); if(!rubberBand) rubberBand = new QRubberBand(QRubberBand::Rectangle, this); rubberBand->setGeometry(QRect(origin,QSize())); rubberBand->show(); QGraphicsView::mousePressEvent(ev); } void MyGraphicsView::mouseMoveEvent(QMouseEvent *ev) { rubberBand->setGeometry(QRect(origin,ev->pos()).normalized()); QGraphicsView::mouseMoveEvent(ev); } void MyGraphicsView::mouseReleaseEvent(QMouseEvent *ev) { rect = rubberBand->geometry(); update(); QGraphicsView::mouseReleaseEvent(ev); } void MyGraphicsView::paintEvent(QPaintEvent *ev) { QGraphicsView::paintEvent(ev); QPainter painter(this->viewport()); painter.setBrush(Qt::red); if(!rect.isNull()) painter.drawRect(rect); }

Hi @f.serreau
I have tested your example, and it works correctly with no errors.
I tested it on Windows 10 (Fall Update), MS VS2017 and Qt 5.11.0.

Sorry, I don't have the XPS printer available. Can you check whether you have the same result with the QPdfWriter class ?

@mdma2
Can you post the full code of this short example, so that it can be tried out immediately using simple copy/paste...

In your MyView::paintEvent write QPainter painter(viewport()); instead of QPainter painter(this);.

@dalishi
You should not mix painter and gl calls like that. You should put gl calls between calls to beginNativePainting() and endNativePainting(), which are painter functions. It might also help to save the painter state before and restore it after depending on how it messes up your gl stuff.
the docs here tell you what the painter does to the states.

Ok, I will try that...
Maybe I should have been more clearly what I intend to get...
I had it all working already, just without the scaling issue

I get camera images and I let opencv compute min/max values and a centerpoint. Somethimes the camera is rotated in a undesired position so the user should be able to rotate the view, including the measurepoints, and more complex including the point values so that the stay at the points but do not rotate staying readable...

I keep trying with qpainter draw

@VRonin Yes, they are, but the pixmap is just grabbed QGraphicsView's content using grab() method.

@mrjj Thanks! I'll look into that. I got another reply from stackoverflow which I will also give a shot. I'll keep everyone in both forums posted as soon as I have some results.

Hi,

Glad you found out and thanks for sharing !

What version of Qt are you using ?
On which platform ?
Did you check the bug report system to see if it's something known ?

It uses the raster engine. See here for historical informations.

The following solution worked:

#include "mainwindow.h" #include "ui_mainwindow.h" MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent), ui(new Ui::MainWindow) { ui->setupUi(this); } MainWindow::~MainWindow() { delete ui; } void MainWindow::paintEvent(QPaintEvent *event) { QPainter painter(this); QFont font = painter.font (); font.setFamily ("Arial"); font.setPixelSize (25); painter.setFont (font); painter.setPen (Qt::white); QRect rect(100, 120, 200, 50); painter.setBrush (Qt::blue); painter.drawRect (rect); painter.drawText (rect, Qt::AlignCenter, tr("Record deleted.")); painter.end(); QTimer::singleShot(3000,this,&MainWindow::hideRect); update(); }

Thank you for your help.

Yeah i feel like i do sudo apt-get update && upgrade at least once a day. I just checked about SIP and i compiled it from source but i cant remember if i actually used the command make and make install. Does that mean i have to reinstall pyqt5 again? I guess i just open a new thread. Thanks for all the help @SGaist

I have found a solution that kind of does the job, although takes three times as much memory.
Every update I calculate the difference between the new and the old saturation matrix, this eliminates 60% of the nonzero values, which means it gets drawn that much faster.

As a second layer of optimization I call repaint with a rectangle describing the most important zone, and call update every 60ms.

@joeQ
As per your suggesttion i'll make changes in the code on my own.Once again thanks for your help.

Not all paint devices (widgets, images, etc) support all composition modes. QImage does, but QWidget might not, which is what I suppose is happening here.

@VRonin Depends on what he wants to achieve.
Is it a widget, than I agree with you.
Is it some CAD-drawing (looks like a diagram symbol to me), then I would prefer drawing.
For the latter the Graphics View Framework would be more practical though.

Thanx, it really solved my problem

@pauledd
:)
class MyWidget : public QWidget << yes that is subclassing :)

Also, a widget can only paint on it self.
its not possible to paint on other widget from inside a
paintevent.
Thats why the first sample didnt work. You asked painter to paint on tab from inside mainwindows paintEvent :)
I guess you already found out but its just to make clear for other readers.

Hi,

You should check the bug report system to see if it's something known. If not please consider opening a new report providing your example and as many details as possible about your setup.

@mrjj
Yeah make sense.

Thanks!

OK, so now that I have launched the program with admin rights and came back with user session, the application is working (?!?)

So it was neither a DLL or plugins problem, but just a weird right access blocking. I am sad I can't explain why I had to launch it once with admin session, but at least I am glad I solved my problem.

Thanks anyway to those who give their time to help.

@Walux :-)

There are a couple of errors here and some weird unused code. Lets go over it:

setGeometry: Unable to set geometry (...) - this is just a warning. A widget has a 0 minimum size and minimum size hint by default. You used a layout on it so it has some margins (11 px in this case) Because of the frame of the window a window manager can't create a window that small so it resizes it to a smallest possible size and lets you know with a warning. To fix it either use setMinimumSize() or override minimumSizeHintfor your widget.

QPainter related messages - you're creating a painter as a class member variable. Don't do that. Create the painter locally in the paint event with a proper surface to use it on:

void DrawLine::paintEvent(QPaintEvent *) { QPainter p(this); //paint using p ...

There's no such thing as a lineEvent() - what is that?

What's the layout for? Are you gonna put something in it? If yes then remember that it will obscure whatever you paint in your paint event.

You're not deleting the dLine variable in main(). It's leaking. Either create it on the stack (which is the easiest and recommended), set a Qt::WA_DeleteOnClose attribute on it or delete it manually after the a.exec() call returns (pointless manual work).

just a hint - when overriding virtual methods use override specifier. It will save you a lot of typo related bugs:

void paintEvent(QPaintEvent *); //compiles fine and overrides the base implementation void pAintEvent(QPaintEvent *); //compiles fine and never gets called because of a typo void pAintEvent(QPaintEvent *) override; //doesn't compile and lets you know. yay!

@kevin32 Great :)

@AlvaroS
ok. seems a bit messy.
Do you really need a QGraphicsScene then ?
You could also draw it as a custom control. you need zooming etc ?

In any case, did u see this example
http://doc.qt.io/qt-5/qtwidgets-graphicsview-diagramscene-example.html

it even has arrowhead class :)

@kshegunov

Awesome! I'll give it a try!

Ok I found the answer. I added
setAttribute(Qt::WA_PaintOutsidePaintEvent, true);
flag to the widget I am repainting