2560 x 1440 Resolution on BetterDummy from 4K monitor is different from 'native' macbook 2560 x 1440 setting?

[mahsoo1172]

Hi! First, thank you for providing a way to fix the resolution limitations on the M1 macs!
Right now I am using a docking station (plugable UD-6950Z) to connect two external monitors to an M1 macbook. One of the monitors is a 4K monitor, and BetterDummy has helped me get 2560 x 1440 resolution, which was originally not an option when using the dock (only 2560 x 1440 'low resolution' mode was originally available with the dock).

I had a question though. The 2560x1440 resolution through BetterDummy is definitely better, however, it still seems a little grainy/blurry compared to when I plug in the 4k monitor directly through HDMI. Is this a hardware limitation I am facing, or is it a limitation of big sur/ DisplayLink support on big sur?

What I mean is:

The 2560x1440 resolution from BetterDummy seems grainy/blurry (screenshot of BetterDummy settings below),

compared to the 2560x1440 resolution from plugging in HDMI (screenshot of display settings thru HDMI below):

Is there any setting I can change through BetterDummy to get the same quality? Or is it an issue with the hardware? The dock should support 2 external monitors at 4k 60 hz, so I am not sure.
I noticed these two posts on the discussion thread, I wonder if it is the same issue?

#170
#135

Thanks!

[waydabber]

Hmm. I don't have a DisplayLink adapter so I am not entirely sure what is happening. There should not be quality difference between a scaled BetterDummy display and a 4K display. But I am not entirely sure why BetterDummy is needed to produce 2560x1440 hidpi with a 4K display as that should be supported by macOS without BD as well.

Maybe somebody with DisplayLink can help with this better.

[jb-ks]

It would be great if the handling of (3) could be as easy as the current mirroing (automatic switch to fullscreen stream at an associated display.

[jb-ks]

Are the two scaling methods result in different image when you try the native display output (I know, I could test this one myself... :))?

I‘ll try to make a mirror also on my native connected screen…

[jb-ks]

When I display a Mirror or a Stream on my natively connected display, I get always a perfect result (1). So the issue seems to be related to DisplayLink.

[jb-ks]

Strange:
If I set the Displaylink resolution to 3860 x 2160 px, the image is displayed perfect (1), but due to the lack of HiDPI the desktop is not usable...

[mashuuuu]

Hey @waydabber and @jb-ks . First, thank you both for looking into this issue so much! Waydabber, thanks for working on a beta with the new method and jb-ks, thanks for testing it!
Some of the technical discussion has gone over my head, but it is reassuring to see that #3 (the new method) seems to be very close to 'native' from jb-ks's testing and I'll be able to use the dual monitor dispaly with the M1 macbook.

I'll be looking forward to when the beta is released!

[jb-ks]

I have another question that might be related to this problem.

When I go to "About this Mac" -> "Displays" I get the following window:

The BetterDummy display is shown with a resolution of 5k (5120 x 2880). Is this used internally as an interim resolution? Could this be the cause of the scaling issue?

When creating a dummy, would it make sense to specify not only the aspect ratio, but the concrete dimensions (here, for example, 3860 x 2160)?

[waydabber]

yes, that is the render resolution for 2560x1440 hidpi.

[jb-ks]

Ok, that could be an explanation:

I once tried what happens when I upscale my 4k test image to 5K and then downscale it to 4k again (simply with "Preview"). The result is pretty much identical to the photo (3) I took.

Can I manually adjust the render resolution somewhere in a config file for a test?

I would then enter 4k (as a first test) or 8k if necessary...

[jb-ks]

Maybe it would be a good idea to have an option to set the render resolution (independent to the desktop resolution "looks like") to optimize image quality.

[waydabber]

I mean the render resolution if always the double of the desired resolution (so 2560x1440 hidpi will be rendered at 5120x2880), this then gets scaled to the display's base (which is everything is ok is the same as the physical) resolution which is 3840x2160 for a 4K display. The 1-0-1-0 vertical lines are scrambled a bit because of this scaling. You can get perfect lines with perfect spacing only if the render resolution matches the display's physical resolution.

[jb-ks]

Of course, if you can't set the render resolution, it's a dead end.

That said, I am now very much looking forward to straeming/pip. :-)