High DPI scaling issues with Qt::AA_EnableHighDpiScaling (macbook pro retina ubuntu 16.04)

[rlober]

tl;dr
I am raising this issue as a follow up to #1077 which was partially fixed in #1078. Qt::AA_EnableHighDpiScaling causes RenderWidget resize event to be 1/2 the proper size. Proposed fix in #1263.

Setup:

• macbook pro late 2011
• retina display
• ubuntu 16.04
• branch: kinetic-devel
• rviz at commit: 87742e4
• rviz version 1.12.16
• compiled against Qt version 5.11.0
• compiled against OGRE version 1.9.0 (Ghadamon)

Problem:
Since Qt 5.6 (see here), high DPI scaling can be handled automatically by setting the application attribute Qt::AA_EnableHighDpiScaling in the main.cpp of a Qt application.

// Enable support for high DPI screens 
QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

This makes gui design more homogeneous across screens (at least where pixel coordinates are concerned).

Unfortunately when I enable this attribute and am using librviz, the RenderWidget only fills 1/4 of the RenderPanel:

The rest of the space is filled with whatever was behind the RenderPanel upon a resize event.

When I do not set the AA_EnableHighDpiScaling attribute the RenderWidget displays properly, but the rest of the gui (not shown) is far too small:

The image above is scaled here but on my screen it is 2x smaller than it should be. This is because AA_EnableHighDpiScaling is not set.

Resolution:

I say this problem was partially fixed in #1078 because the AA_EnableHighDpiScaling is never used in rviz and the RenderWidget displays as expected.

I traced the problem to the resizeEvent() handler in RenderWidget:

void RenderWidget::resizeEvent(QResizeEvent *e)
{
  if( render_window_ )
  {
    // render_window_->writeContentsToFile() (used in
    // VisualizationFrame::onSaveImage()) does not work right for
    // window with an odd width, so here I just always force it to be
    // even.
    render_window_->resize( width() + (width() % 2), height() );
    render_window_->windowMovedOrResized();
  }
}

When using AA_EnableHighDpiScaling, the functions width() and height() return the widget width and height with respect to a virtual pixel coordinate system and not the real pixel coordinate system. The Ogre::RenderWindow on the other hand is expressed in the real pixel coordiante system (I think...) and therefore only renders at 1/devicePixelRatio().

I have proposed a fix in #1263 which simply multiplies the width and height by the pixel ratio:

render_window_->resize( width()*pixel_ratio_ + (width()*pixel_ratio_ % 2), height()*pixel_ratio_ );

This results in a properly scaled RenderWidget:

When not using AA_EnableHighDpiScaling, the widget fills the panel but it the whole gui is half the size, which is the expected behavior on high DPI screens.

[mgrrx]

I tried the proposed fix on my Mac and it seems to work. Not sure if it is related to the issue described here when I try to do e.g. a Pose Estimate it only works in the upper right corner of the rendering area.

[tompe17]

I see this issue on Ubuntu 18.04 and melodic. Did not have the issue on 16.04 and kinetic.

[ahoarau]

Same issue with a Dell XPS 4k (16.04 / kinetic). #1263 fixes it.

[rhaschke]

Fixed via #1263.