this code redraw two rectangles, every 10ms.
its looks ugly.
ezgif-2422137665.gif
maybe need use some interpolation? if need, how to?

All project in attached file
TestQPainterSlowdrawing.zip


maybe have better solution for fast drawing?


mainwindow.cpp

Qt Code: 
  1. #include "mainwindow.h"
  2. #include "ui_mainwindow.h"
  3. #include "qaudiolevel.h"
  4. #include <QThread>
  5. #include <QDebug>
  6.  
  7. QList<QAudioLevel*> audioLevels;
  8.  
  9. MainWindow::MainWindow(QWidget *parent) :
  10.     QMainWindow(parent),
  11.     ui(new Ui::MainWindow)
  12. {
  13.     ui->setupUi(this);
  14.     ui->levelsLayout->setSpacing(0);
  15.  
  16.     for(int i=0;i<2;i++)
  17.     {
  18.         QAudioLevel *level = new QAudioLevel(ui->centralWidget);
  19.         audioLevels.append(level);
  20.         ui->levelsLayout->addWidget(level);
  21.     }
  22.     trs = new Threadl();
  23.     connect(trs,SIGNAL(SetLevels()),this,SLOT(SetLevels()));
  24.     trs->start();
  25.  
  26. }
  27.  
  28. MainWindow::~MainWindow()
  29. {
  30.     delete ui;
  31. }
  32.  
  33. void MainWindow::SetLevels()
  34. {
  35.     float val1=((0.26-0.21)*((float)rand()/RAND_MAX))+0.21;
  36.     float val2=((0.26-0.21)*((float)rand()/RAND_MAX))+0.21;
  37.     qDebug() << val1 << val2;
  38.     audioLevels.at(0)->setLevel(val1);
  39.     audioLevels.at(1)->setLevel(val2);
  40. }
  41.  
  42. void Threadl::run()
  43. {
  44.     while(true)
  45.     {
  46.         emit SetLevels();
  47.         QThread::msleep(10);
  48.     }
  49. }
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qpainter.cpp
Qt Code: 
  1. #include "qaudiolevel.h"
  2. #include <QPainter>
  3. #include <QDebug>
  4.  
  5. QAudioLevel::QAudioLevel(QWidget *parent)
  6.   : QWidget(parent)
  7.   , m_level(0.0)
  8. {
  9.     setMinimumHeight(15);
  10.     setMaximumHeight(180);
  11. }
  12.  
  13. void QAudioLevel::setLevel(qreal level)
  14. {
  15.     if (m_level != level) {
  16.         m_level = level;
  17.         update();
  18.     }
  19. }
  20.  
  21. void QAudioLevel::paintEvent(QPaintEvent *event)
  22. {
  23.     Q_UNUSED(event);
  24.  
  25.     QPainter painter(this);
  26.     qreal widthLevel = m_level * height();
  27.     painter.fillRect(0,0, width(),height(), Qt::black);
  28.     painter.fillRect(0,height()-widthLevel,50, height(), Qt::red);
  29.  
  30.  
  31.  
  32.  
  33. }
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