I have been stuck on this issue for 2 days now. I'm using the Qt plugin for VisualStudio2013 on Window 7-64bit.

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

Qt Code: 
  1. #ifndef DISPLAYWIDGET_H
  2. 	#define DISPLAYWIDGET_H
  3.  
  4. 	#include <QtWidgets/QWidget>
  5. 	#include <qpixmap.h>
  6. 	#include <QGraphicsScene>
  7. 	#include <QGraphicsView>
  8. 	#include <QGraphicsPixmapItem>
  9. 	#include <QPushButton>
  10. 	#include <QHBoxLayout>
  11. 	#include <qtimer.h>
  12. 	#include <qlabel.h>
  13.  
  14.  
  15. 	#define FULLSCALE 255
  16. 	#define IM_X_MIN -5.0
  17. 	#define IM_X_MAX 5.0
  18. 	#define IM_Z_MIN 0.0
  19. 	#define IM_Z_MAX 15.0
  20. 	#define IM_PIXEL_WIDTH 200
  21. 	#define IM_PIXEL_HEIGHT IM_PIXEL_WIDTH * (IM_Z_MAX-IM_Z_MIN)/(IM_X_MAX - IM_X_MIN)
  22. 	#define BORDER_WIDTH	10
  23. 	#define RAND_SEED 7
  24.  
  25. 	class DisplayWidget : public QWidget
  26. 	{
  27. 		Q_OBJECT
  28.  
  29. 	public:
  30. 		DisplayWidget(int width, int height, QWidget *parent = 0);
  31. 		~DisplayWidget();
  32.  
  33. 	private:
  34. 		QLabel *bimageLabel;
  35. 		QLabel *dimageLabel;
  36. 		QImage * bImage;
  37. 		QImage * dImage;
  38. 		QTimer * frameGrab;
  39.  
  40. 		QPushButton * debugButton;
  41.  
  42. 		void CreateWidgets();
  43. 		void SetupGui();
  44.  
  45. 		int w, h;
  46.  
  47. 	public slots:
  48. 		void GenerateNewData();
  49. 	};
  50.  
  51. 	#endif // DISPLAYWIDGET_H
To copy to clipboard, switch view to plain text mode 

and the .cpp.

Qt Code: 
  1. #include "displaywidget.h"
  2.  
  3. 	DisplayWidget::DisplayWidget(int width, int height, QWidget *parent): QWidget(parent)
  4. 	{
  5. 		//ui.setupUi(this);
  6.  
  7. 		w = width;
  8. 		h = height;
  9.  
  10. 		CreateWidgets();
  11. 		SetupGui();
  12.  
  13. 		// seed the random number generator
  14. 		srand(RAND_SEED);
  15. 		GenerateNewData();
  16. 	}
  17.  
  18. 	DisplayWidget::~DisplayWidget()
  19. 	{
  20. 	}
  21. 	void DisplayWidget::CreateWidgets()
  22. 	{
  23. 		bImage = new QImage(w, h, QImage::Format_ARGB32);
  24. 		dImage = new QImage(w, h, QImage::Format_ARGB32);
  25. 		bimageLabel = new QLabel(this);
  26. 		dimageLabel = new QLabel(this);
  27.  
  28. 		debugButton = new QPushButton("DEBUG", this);
  29. 		bimageLabel->setStyleSheet("QLabel {background-color: black};");
  30. 		dimageLabel->setStyleSheet("QLabel {background-color: white};");
  31.  
  32. 		frameGrab = new QTimer(this);
  33. 	}
  34. 	void DisplayWidget::SetupGui()
  35. 	{
  36. 		QHBoxLayout * layout = new QHBoxLayout();
  37. 		setLayout(layout); // commenting this line out makes it refresh
  38. 		layout->addWidget(bimageLabel);
  39. 		layout->addWidget(dimageLabel);
  40. 		layout->addWidget(debugButton);
  41.  
  42. 		connect(frameGrab, SIGNAL(timeout()),this, SLOT(GenerateNewData()));
  43. 		connect(debugButton, SIGNAL(clicked()), this, SLOT(GenerateNewData()));
  44. 		frameGrab->start(50);
  45. 	}
  46. 	void DisplayWidget::GenerateNewData()
  47. 	{
  48. 		QRgb * bImageData = (QRgb *)bImage->scanLine(0);
  49. 		QRgb * dImageData = (QRgb *)dImage->scanLine(0);
  50.  
  51. 		for (int i; i < w * h; i++)
  52. 		{
  53. 			bImageData[i] = qRgba(rand() % FULLSCALE, 0, 0, FULLSCALE);
  54. 			dImageData[i] = qRgba(0, 0, rand() % FULLSCALE, FULLSCALE);
  55. 		}
  56.  
  57. 		bimageLabel->setPixmap(QPixmap::fromImage(*bImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
  58. 		dimageLabel->setPixmap(QPixmap::fromImage(*dImage).scaled(QSize(IM_PIXEL_WIDTH, IM_PIXEL_HEIGHT)));
  59.  
  60. 		//this->update(); // this breaks it again
  61. 	}
To copy to clipboard, switch view to plain text mode 

Losing my mind here. I have very limited experience with Qt, but I believe I have the right approach.

Please help!