Fast Painting region by region

aureshinite

I need to paint on my widget region by region and I want to get some suggestion on how to do so in the most efficient way possible. I have a video playing by my custom player, let's say at a rate around 5fps to 30 fps. On top of the display I have another widget I use as overlay, which receive information about each 16x16 blocks of pixels. For each of this block I need to apply a transparent color and vector.

Right now I have the following method called inside paint event:

@
void OverlayDrawer::drawVectors(QPainter painter){
int x,y;
int vx,vy;
QLineF line1,line2,line3;
QVector<QLineF> arraylines;
BLOCK blk;

float ratiox = 1;//((float)width())/defaultSize.width();  don't mind this, it is just in case of resizing
float ratioy = 1;//((float)height())/defaultSize.height();
float arrowsize = 3.0f*(ratiox+ratioy)/2;

QListIterator<BLOCK*> mbIterator(pictureInformation.blockList()); // get list of block info
while(mbIterator.hasNext()){
    blk = mbIterator.next();
    x = blk->X();
    y = blk->Y();

    vx = blk->vectorX();
    vy = blk->vectorY();

    // cellsize = 16x16 when the screen is not resized
   // I also draw the rectagle (not displayed here), that's why there is a QrectF
     QRectF rect(x*cellsize.width(),y*cellsize.height(),cellsize.width(),cellsize.height()); 
     makeArrow(QPointF(rect.center().x() +vx*ratiox  ,rect.center().y()+vy*ratioy),
        QPointF(rect.center().x(), rect.center().y()),
        arrowsize,
        line1,
        line2,
        line3
        );

if (!qFuzzyCompare(line1.length(), qreal(0.))){
arraylines.append(line1);
arraylines.append(line2);
arraylines.append(line3);
}
}
painter->setPen(Qt::blue);
painter->setRenderHint(QPainter::Antialiasing);
painter->drawLines(arraylines);
}
@

I want to optimize this as much as possible. I already thought about computing (makeArrow())all the arrows at construction, and only creating the appropriate ones by translation. But I want to know the BEST TOOL in the framework for this kind of task. Is it openGL, some function I didn't notice? etc.. etc..
Thanks

aureshinite

For those who might be interested I figured out a simple way :

Drawing outside the paint event in a picture with alpha enabled
Painting the picture on top of the video. Which means before displaying the actual video, I paint the overlay image on the video image. Now I dropped superposition of child widgets and everything.

The advantage is that I can create the overlay while the application is busy doing some other things.