using Qt to paint a parabola or any other polynomial - c++

Question

Is there a way to use Qt library to paint parabola or any other polynomial?
I tried using QPainter but there is no such option there.
Thank you

Answer 1

If you want to use QPainter then you should to use an QImage or QPixmap to display or save your output.
This is a simple code to show how it's done in a Qt Widgets Application.
This will be the MainWindow.h file.
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QImage>
#include <QPainter>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
void drawMyFunction(qreal xMin,qreal xMax);
qreal myFunction(qreal x);
QImage * pic;
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
and this will be your MainWindow.cpp:
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
drawMyFunction(-3,3);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::drawMyFunction(qreal xMin, qreal xMax)
{
qreal step = (xMax - xMin) / 1000;
QPainterPath painterPath;
QSize picSize(300,300);
for(qreal x = xMin ; x <= xMax ; x = x + step)
{
if(x == xMin)
{
painterPath.moveTo(x,myFunction(x));
}
else
{
painterPath.lineTo(x,myFunction(x));
}
}
// Scaling and centering in the center of image
qreal xScaling = picSize.width() / painterPath.boundingRect().width();
qreal yScaling = picSize.height() / painterPath.boundingRect().height();
qreal scale;
if(xScaling > yScaling)
scale = yScaling;
else
scale = xScaling;
for(int i = 0 ; i < painterPath.elementCount() ; i++ )
{
painterPath.setElementPositionAt(i,painterPath.elementAt(i).x*scale + picSize.width()/2,-painterPath.elementAt(i).y*scale + picSize.height()/2);
}
// Drawing to the image
pic = new QImage(picSize,QImage::Format_RGB32);
pic->fill(Qt::white);
QPainter picPainter(pic);
QPen myPen;
myPen.setColor(Qt::black);
myPen.setWidth(10);
picPainter.drawPath(painterPath);
ui->label->setPixmap(QPixmap::fromImage(*pic)); // label is an added label to the ui
// you can also do this instead of just showing the picture
// pic->save("myImage.bmp");
}
qreal MainWindow::myFunction(qreal x)
{
return x*x; // write any function you want here
}

Answer 2

Have you tried QPainterPath? It has QPainterPath::quadTo and QPainterPath::cubicTo member functions that could be of help.

Answer 3

compute discrete points from your polynomial function and use QPainter::drawLines to draw the figure.
For instance, y = x^2:
float xmin = 0;
float xmax = 2;
float step = 0.1; // experiment with values
QVector<QPointF> points;
float x = xmin;
while(x < xmax)
{
float y = x^x; //f(x)
lines.push_back(QPointF(x,y));
x+= step;
}
painter.drawLines(points);
Because the top corner in qt coordonates is (0,0) You need to make a geometric translation for x and y after computing y.