Qt rotating shapes - c++

Question

I'm experimenting with drawing and rotating shapes in Qt, but I'm really at a loss as to how it works. Currently I have code which draws a rectangle with a small triangle on top of it. I want to rotate the shape by 35 degrees, so I try this:
void Window::paintEvent(QPaintEvent *event)
{
QPainter painter(this);
QBrush brush;
brush.setStyle(Qt::SolidPattern);
brush.setColor(Qt::white);
painter.setBrush(brush);
painter.setPen(Qt::NoPen);
painter.fillRect(0,0,800,800,brush);
brush.setColor(Qt::red);
painter.translate(s.getX()-5,s.getY()-8);
painter.rotate(35);
painter.fillRect(0,0,10,16,brush);
QPolygon pol;
pol.setPoints(3,0,0,10,0,5,10);
QPainterPath p;
p.addPolygon(pol);
painter.fillPath(p,brush);
}
(Ignore the s.getX/Y() calls, for now x is 150 and y is 750.)
Without the rotating and translating the code works fine and draws the shape. With the current code only the rectangle is displayed, not the polygon. How do I rotate these shapes?

Answer 1

You need to properly understand how affine transformations work. Without the proper understanding you will have hard times achieving what is needed.
rotate rotates everything around center of coordinates: (0,0)
translate moves center of coordinates to a new position
Your code rotates everything around the point (s.getX() - 5, s.getY() - 8).
So here's the code that will rotate both shapes 35 degrees around the center of red rectangle:
QPainter painter(this);
QBrush brush;
brush.setStyle(Qt::SolidPattern);
brush.setColor(Qt::white);
painter.setBrush(brush);
painter.setPen(Qt::NoPen);
painter.fillRect(0, 0, 800, 800, brush);
brush.setColor(Qt::red);
painter.translate(150, 750);
painter.translate(5, 8); // move center of coordinates to the center of red rectangle
painter.rotate(35); // rotate around the center of red rectangle
painter.translate(-5, -8); // move center of coordinates back to where it was
painter.fillRect(0, 0, 10, 16, brush);
QPolygon pol;
pol.setPoints(3, 0, 0, 10, 0, 5, 10);
QPainterPath p;
p.addPolygon(pol);
brush.setColor(Qt::blue);
painter.fillPath(p, brush);
Without transformations:
With transformations: