Enhancing performance drawing polygons with many holes

alan73

I'm drawing complex polygons with many vertices and holes.
Code sample:

QImage image;    // some image
QPainter painter;
painter.begin(&image);

// overall region
QPolygon polygon;
polygon.setPoints(polyCounts, (int*)polyPoints);
QRegion region(polygon);

// cutting holes
while ( holesCount-- > 0 )
{
    QPolygon subtractedPolygon;
    subtractedPolygon.setPoints(holePointsCount, (int*)holePoints);
    QRegion subtractedRegion(subtractedPolygon);
    region = region.subtract(subtractedRegion);

  // defining next hole properties...
}

// setting clip region
painter.setClipRegion(region, Qt::IntersectClip);
// painting
painter.fillRect( image.size(), Qt::red );

It seems to be that main performance leaks are caused by copying points into QPolygon.
I've tried using composition mode QPainter::CompositionMode_SourceOut: it works really faster, but I can't use it because I have to save original image contents.
In Windows API PolyPolygon function seems to be useful in this situation:
https://msdn.microsoft.com/en-us/library/windows/desktop/dd162818(v=vs.85).aspx
Is there any way in Qt to improve performance in this case?

mrjj

Hi
would it possible to setup the QPolygon and QRegion outside of
paint so it didnt have to recreate it over and over for each paint?

alan73

Hi
I use different points every rendering so it doesn't prevent from adding new points into QPolygon every painting operation.

mrjj

@alan73
ok so not really possible to cache it ?

KeithS

If you want better performance, you might want to look at using openGL.