c++ Qt QGraphicsItemAnimation model of thrown ball - c++

Question

I have problem with the animation of a ball, which flies according to the equations of motion
x = speed*cos(angle) * time;
y = speed*sin(angle) * time - (g*pow(time,2)) / 2;
I create a QGraphicsScene with QGraphicsEllipseItem
QGraphicsScenescene = new QGraphicsScene;
QGraphicsEllipseItemball = new QGraphicsEllipseItem(0,scene);
then I try to animate ball
scene->setSceneRect( 0.0, 0.0, 640.0, 480.0 );
ball->setRect(15,450,2*RADIUS,2*RADIUS);
setScene(scene);
QTimeLine *timer = new QTimeLine(5000);
timer->setFrameRange(0, 100);
QGraphicsItemAnimation *animation = new QGraphicsItemAnimation;
animation->setItem(ball);
animation->setTimeLine(timer);
animation->setPosAt(0.1, QPointF(10, -10));
timer->start();
But I can't understand how setPosAt works and how I can use my calculated x,y in this case.
The official Qt documentation for setPosAt is very short and incomprehensible.

Answer 1

You need to call setPosAt() multiple times with various values of (step) between 0.0 and 1.0. Then when you play the animation, Qt will use linear interpolation to animate smoothly between the points you set, as Qt increases its "current step" value from 0.0 to 1.0.
For example, to make the ball move in a straight line you could do something like:
animation->setPosAt(0.0, QPointF(0,0));
animation->setPosAt(1.0, QPointF(10,0));
... or to make the ball go up, and then down, you could do:
animation->setPosAt(0.0, QPointF(0,0));
animation->setPosAt(0.5, QPointF(0,10));
animation->setPosAt(1.0, QPointF(0,0));
... so to get the arc you want you could do something like:
for (qreal step=0.0; step<1.0; step += 0.1)
{
qreal time = step*10.0; // or whatever the relationship should be between step and time
animation->setPosAt(step, QPointF(speed*cos(angle) * time, speed*sin(angle) * time - (g*pow(time,2)) / 2);
}