c++ - Make polygonal hole in QImage alpha channel -


i'm trying make polygonal hole in qimage alpha channel. current implementation use deprecated 'alphachannel' method , works slow (because use containpoint every image pixel instead of draw polygon).

qimage makeimagewithhole(const qimage & image, const std::vector<qpoint> & hole_points) {   qimage newimage = image.converttoformat(qimage::format_argb32);    qimage alpha = newimage.alphachannel();   qpolygon hole(qvector<qpoint>::fromstdvector(hole_points));   (int x = 0; x < image.width(); x++)   {     (int y = 0; y < image.height(); y++)     {       if (hole.containspoint(qpoint(x, y), qt::oddevenfill))       {         alpha.setpixel(x, y, 0);       }     }   }   newimage.setalphachannel(alpha);    return newimage; }

i trying implement using painter , proper composition mode, in result have white artifacts on polygon borders.

qimage makeimagewithhole(const qimage & image, const std::vector<qpoint> & hole) {   qimage newimage = image.converttoformat(qimage::format_argb32);    qpainter p(&newimage);   p.setcompositionmode(qpainter::compositionmode_sourceout);   p.setpen(qcolor(255, 255, 255, 255));   p.setbrush(qbrush(qcolor(255, 255, 255, 255)));   p.drawpolygon(hole.data(), hole.size());   p.end();    return newimage; }

what proper way this?

i think should enable antialiazing this:

qpainter p(&newimage); p.setrenderhints(qpainter::antialiasing);

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