Program doesn't draw anything, why?

[guaranito93]

Hi all,
I'm programming a a kind of "virtual tennis" game, but when I run it there isn't anything but the grey background. Can you please help understand why?

objects.cpp

@#include "objects.h"

Objects::Objects(QWidget *parent) :
QWidget(parent)
{
setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
//angoli campo da gioco
Border[0].setY(-height()/2);
Border[0].setX(-width()/2);
Border[1].setY(-height()/2);
Border[1].setX(+width()/2);
Border[2].setY(+height()/2);
Border[2].setX(+width()/2);
Border[3].setY(+height()/2);
Border[3].setX(-width()/2);

//dimensioni oggetti
WidthPad=13;
HeightPad=30;
sizeBall=10;

//coordinate Pad1
Pad1.setX(-width()+WidthPad);
Pad1.setY(-height()/2+HeightPad);

//coordinate Pad2
Pad2.setX(width()-WidthPad);
Pad2.setY(+height()/2-HeightPad);

//coordinate pallina
Ball.setX(width()/2);
Ball.setY(0);

}

QPolygon Objects::BuildRectangle(QPoint location, int Large, int Tall){
QPolygon Rectangle;
QPoint TopRight, BottomRight, BottomLeft;
TopRight.setX(location.x()+Large);
TopRight.setY(location.y());
BottomRight.setX(TopRight.x());
BottomRight.setY(TopRight.y()-Tall);
BottomLeft.setX(location.x());
BottomLeft.setY(BottomRight.y());
Rectangle << location;
Rectangle << TopRight;
Rectangle << BottomRight;
Rectangle << BottomLeft;
return Rectangle;
}

void Objects::SetAliasing(bool antialiasing){
this->antialiased = antialiasing;
update();
}

void Objects::PaintObjects(QPaintEvent *){
QPainter PaintIt(this);
QPainter PaintThem(this);
QPolygon BorderLine, Plat1, Plat2;

//Creazione oggetto grafico per coordinate del confine di giuoco
BorderLine << Border[0];
BorderLine << Border[1];
BorderLine << Border[2];
BorderLine << Border[3];

//????????????????????????????
PaintIt.setRenderHint(QPainter::Antialiasing,antialiased);
PaintThem.setRenderHint(QPainter::Antialiasing,antialiased);

//Creazione oggetti grafici Pad di giuoco
Plat1 = BuildRectangle(Pad1,WidthPad,HeightPad);
Plat2 = BuildRectangle(Pad2,WidthPad,HeightPad);

//Setting di disegno
PaintIt.setPen(Qt::black);
PaintThem.setPen((Qt::black));
PaintThem.setBrush(Qt::black);

//Translazione
PaintIt.translate(width(), height());
PaintThem.translate(width(), height());

//Muciaccia
PaintIt.drawPolygon(BorderLine);
PaintThem.drawPolygon(Plat1);
PaintThem.drawPolygon(Plat2);
PaintThem.drawEllipse(Ball,sizeBall,sizeBall);

}
@

virtualtennis.cpp

@#include "virtualtennis.h"

VirtualTennis::VirtualTennis(){
//vettori di spostamento della pallina;
VectorX=5;
VectorY=3;

//Oggetti del giuoco
mambo=new Objects;

//Creazione e settaggio del layout
QGridLayout *Layout = new QGridLayout;
Layout->addWidget(mambo);
setLayout(Layout);
setWindowTitle(tr("VIRTUAL TENNIS SINGLE PLAYER"));

//Creazione del timer;
QTimer *Clock = new QTimer(this);
Clock->start(500);
connect(Clock, SIGNAL(timeout()),this, SLOT(MoveDaBall(VectorX,VectorY)));

}
@

Thanks

[SGaist]

Hi,

You don't call start nor end on your QPainter. These are need to tell Qt that you are painting something and when you did end the painting

Hope it helps

[guaranito93]

Ehm... I didn't get it... I'm a newbie and I still need to learn a lot.

When I say:
@
mambo=new Objects;
QGridLayout *Layout = new QGridLayout;
Layout->addWidget(mambo);
setLayout(Layout);
@

It doesn't mean that I the program draw everything in the layout? If not how do I solve the problem?

[SGaist]

It will show the widget, but you must ensure that the painting is done correctly first.

I've re-read QPainter's documentation, begin/end are not mandatory the way you use QPainter. However, I have not seen implementation using two QPainter on the same device, so it might be a source of problem.

Since you are completely new, you should go step by step. As silly as it may sound, start by painting a rect, once you have it on screen properly. Add the different pieces gradually.

[Chris Kawa]

Few things here.

First - you should paint in
@void Objects::paintEvent(QPaintEvent *) override@
Qt doesn't know anything about
@void Objects::PaintObjects(QPaintEvent *)@
so unless you're not giving us the whole code this will never be called.

Second, as SGaist said, you should use only one painter on a paint device (in your case "this" widget). There's just no point in having two there.