How can I copy image from anohter image

[ChrisW67]

Try something like this for a slow approach

for (int x = 0; x < oldImage.width(); ++x) {
  for (int y = 0; y < oldImage.width(); ++y) {
    QPoint p(x, y);
    if (polygon.containsPoint(p, Qt::OddEvenFill)) {
      QRgb color = oldImage.pixel(p);
      newImage.setPixel(p, color);
    } 
  }
}

for (int x = 0; x < oldImage.width(); ++x) {
  for (int y = 0; y < oldImage.width(); ++y) {
    QPoint p(x, y);
    if (polygon.containsPoint(p, Qt::OddEvenFill)) {
      QRgb color = oldImage.pixel(p);
      newImage.setPixel(p, color);
    } 
  }
}

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(Untested code)

You should investigate ways to use the image composition modes to draw a filled version of your polygon over a copy of the original leaving only the intersection. This would be much faster.