Pixmap not scaling on hdpi

Christian Ehrlicher

@Kite-R said in Pixmap not scaling on hdpi:

How come pixmaps that run through this function don't display correctly?

QPixmap::setDevicePixelRatio()

Kite R

That works on pixmaps generated from QIcon (mostly, there are still some with issues), but it only very slightly increases the size from pixmaps generated from Qicon::fromTheme.

QPixmap generateOpacity(QPixmap &input) {
  QPixmap normalPixmap(input);
  normalPixmap.setDevicePixelRatio(getDevPixRatio());

  QPixmap disabledPixmap(normalPixmap.size());
  disabledPixmap.setDevicePixelRatio(getDevPixRatio());

  disabledPixmap.fill(Qt::transparent);

  QPainter p(&disabledPixmap);

  p.setBackgroundMode(Qt::TransparentMode);
  p.setBackground(QBrush(Qt::transparent));
  p.eraseRect(normalPixmap.rect());
  p.setOpacity(0.2);
  p.drawPixmap(0, 0, normalPixmap);
  p.end();

  return disabledPixmap;
}

Christian Ehrlicher

@Kite-R said in Pixmap not scaling on hdpi:

getDevPixRatio()

what does this function do?

Kite R

@Christian-Ehrlicher Oh, forgive me for missing that out.

int getDevPixRatio() {
  static int devPixRatio = QApplication::desktop()->devicePixelRatio();
  return devPixRatio;
}

Christian Ehrlicher

You forgot to multiply the pixel size of the pixmap with the device pixel ratio:

For example, painting on a 200x200 image if with a ratio of 2.0 will result in effective (device-independent) painting bounds of 100x100.

Kite R

@Christian-Ehrlicher Hmm, I tried this but it produces corrupted looking pixmaps, most of them display nothing. Is this right?

QPixmap generateOpacity(QPixmap &input) {
  qreal ratio = getDevPixRatio();
  QPixmap disabledPixmap(input.width() * ratio, input.height() * ratio);
  disabledPixmap.setDevicePixelRatio(ratio);
  disabledPixmap.fill(Qt::transparent);

  QPixmap normalPixmap(input.width() * ratio, input.height() * ratio);
  normalPixmap.setDevicePixelRatio(ratio);

  QPainter p(&disabledPixmap);
  p.setBackgroundMode(Qt::TransparentMode);
  p.setBackground(QBrush(Qt::transparent));
  p.eraseRect(normalPixmap.rect());
  p.setOpacity(0.2);
  p.drawPixmap(0, 0, normalPixmap);
  p.end();

  return disabledPixmap;
}

Christian Ehrlicher

@Kite-R said in Pixmap not scaling on hdpi:

most of them display nothing. Is this right?

QPixmap normalPixmap(input.width() * ratio, input.height() * ratio);

You create an empty pixmap...

Kite R

@Christian-Ehrlicher Setting this does the same as before, tiny pixmaps.

QPixmap normalPixmap(input);
normalPixmap.setDevicePixelRatio(ratio);

Christian Ehrlicher

@Kite-R said in Pixmap not scaling on hdpi:

Setting this does the same as before, tiny pixmaps.

You have to scale it up - otherwise you only create a big pixmap with a small icon inside.

Kite R

@Christian-Ehrlicher Aaah... thanks so much man, it's been a long day, this worked:

QPixmap normalPixmap = input.scaled(input.width() * ratio, input.height() * ratio, Qt::KeepAspectRatio);

Christian Ehrlicher

Yeah, the high-dpi stuff is not easy... even in the qt styles there are still a lot of pitfalls, esp. due to rounding errors.