I am trying to make a class that will make a dot graphic. The class inherits from QWidget. I want it to draw lines and dots on a QPixmap that will be displayed in a QLabel.
The constructor of the class looks like this:
MyClass::MyClass()
{
calcul_proprietes(); // Function that makes calculation of what to draw.
pix = new QPixmap(760,350);
dessiner_graphique(); // Function that does the drawing.
//Displaying the qpixmap
layout_principal = new QVBoxLayout(this);
label_pix = new QLabel(this);
label_pix->setPixmap(*pix);
layout_principal->addWidget(label_pix);
this->setLayout(layout_principal);
}
And a short part of the function that does the drawing
void MyClass::dessiner_graphique()
{
// ...
QPainter painter(pix);
QRect contour(x_depart,y_depart,largeur_grille,hauteur_grille);
painter.drawRect(contour);
// ...
}
I don't want to use the paintEvent function because it gets called all the time and i only need my graphic to be painted once. What do i do wrong?
Did you call the default base class constructor before Your class constructor ?
MyClass( QObject *parent )
: QWidget( parent )
{
// Your posted code.
}
Related
What would the syntax look like if I wanted to even use QGraphicsView::scene() in the first place? My goal is to change the scale of a pixmap in my graphics view object with sliders. I want to use QgraphicsView::scene() and QScene::itemsAt() etc. to find the original pixmap and then use QPixmap::scaled() (this I found would be the only way to ensure my cosmetic property set for the pixmap holds true). However I am having issues with the syntax of QGraphicsView::scene(). My attempt is below. I am also creating a Qt widget application for context.
QGraphicsViewScene graphicsScene = ui->PixmapView->scene();
QGraphicsPixmapItem graphicsPixmapItem = graphicsScene.itemAt(0, 0);
edit
If I was to store my QPixmap pixmap* as a member variable I am not entirely sure how to implement that where it remains in scope for my slots.
edit
static member variables?
You can make your QGraphicsPixmapItem object a member variable of your class. Then you would be able to access it from any of your class member functions.
Here is a quick example:
class MyClass : public QWidget
{
Q_OBJECT
public:
MyClass(QWidget *parent = nullptr) : QWidget(parent)
{
// create graphics view, scene, etc..
}
public slots:
void openActionTriggered()
{
...
myItem = scene->addPixmap(myPixmap); // you can create your item however you want.. this is just an example
}
void mySlot()
{
if(myItem)
{
// do something with myItem
}
}
private:
QGraphicsPixmapItem *myItem = nullptr; // myItem is a member variable of
QGraphicsScene *scene = nullptr; // I made scene a member variable so it can be accessed from any member functions
}
I'm having a question about a weird (At least it was unexpected for me) behavior (It crashes) of qt when initializing pointers on a member class different than the constructor. I am attaching part of my code:
In mainwindow.h:
class MainWindow : public QMainWindow
{
...
private:
QPixmap *qpm_s1_yaw;
QPainter *s1_yaw_painter;
...
}
In mainwindow.cpp:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
...
initGraph(qpm_s1_yaw, s1_yaw_painter, ui->YAW1);
...
}
void MainWindow::initGraph(QPixmap *map, QPainter *painter, QLabel *label)
{
map = new QPixmap(label->size());
map->fill(Qt::white);
painter = new QPainter(map);
... doing some stuff ...
label->setPixmap(*map); // ++(Remember this LINE)++
}
That actually works, but when I comment the line:
label->setPixmap(*map)
and instead set the Pixmap in the constructor (MainWindow::MainWindow) by writing:
ui->YAW1->setPixmap(*qpm_s1_yaw)
I got a segmentation Fault.
Could someone explain what is wrong with it? To make it work I had to initialize all the pointers in the constructor (and commenting those line in the classs member initGraph), like this:
qpm_s1_yaw = new QPixmap(ui->YAW1->size());
s1_yaw_painter = new QPainter(qpm_s1_yaw);
initGraph(qpm_s1_yaw, s1_yaw_painter, ui->YAW1);
ui->YAW1->setPixmap(*qpm_s1_yaw);
Thanks
This is a trivial misunderstanding of how C++ works, nothing to do with Qt.
Your code lies to you: you can equally well write: initGraph(0, 0, ui->YAW1). You're initializing local variables instead of class members. The values you pass as the first two arguments are not used for anything.
It's also completely unnecessary to hold the pixmap and the painter by pointer. Hold the pixmap by value, and only instantiate a painter for it when you do the painting.
Holding a painter to a pixmap when you're not painting on it can cause unnecessary copies of the pixmap to be made when the pixmap is consumed (read from): a pixmap with an active painter is considered "dirty".
What you should do then is to hold pixmaps by value and you can return the new value from initGraph - this decouples initGraph from the detail of the surrounding class where the pixmaps are stored. The user of initGraph has an option of not storing the pixmap, and e.g. querying the label itself for it.
class MainWindow : public QMainWindow
{
Ui::MainWindow ui; // hold by value
...
QPixmap qpm_s1_yaw; // hold by value
QPixmap initGraph(QLabel *label) {
QPixmap pixmap{label->size()};
pixmap.fill(Qt::white);
QPainter painter{&pixmap};
//... doing some stuff ...
label->setPixmap(pixmap);
return pixmap;
}
public:
explicit MainWindow(QWidget *parent = nullptr) : QMainWindow(parent) {
ui.setupUi(this);
gpm_s1_yaw = initGraph(ui.YAW1);
}
};
I'm a Chinese and poor in English,so I show the code to express myself.
class Widget:public QWidget
{
private:
//As a child object of Widget object
QLabel* label;
};
// Override the paintEvent
void Widget::paintEvent(QPaintEvent* event)
{
//How to draw a picture in the QLabel not in the full Widget's view,thx!!
}
First, you need to initialize your label in constructor, add something like this:
void Widget::Widget()
{
label=new QLabel(this);
}
and then your paintEvent do something like this -
void Widget::paintEvent(QPaintEvent* event)
{
// create QPixmap or QImage object or how else you store it and load it to label
QImage img("./myimage.jpg");
label->setPixmap(img.pixmap());
}
and it will be drawn on tha label, but by default label has growing sizePolicy - it will be resized to fill as many space as possibly if your widget doesn't have any other objects - your label will take the whole space, so it will be good to add your QLabel to some of the layouts (QGridLayout) and you need to setSizePolicy of your QLabel to QSizePolicy::Maximum
In Qt Creator, I have a couple of widgets declared like so:
Header File:
class MapViewer : public QGraphicsView
{
Q_OBJECT
public:
explicit MapViewer(QGraphicsScene *scene, QWidget *parent = 0);
~MapViewer();
public slots:
void mousePressEvent(QMouseEvent *event);
};
// Declaration for the map editor window.
class MapEditor : public QMainWindow
{
Q_OBJECT
public:
explicit MapEditor(QWidget *parent = 0);
~MapEditor();
public:
QLayout *editorLayout;
QPushButton *btn;
QGraphicsScene *mapScene;
MapViewer *mapView;
private:
Ui::MapEditor *ui;
};
CPP File:
MapEditor::MapEditor(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MapEditor)
{
ui->setupUi(this);
this->setWindowTitle("2DXY :: Map Editor");
this->setGeometry(10,10,1170,750);
editorLayout = new QVBoxLayout; // Create a new layout
this->setLayout(editorLayout); // Set the widget's layout to our newly created layout.
mapScene = new QGraphicsScene(); // Create a new graphics scene to draw upon.
mapView = new MapViewer(mapScene,this); // Create a new graphics view to display our scene - set its parent to 'this' so that it doesn't open in a new window.
mapView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
mapView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
mapView->setGeometry(20,20,1178,546); // Width first, then height.
AND:
void MapViewer::mousePressEvent(QMouseEvent *event)
{
// Show an empty message box, just to check that the event handler works!
QMessageBox *notification = new QMessageBox();
notification->show();
notification->exec();
// Some how access the same QGraphicsScene and View (mapScene, mapView) as above, so
// I can update their contents on the open form / window.
}
And as you can see, I wish to access the Graphics Scene again to update it, then redraw it (or whatever). But I'm not able to access the graphics scene at all, despite a few hours of trial and error with declaring widgets in different ways.
I know that the listener itself works, because if it's set to open a new message box, or output to the debug window, then it works, it's just that I can't access the widgets I've already defined.
I feel that there is a (relatively) easy solution to this problem, and that I'm just missing something obvious.
You passed the QGraphicsScene to your MapRender object's constructor. What do you do with the scene within its constructor? Ideally, you should be storing it as a data member of MapRender. For example:
class MapRender {
public:
MapRender(QGraphicsScene* scene)
: scene_(scene)
{
}
public slots:
void mousePressEvent(QMouseEvent *event);
private:
QGraphicsScene* scene_;
}
Now in your implementation of mousePressEvent, you can access to the scene member:
void MapRender::mousePressEvent(QMouseEvent *event) {
int CursorX = event->globalX();
int CursorY = event->globalY();
QGraphicsRectItem *clickedBox = new QGraphicsRectItem(40,40,32,32);
clickedBox->setBrush(QBrush(Qt::blue));
scene_->addItem(clickedBox);
}
Keep in mind you should ideally be putting the implementation of the constructor in your cpp file, but my example does it in the declaration for brevity.
void MapViewer::mousePressEvent(QMouseEvent *event)
{
// Show an empty message box, just to check that the event handler works!
QMessageBox *notification = new QMessageBox();
notification->show();
notification->exec();
// To add something whenever the user clicks, you don't need the view,
// just the scene.
scene()->addItem( new MyItem() );
}
Remember MapViewer derives from QGraphicsView and the view must know about the scene it belongs to - so it has a scene() method to return it, which you inherited.
I'm trying to write a QML plugin that reads frames from a video (using a custom widget to do that task, NOT QtMultimedia/Phonon), and each frame is converted to a QImage RGB888, and then displayed on a QGLWidget (for performance reasons). Right now nothing is draw to the screen and the screen stays white all the time.
It's important to state that I already have all of this working without QGLWidget, so I know the issue is setting up and drawing on QGLWidget.
The plugin is being registered with:
qmlRegisterType<Video>(uri,1,0,"Video");
so Video is the main class of the plugin. On it's constructor we have:
Video::Video(QDeclarativeItem* parent)
: QDeclarativeItem(parent), d_ptr(new VideoPrivate(this))
{
setFlag(QGraphicsItem::ItemHasNoContents, false);
Q_D(Video);
QDeclarativeView* view = new QDeclarativeView;
view->setViewport(&d->canvas()); // canvas() returns a reference to my custom OpenGL Widget
}
Before I jump to the canvas object, let me say that I overloaded Video::paint() so it calls canvas.paint() while passing QImage as parameter, I don't know if this is the right way to do it so I would like some advice on this:
void Video::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget)
{
Q_UNUSED(painter);
Q_UNUSED(widget);
Q_UNUSED(option);
Q_D(Video);
// I know for sure at this point "d->image()" is valid, but I'm hiding the code for clarity
d->canvas().paint(painter, option, d->image());
}
The canvas object is declared as GLWidget canvas; and the header of this class is defined as:
class GLWidget : public QGLWidget
{
Q_OBJECT
public:
explicit GLWidget(QWidget* parent = NULL);
~GLWidget();
void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QImage* image);
};
Seems pretty simple. Now, the implementation of QGLWidget is the following:
GLWidget::GLWidget(QWidget* parent)
: QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
{
// Should I do something here?
// Maybe setAutoFillBackground(false); ???
}
GLWidget::~GLWidget()
{
}
And finally:
void GLWidget::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QImage* image)
{
// I ignore painter because it comes from Video, so I create a new one:
QPainter gl_painter(this);
// Perform drawing as Qt::KeepAspectRatio
gl_painter.fillRect(QRectF(QPoint(0, 0), QSize(this->width(), this->height())), Qt::black);
QImage scaled_img = image->scaled(QSize(this->width(), this->height()), _ar, Qt::FastTransformation);
gl_painter.drawImage(qRound(this->width()/2) - qRound(scaled_img.size().width()/2),
qRound(this->height()/2) - qRound(scaled_img.size().height()/2),
scaled_img);
}
What am I missing?
I originally asked this question on Qt Forum but got no replies.
Solved. The problem was that I was trying to create a new GL context within my plugin when I should be retrieving the GL context from the application that loaded it.
This code was very helpful to understand how to accomplish that.
By the way, I discovered that the stuff was being draw inside view. It's just that I needed to execute view->show(), but that created another window which was not what I was looking for. The link I shared above has the answer.
I think that you have to draw on your glwidget using the opengl functions.
One possible way is to override your paintGL() method in GLWidget
and then draw your image with glDrawPixels().
glClear(GL_COLOR_BUFFER_BIT);
glDrawPixels(buffer.width(), buffer.height(), GL_RGBA, GL_UNSIGNED_BYTE, buffer.bits());
Where buffer is a QImage object that needs to be converted using QGLWidget::converrtToGLFormat() static method.
Look at this source for reference:
https://github.com/nayyden/ZVector/blob/master/src/GLDebugBufferWidget.cpp