How to draw smootly?

this code redraw two rectangles, every 10ms.
its looks ugly.
Attachment 11997
maybe need use some interpolation? if need, how to?

All project in attached file
Attachment 11996

maybe have better solution for fast drawing?

mainwindow.cpp

Code:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "qaudiolevel.h"
#include <QThread>
#include <QDebug>
 
QList<QAudioLevel*> audioLevels;
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->levelsLayout->setSpacing(0);
 
    for(int i=0;i<2;i++)
    {
        QAudioLevel *level = new QAudioLevel(ui->centralWidget);
        audioLevels.append(level);
        ui->levelsLayout->addWidget(level);
    }
    trs = new Threadl();
    connect(trs,SIGNAL(SetLevels()),this,SLOT(SetLevels()));
    trs->start();
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void MainWindow::SetLevels()
{
    float val1=((0.26-0.21)*((float)rand()/RAND_MAX))+0.21;
    float val2=((0.26-0.21)*((float)rand()/RAND_MAX))+0.21;
    qDebug() << val1 << val2;
    audioLevels.at(0)->setLevel(val1);
    audioLevels.at(1)->setLevel(val2);
}
 
void Threadl::run()
{
    while(true)
    {
        emit SetLevels();
        QThread::msleep(10);
    }
}

qpainter.cpp

Code:

#include "qaudiolevel.h"
#include <QPainter>
#include <QDebug>
 
QAudioLevel::QAudioLevel(QWidget *parent)
  : QWidget(parent)
  , m_level(0.0)
{
    setMinimumHeight(15);
    setMaximumHeight(180);
}
 
void QAudioLevel::setLevel(qreal level)
{
    if (m_level != level) {
        m_level = level;
        update();
    }
}
 
void QAudioLevel::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event);
 
    QPainter painter(this);
    qreal widthLevel = m_level * height();
    painter.fillRect(0,0, width(),height(), Qt::black);
    painter.fillRect(0,height()-widthLevel,50, height(), Qt::red);
 
 
 
 
}