Set canvas rendering dpi from window dpi

[m-kuhn]

@elpaso I think you have a high dpi device to verify the patch?

[elpaso]

@m-kuhn sure, I'll check it ASAP, what exactly should I test?

[m-kuhn]

It doesn't work apparently qgis/homebrew-qgisdev#5 (comment). I think there's something else required to make it work, but don't know what...

[m-kuhn]

@elpaso possibly setting the env variable QT_AUTO_SCREEN_SCALE_FACTOR=1 http://doc.qt.io/qt-5/qt.html and check what happens with/without this patch... I guess it just needs someone with appropriate hardware to play around a bit :)

[elpaso]

@m-kuhn I can test it only on a retina Macbook pro running Kubuntu xenial (I cannot build on MacOS, sorry and my other 4k machine has only a Qt4 toolchain), but what exactly does not work in current master and it's supposed to be fixed with your patch?

I need the procedure for what to test.

The map canvas DPIs?
Do you I need a particular symbology/labels to test with?
Should I try to save and export the canvas from master and with your patch and compare the output?

I'll have time to test it right after christmas.

[m-kuhn]

I don't know anything more than is written in here: qgis/homebrew-qgisdev#5

I assume it's a matter of making sure that

1. the canvas graphicsscene draws on every single pixel (and doesn't use some "backwards-compatible" upscaling logic)
2. the mapcanvasmap item really draws in the appropriate resolution (so fonts and symbology is not too small)

I'm sorry I really cannot be of anymore help.

[elpaso]

I've checked this PR on kubuntu xenial QT 5.5.1 on a retina screen 220dpi and makes no difference: the mDPI is already set at 220, so that line does nothing.
The canvas is rendered correctly @220 dpi, unless you set QT_DEVICE_PIXEL_RATIO=2 (deprecated on 5.6), that makes pixels doubled (i.e. prints 4 pixels groups of equal color).

[m-kuhn]

Thanks. So it's already rendered in HighDpi. In this case it's maybe a mac issue or only happens in newer Qt versions, but unlikely that this patch fixes anything.