Can't refesh pixmap of QLabel - c++

Question

I have been stuck on this issue for 2 days now. I'm using the Qt plugin for Visual Studio 2013 on Window 7-64 bit.
I have been trying to display a pair of images in QLabels. I need to manipulate the pixel data regularly, so I store them in QImages, and every time I want to refresh the display I set the QPixmap of a QLabel. The problem is, it only seems to refresh if I change/move the window in some way.
This problems goes away if I just make the QLabels children of my QWidget, but never set a layout. If I then add repaint() or update(), the problem comes back.
(this is a very similar post to one I posted using QGraphicsScene, but the problem seems to be more fundamental than that, so I am reposting)
Here is my code. First the .h
#ifndef DISPLAYWIDGET_H
#define DISPLAYWIDGET_H
#include <QtWidgets/QWidget>
#include <QPixmap>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsPixmapItem>
#include <QPushButton>
#include <QHBoxLayout>
#include <QTimer>
#include <QLabel>
#define FULLSCALE 255
#define IM_X_MIN -5.0
#define IM_X_MAX 5.0
#define IM_Z_MIN 0.0
#define IM_Z_MAX 15.0
#define IM_PIXEL_WIDTH 200
#define IM_PIXEL_HEIGHT IM_PIXEL_WIDTH * (IM_Z_MAX-IM_Z_MIN)/(IM_X_MAX - IM_X_MIN)
#define BORDER_WIDTH 10
#define RAND_SEED 7
class DisplayWidget : public QWidget
{
Q_OBJECT
public:
DisplayWidget(int width, int height, QWidget *parent = 0);
~DisplayWidget();
private:
QLabel* bimageLabel;
QLabel* dimageLabel;
QImage* bImage;
QImage* dImage;
QTimer* frameGrab;
QPushButton* debugButton;
void CreateWidgets();
void SetupGui();
int w, h;
public slots:
void GenerateNewData();
};
#endif // DISPLAYWIDGET_H
and the .cpp.
#include "displaywidget.h"
DisplayWidget::DisplayWidget(int width, int height, QWidget *parent): QWidget(parent)
{
//ui.setupUi(this);
w = width;
h = height;
CreateWidgets();
SetupGui();
// seed the random number generator
srand(RAND_SEED);
GenerateNewData();
}
DisplayWidget::~DisplayWidget()
{
}
void DisplayWidget::CreateWidgets()
{
bImage = new QImage(w, h, QImage::Format_ARGB32);
dImage = new QImage(w, h, QImage::Format_ARGB32);
bimageLabel = new QLabel(this);
dimageLabel = new QLabel(this);
debugButton = new QPushButton("DEBUG", this);
bimageLabel->setStyleSheet("QLabel {background-color: black};");
dimageLabel->setStyleSheet("QLabel {background-color: white};");
frameGrab = new QTimer(this);
}
void DisplayWidget::SetupGui()
{
QHBoxLayout * layout = new QHBoxLayout();
setLayout(layout); // commenting this line out makes it refresh
layout->addWidget(bimageLabel);
layout->addWidget(dimageLabel);
layout->addWidget(debugButton);
connect(frameGrab, SIGNAL(timeout()),this, SLOT(GenerateNewData()));
connect(debugButton, SIGNAL(clicked()), this, SLOT(GenerateNewData()));
frameGrab->start(50);
}
void DisplayWidget::GenerateNewData()
{
QRgb * bImageData = (QRgb *)bImage->scanLine(0);
QRgb * dImageData = (QRgb *)dImage->scanLine(0);
for (int i; i < w * h; i++)
{
bImageData[i] = qRgba(rand() % FULLSCALE, 0, 0, FULLSCALE);
dImageData[i] = qRgba(0, 0, rand() % FULLSCALE, FULLSCALE);
}
bimageLabel->setPixmap(QPixmap::fromImage(*bImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
dimageLabel->setPixmap(QPixmap::fromImage(*dImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
//this->update(); // this breaks it again
}
Losing my mind here. I have very limited experience with Qt, but I believe I have the right approach.
Please help!

Answer 1

I've tested your code on Ubuntu. I can unfortunately not comment on windows. I've modified the code slightly (See comments marked by //#w:):
#include "displaywidget.h"
DisplayWidget::DisplayWidget(int width, int height, QWidget *parent): QWidget(parent)
{
w = width;
h = height;
CreateWidgets();
SetupGui();
// seed the random number generator
srand(RAND_SEED);
GenerateNewData();
}
DisplayWidget::~DisplayWidget()
{
}
void DisplayWidget::CreateWidgets()
{
bImage = new QImage(w, h, QImage::Format_ARGB32);
dImage = new QImage(w, h, QImage::Format_ARGB32);
bimageLabel = new QLabel(this);
bimageLabel->setScaledContents(true);
dimageLabel = new QLabel(this);
dimageLabel->setScaledContents(true);
debugButton = new QPushButton("DEBUG", this);
bimageLabel->setStyleSheet("QLabel {background-color: black};");
dimageLabel->setStyleSheet("QLabel {background-color: grey};");
frameGrab = new QTimer(this);
}
void DisplayWidget::SetupGui()
{
//#w: Adding vertical layout as button below seems like better usage of space...
QVBoxLayout * vlay = new QVBoxLayout();
QHBoxLayout * hlay = new QHBoxLayout();
hlay->addWidget(bimageLabel);
hlay->addWidget(dimageLabel);
vlay->addLayout(hlay);
vlay->addWidget(debugButton);
//#w: Removing size constraints on top layout allows me to resize window and see effect.
vlay->setSizeConstraint(QLayout::SetNoConstraint);
setLayout(vlay); // commenting this line out makes it refresh
connect(frameGrab, SIGNAL(timeout()),this, SLOT(GenerateNewData()));
//#w: I suppose we can chuck the button.... Currently it serves no purpose
connect(debugButton, SIGNAL(clicked()), this, SLOT(GenerateNewData()));
//#w: Timer slower initially, then increase to see where performance degrades.
frameGrab->start(200);
}
void DisplayWidget::GenerateNewData()
{
QRgb * bImageData = (QRgb *)bImage->scanLine(0);
QRgb * dImageData = (QRgb *)dImage->scanLine(0);
//#w: This code just varies the contents by having a b and d selector that
// alternates colour... Simple stuff...
int bSelect = rand() % 3,
dSelect = (bSelect==2) ? 0 : bSelect+1;
for (int i = 0; i < w * h; i++)
{
//#w: 3 colours, only two being selected - can be improved, I suppose.
QRgb rgb[3] =
{
(bSelect == 0) || (dSelect==0) ? qRgba(rand() % FULLSCALE, 0, 0, FULLSCALE) : 0,
(bSelect == 1) || (dSelect==1) ? qRgba(0, rand() % FULLSCALE, 0, FULLSCALE) : 0,
(bSelect == 2) || (dSelect==2) ? qRgba(0, 0, rand() % FULLSCALE, FULLSCALE) : 0,
};
bImageData[i] = rgb[bSelect];
dImageData[i] = rgb[dSelect];
}
//#w: Removed scaling, as it depends on layout...
bimageLabel->setPixmap(QPixmap::fromImage(*bImage));
dimageLabel->setPixmap(QPixmap::fromImage(*dImage));
}
Initially the labels take its size from the pixmaps. Thereafter the labels adhere to the layout, which adheres to resizing of the form (main/parent widget)

Answer 2

QtForum solved this one for me, and boy is it embarassing.
I forgot to initialize the loop counter. Fixed everything.