Slow performance graphics on Mac in certain resolutions

[Tone2 Synthesizers]

I did some measurements with the GUI performace on Mac. M1, VSTGUI4, Big Sur.
I noticed that the perfomance depends a lot from the 'desktop zoom level' of the OS which you can select in the system settings.

- If you select 'more space' (100% zoom) performance is fast. Graphics are crisp.

- If you select 'larger text' (200% zoom) performance is also fast. Graphics are crisp.

- If you select a zoom level in between (like 150%) GUI performace is a lot slower. Graphics are a bit blurry. The difference is quite noteable on the M1 mini with a fast 4K screen connected.
It seems they first stretch anything up for 300% and then downsample it to 50%. This does seem to affect all(?) software.

[BertKoor]

You ask it to do extra work (scaling) so yeah that takes some effort which is measurable

[Tone2 Synthesizers]

The weird thing is that performance with 200% scaling is a lot better than with 150%

[BertKoor]

And you find that surprising?

200% scaling means each logical pixel is painted by 2x2 physical pixels. No interpolation needed and thus razor sharp.

Any other percentage means lots of pixels sit somewhere inbetween, which looks blurry.

Funny thing is, the same happens in audio when you load a 44.1kHz wav into a 48kHz project. This is just resampling. But apparently our eyes are more sensitive to sharp edges than our ears are sensitive to high frequencies.

[Tone2 Synthesizers]

What I find surprising is that scaling to 150% needs 3x more performance than scaling to 200%. We're not talking about resampling with a polyphase filter. This is simple linear or spline interpolation.

[BertKoor]

Scaling by factor two does not require any interpolation at all, hence it's fast.

[Tone2 Synthesizers]

I did some research. In mosts cases the weak performance becomes only obvious if you use a non-Apple 4K screen. It affects lots of users.
It appears in scaling factors which are not 1x or 2x results from Apple's lazyness (and the intention of offering sluggish graphics on non-Apple displays?)

- They first upscale everything by a factor of 2x. The RAM requirement and bandwidth for this is HUGE. It should be up to 64MB that need to be refreshed and scaled @ 60Hz per second.
- Then they downscale everything to desired screen resolution

Instead, they could do up-scaling to 1.5x in a single step without wasting lots of RAM, GPU load and energy. But this would mean a little more effort. Especially if you want to support a wide range of different resolutions and aspect ratios.

Reference - take a look especially at 3:10 :
https://www.youtube.com/watch?v=5HZO-tfsQ-A&t=765s

[midi_transmission]

It's a known annoyance since years and in my opinion only the display industry is to blame.

4k is simply a bad choice for displays larger than 24" with most operating systems, because you run into these kind non integer scaling issues. The Apple 4k is nothing special, it's just physically smaller so you get a high PPI and will still use it at 200% Larger Apple screen use 5k or 6K.

As you said, it's not only a performance problem, but also the final result is missing sharpness and 1px structures are a problem. Windows has has some parts that renders without rescaling on the pixel layer, but in many areas and programs they have very similar problems.

The overall experience in Windows with larger 4k screens is just as bad in my opinion.

To my knowledge, Apple renders with a much higher than used resolution and then downscale. I forgot about the details though. Depending on what they do exactly, this could be a possibility for an unexpected drop in performance. But please understand this as an idea, not as a fact.

The only solution to this problem is to use screens with a larger PPI than the "typical" 4k screens so that you can use 200% scaling.

Outside of Apple there are unfortunately only very few options.