Fix icon scaling for HiDPI displays

[2xsaiko]

Type of change

• ✅ Bug fix (non-breaking change which fixes an issue)

Description and Context

This PR fixes the way icons are scaled for displays with a scale level >1 according to the method described in https://andrewcrouthamel.wordpress.com/2018/10/03/making-plasma-superb-in-hidpi/. Before, icons displayed severe scaling artifacts (see screenshots below)

Screenshots

The top image is taken from this PR with fixed scaling. (Open in fullscreen view/zoom in to see the change better!)

Testing strategy

Tested with scale factor 1, no change from upstream. Tested with scale factor >1, fix works as expected.

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
• ✅ I have compiled and verified my code with -DWITH_ASAN=ON. [REQUIRED]

[phoerious]

I think this is like the fourth PR or so that proposes this change. I don't like it unless we finally get new icons that actually support higher resolutions.

Right now the icons don't have "severe artifacts" (except those that actually have high resolution), they are just scaled with nearest neighbour interpolation. It makes them look blocky but sharp. Bilinear interpolation, however, makes them quite mushy and unpleasant.

[2xsaiko]

It does seem to be nearest neighbor interpolation, but scaling using an uneven factor using that always makes the result look pretty bad, since some of the pixels on the icons will have different sizes, which is the case here. Not to mention downscaling using nearest makes the image lose way more detail than any interpolation (which also seems to be happening here, I think?) If I open for example share/icons/application/32x32/actions/password-generator.png in an image editor and compare it to a screenshot taken from the KeePassXC gui, the original image looks way better. And it also happens for system icons (most of the icons in the icon bar here) provided by KDE, that do have high resolution icons.

Here's the original icon (left) and the way it is displayed in the application without this patch (scaled to both 256x256 with nearest from 32x32 source images). You'll see that the original looks way better. (If it'd just keep the original image as is, it would most likely look fine, but not like this.)

[droidmonkey]

I don't see the harm in adding this application property as long as it doesn't degrade non-hidpi experience.

[phoerious]

I cannot really speak for Linux, because I only have a HiDPI screen on my Windows laptop. But on Windows at least not having that option looked better IMHO. At least the last time I tested it, everything looked blurry after applying this option. I do agree that having proper scaling will improve appearances on Linux where most of the icons are taken from your system theme which does have high-resolution versions of them. But on Windows we only have our own low-res icons which are hard to scale properly. I will test it again, though. In the end the only real solution is new icons and then we will definitely turn pixmap scaling on.

[droidmonkey]

We can limit this to Linux-only then, until you get our new icon set in order 😉.