Qt – Basic Graphics Drawing

1. Basic graphics drawing

A. The key role in the Qt graphics system
QPainter–The painter in Qt can draw various foundations Graphics, with brushes, brushes, and fonts needed for drawing
QPaintDevice–Canvas in Qt, painter’s drawing board, all QWidget classes inherit from QPaintDevice
The key role in the Qt graphics system
< br>Tools used in Qt
1.QPen-Used to draw the edges of geometric figures, composed of parameters such as color, width, line style, etc.
2. QBrush-a palette used to fill geometric figures, composed of colors and filling styles
3.QFont-used for text drawing, composed of font attributes

Note that-only in Drawing graphics in QWidget::paintEvent

B. Coordinate system in Qt graphics system
1. Physical coordinate system-the position of the origin (0,0) in the upper left corner, the unit is pixels and the X coordinate increases to the right, and the Y coordinate goes down Growth
2. Logical coordinate system–the abstract coordinate system in the mathematical model. The unit is determined by the specific problem, and the growth direction of the coordinate axis is determined by the specific problem.
What we need to pay attention to is that QPainter uses the logical coordinate system to draw graphics. The size and position of the graphics in the logical coordinate system are converted and drawn on the specific device, and the logical coordinates by default are exactly the same as the physical coordinate system.
C. Viewport and window
1. Viewport-a arbitrarily specified rectangle in the physical coordinate system
2. Window-the logical coordinate system corresponds to the same rectangle in the physical coordinate system
Understand the view Port and window
1. The viewport and the window are the same rectangle in different coordinate systems
2. There is a one-to-one mapping relationship between the viewport and the coordinate points in the window
3. Port and window can be converted mutually through coordinate conversion

The transformation method of viewport and window
1. Define the viewport-the coordinates of the upper left corner, the coordinates of the lower right corner, calculate the width and height
2. Define the window-the coordinates of the upper left corner, the coordinates of the lower right corner, calculate the width and height
Code example

Widget.cpp
#include "Widget.h"
#include 
#include 
#include 
 #include 

Widget::Widget(QWidget *parent)
: QWidget(parent)
{
}

void Widget::paintEvent(QPaintEvent *)
{
 //Basic settings of drawing tools
 QPainter painter(this);
 QPen pen;

 pen.setColor(Qt::green);
 pen.setStyle(Qt::SolidLine);
 pen.setWidthF(0.01);

 painter.setPen(pen) ;

 painter.setVie wport(50, 50, width()-100, height()-100);//View
 painter.setWindow(-10, 2, 20, -4); //Window

 painter.fillRect(-10, 2, 20, -4, Qt::black);

 painter.drawLine(QPointF(-10, 0), QPointF(10, 0)); // x
 painter.drawLine(QPointF(0, 2), QPointF(0, -2)); // y

 for(float x=-10; x<10; x+=0.01)
 {
 float y = qSin(x);

 painter.drawPoint(QPointF(x, y));
 }
 
}

Widget::~Widget()
{

}