Why the image is blurry on 4K display if I set 1920*1080 resolution?

Question

I have 2 27'' displays: 4K and FullHD native resolutions. If I set 1920*1080 on the 4K display, the image looks blurry in comparison with FullHD native display.

I would not be surprised with any other resolutions, because I understand that pixels are physical squares, so if a ratio between 2 numbers is not a whole number, we could see artifacts.

But in this particular case FullHD is exactly 2 times smaller than 4K on each dimention. So each dot in 1920*1080 image should take exactly 4 physical pixels on 4K display and I do not see any reason for blur.

So why the blurriness appears?

Answer 1

This problem has a long history. You may see an NVIDIA feature request dating from the year 2012 at Nonblurry Upscaling at Integer Ratios, still today without any answer or solution.

In short: The video adapter uses a more sophisticated algorithm than simple pixel multiplication (two pixels for one), which works well for many tasks but not very good for this too-simple task.

Most advice you will find for the problem would be to replace the display adapter and/or monitor, which is not very practical.

One person has claimed success in this task. I quote below part of his advice (with small modifications):

You have to convince Windows that your display supports HD as a native resolution. This can easily be done by using CRU (Custom Resolution Utility) authored by ToastyX and can be found here:

https://www.monitortests.com/forum/Thread-Custom-Resolution-Utility-CRU

It was as simple as adding 1920x1080, rebooting, and selecting that resolution. I added it to detailed and standard resolutions and all is well in that department.

I have not tried this solution and so cannot vouch that it works.