GUI update in Qt - qt

Question

I'm working on a project which I need get some data(points) from a device and show them on the widget. I'm using Qt 5 and C++ language. the data come 12 times in a second and each time it has 895 points. actually its a simulation of PPI display of a Radar. I a have list which I put the point on that, and then I try to paint them on the widget. the problem is when I draw points on the widget it will connects the points and draw lines that obviously are unwanted.
void FormPPI::PPIUpdate()
{
QPainter* pdc = new QPainter();
ObservationList* list = monitoring->getObservationList();
ObservationList::iterator it = list->begin();
// qDebug() << list->size();
while (it != list->end())
{
Observation *o = &(*it);
int xp, yp;
float angle = calcPhi(o->pos().bearing());
//Find the pixel position on the screen
getPixelPos(o->pos().range() * cos(angle * DEG2RAD),
o->pos().range() * sin(angle * DEG2RAD), xp, yp);
//Draw observations here
//o->lifeTime() gets object lifetime which is initialized by 12
//used to reduce the intensity of color like antique radar system displays
pdc->fillRect(xp - 1, yp - 1, 2, 2, QColor(255, 255, 0, o->lifeTime() * 21));
//Decrease the observation life time by 1
o->decLifeTime();
if(o->lifeTime() <= 1)
list->erase(it);
it++;
}
qDebug() << list->size();
update();
list->release();
}

Answer 1

I am not able to explain why
pdc->fillRect(xp - 1, yp - 1, 2, 2, QColor(255, 255, 0, o->lifeTime() * 21));
fill rectanges between two consecutives point (xp1,yp1) and (xp2,yp2) .
But you have an alternative. You can draw points directly with a specific diameter d (seems you want a diameter of approximately 2 pixels).
int diameter = 2;
QPen pen = pdc->pen();
pen.setWidth(2);
pdc.setPen(pen);
while()
{
...
pen.setColor(QColor(255, 255, 0, o->lifeTime() * 21));
pdc->setPen(pen);
pdc->drawPoint(xp, yp);
...
}