How to make QGraphicsView update after changing a QGraphicsPixmapItem - c++

Question

I'm quite a n00b with Qt, but I have seen a lot of posts where people can't get a QGraphicsView to update as expected. I am trying to make very simple widget that will display two arrays of data as images overlapped with some alpha blending. From what I gather the best way to do this is to create a QImage to hold the image data, and each time I want to update the overlay display, you convert the QImage into a QPixmap, and then use that to update the pixmap of a QGraphicsPixmapItem (which is in a QGraphicsScene, which is in a QGraphicsView).
As a minimum working example, I have it setup to generate a random red image and random green image, and with random blending between them. It's setup on a timer to generate new data and, ideally, update the view. No matter how many updates/repaints I drop around, I can't seem to get it to update properly. The timer seems to be working, and the random data generation seems to be working as well, but the scene only updates if I physically change the size of the window.
Here is my code, in a few blocks. First DisplayWidget.h
#include <QtWidgets/QWidget>
#include "ui_displaywidget.h"
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsPixmapItem>
#include <QPushButton>
#include "math.h"
#include <QVBoxLayout>
#include <qsize.h>
#include <qtimer.h>
#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();
void SetData(float * data, float minVal, float maxVal);
void SetTransparency(float * alpha, float minVal, float maxVal);
private:
//Ui::DisplayWidgetClass ui;
QGraphicsView * view;
QGraphicsScene * scene;
QGraphicsPixmapItem * bModeItem;
QGraphicsPixmapItem * dModeItem;
QImage * bImage;
QImage * dImage;
QTimer * frameGrab;
void CreateWidgets();
void SetupGui();
int w, h;
public slots:
void GenerateNewData();
};
And here's the relevant parts of my .cpp. GenerateNewData() is the function that doesn't seem to produce updates to the scene. This is where I have tried view/scene/item updates/repaints.
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();
}
void DisplayWidget::CreateWidgets()
{
view = new QGraphicsView(this);
scene = new QGraphicsScene(this);
bModeItem = scene->addPixmap(QPixmap());
dModeItem = scene->addPixmap(QPixmap());
bImage = new QImage(w, h, QImage::Format_ARGB32);
dImage = new QImage(w, h, QImage::Format_ARGB32);
frameGrab = new QTimer(this);
}
void DisplayWidget::SetupGui()
{
QVBoxLayout * layout = new QVBoxLayout(this);
layout->addWidget(view);
setLayout(layout);
scene->setSceneRect(0, 0, IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT);
view->setGeometry(0, 0, IM_PIXEL_WIDTH + 2*BORDER_WIDTH, IM_PIXEL_HEIGHT + 2*BORDER_WIDTH);
view->setScene(scene);
bModeItem->setPixmap(QPixmap::fromImage(*bImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
dModeItem->setPixmap(QPixmap::fromImage(*dImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
connect(frameGrab, SIGNAL(timeout()),this, SLOT(GenerateNewData()));
frameGrab->start(500);
}
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, 255,0, rand() % FULLSCALE);
}
bModeItem->setPixmap(QPixmap::fromImage(*bImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
dModeItem->setPixmap(QPixmap::fromImage(*dImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
}
And here's my main.
#include "displaywidget.h"
#include <QtWidgets/QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
DisplayWidget w(25,25);
w.show();
return a.exec();
}
I would appreciate any help!