Adjust for Retina and High DPI screens

[stefkeB]

When you set viewports, you need to multiply the sizes with devicePixelRatio() to ensure the view is not "half the size". This happens in many other OSG/Qt examples as well. This is only a problem when running on Retina Resolution (= double the resolution). When I switch to a non-retina resolution, the problem is gone.

This has to be repeated a few times. I did adjust it inside the OSGWidget::onResize() handler, but it happens in a few other places too.

void OSGWidget::onResize( int width, int height )
{
  std::vector<osg::Camera*> cameras;
  viewer_->getCameras( cameras );

  assert( cameras.size() == 2 );

  cameras[0]->setViewport( 0, 0, width / 2 * this->devicePixelRatio(), height * this->devicePixelRatio());
  cameras[1]->setViewport( width / 2 * this->devicePixelRatio(), 0, width / 2 * this->devicePixelRatio(), height * this->devicePixelRatio());
}

However, when doing this, the Pickhandler is not working anymore. I guess this is related to the problem above, since the Pickhandler runs OK on a non-retina resolution.

[Pseudomanifold]

Thanks for reporting this! I will take a look at this. Since I do not have access to a retina display, would you kindly be available for testing?

By the way: is this something time-critical for you? I am presently quite swamped with other work, so I cannot guarantee that I will find the time to address this rapidly.

[stefkeB]

Not time critical. It's a side project, but as I bump into the same problems in many examples, I can as well contribute a bit.

[Pseudomanifold]

Thanks for your patience! I started to implement support for high-DPI screens as you suggested. I have added devicePixelRatio() at multiple places. Would you like to test with the current revision?

[stefkeB]

Current revision has solved the retina problems for me.

[stefkeB]

While viewport display is now correct and the errors (mentioned in other case) are gone, the "pickhandler" code is not working correctly, yet. I can pick a point above the "sphere" in the viewport and picks at the bottom half seem not to be recognised. I guess you have to compensate a bit for hiDPI usage.

To me it seems that the picks are happening half a window upwards (but horizontally they seem correct).

[Pseudomanifold]

Selection handling has been fixed in commit b0ea8eddce3ce14287a1bf292be0a9ef50a4e296. Pick handling still needs some attention, though.

[Pseudomanifold]

I am observing something strange: do you get negative coordinates in the pick handler as well on a high-DPI display? I temporarily added the following code in the handle() function:

std::cout << ea.getX() << "," << ea.getY() << "," << devicePixelRatio_ << "\n";

Under Linux, the minimum value is achieved in the bottom left corner of the window with (0,0). Under Mac OS X with a high-DPI display, I obtain negative y-values. Can you confirm this behaviour? If so, this may be the reason why pick handling does not work correctly.

[stefkeB]

I can confirm. In high-DPI, the "y=0" is in the middle of the view. On normal DPI, "y=0" is at the bottom. Maybe done the pixel ratio mapping once too much? Could be trivial to remap, but I'm a bit lost in the different mappings done already (between OS/qt/osg).

Selection handling (the rectangle selection) seems to work correct.

[Pseudomanifold]

OK, thank you! I will try to prepare a small example program in order to figure out whether this is an issue of OSG or not. Did this work prior to the pixel ratio mappings? Since I am only multiplying values here, I do not yet see how this offset could be created, to be honest.

I will get back to you if I have something to report :smile: