c++ - Qt rotating shapes -

i'm experimenting drawing , rotating shapes in qt, i'm @ loss how works. have code draws rectangle small triangle on top of it. want rotate shape 35 degrees, 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 s.getx/y() calls, x 150 , y 750.)

without rotating , translating code works fine , draws shape. current code rectangle displayed, not polygon. how rotate these shapes?

you need understand how affine transformations work. without proper understanding have hard times achieving needed.

• rotate rotates around center of coordinates: (0,0)
• translate moves center of coordinates new position

your code rotates around point (s.getx() - 5, s.gety() - 8).

so here's code rotate both shapes 35 degrees around 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 center of red rectangle painter.rotate(35); // rotate around center of red rectangle painter.translate(-5, -8); // move center of coordinates  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: