Universal nits based brightness sync mapping system for synchronization

[jhildenbiddle]

I'd love to see BetterDisplay offer a simple form of multi-monitor brightness sync. I've tried Lunar Pro but found for all of its sophisticated controls and auto-adjusting brightness curves it simply does not do what I expect.

Here's my setup:

• MacBook Air 2020
• LG 32UD99 (connected via USB-C)
• BetterDisplay options:
  • Keyboard shortcuts affect: All Suitable Screens

The brightness of these two displays matches when the MacBook Air is at 80% and the LG monitor is @ 100%. This is the brightness ratio I would like to maintain when adjusting the brightness of both displays. A conceptually simple way to accomplish this would be as follows:

1. Allow specifying a max brightness level for each display. In my case, I would set the MBA to 80%.
2. Allow specifying that max brightness levels can by applied 1) when connected to an external monitor (default) or 2) always. This will allow users to enjoy their laptop display at full brightness when not connected to other displays they are trying to sync brightness with.
3. When increasing the brightness, ensure that all displays reach their max brightness at the same time.
4. When decreasing the brightness, ensure that all displays should reach zero at the same time.

Unfortunately, this is not how brightness levels are adjusted as of BetterDisplay v1.3.10b:

	-	↓1	↓2	↓3	↓4	↓5	↓6	↓7	↓8	↓9	↓10	↓11	↓12	↓13	↓14	↓15	↓16
LG	100	94	88	81	75	69	62	56	50	44	38	31	25	19	12	6	0
MBA	80	75	69	62	57	50	44	38	32	25	19	12	6	0	0	0	0

• The MBA and LG do not reach 0% brightness at the same time.
• If I decrease the brightness of the LG below 19%, the brightness ratio is off because the MBA was already at 0%.
• If I increase the brightness of the MBA above 80%, the brightness ratio is off because the LG was already at 100%.

This is how I would like to see BetterDisplay enforce max brightness and and adjust levels using my setup as an example:

	Max	↓1	↓2	↓3	↓4	↓5	↓6	↓7	↓8	↓9	↓10	↓11	↓12	↓13	↓14	↓15	↓16
LG	100	94	88	81	75	69	62	56	50	44	38	31	25	19	12	6	0
MBA	80	75	70	65	60	55	50	45	40	35	30	25	20	15	10	5	0

Thanks!

[waydabber]

Yes, you want to brightness-match displays, that's understandable. However supporting this and various other ways of brightness syncing (keep delta, factor in XDR/HDR upscaling, manage mismatching lower values - for DDC brightness only there is a low treshold, no dimming to zero - etc) makes syncing eventually complicated. :) I'll take a deep breath and dive into it eventually as it's continuously on my TODO list.

[waydabber]

Related to https://github.com/waydabber/BetterDisplay/issues/849

[jhildenbiddle]

Understood on the complexities. I have no doubt that if it was easy it would be done. :)

Appreciate your time and consideration, @waydabber. Keep up the great work and I'll keep my fingers crossed brightness sync gets added to the feature list eventually.

[waydabber]

Hmm. This is a bit more complicated than I thought as the syncing mappings are different for all two pairs of displays + for brightness it further depends on various factors like HDR is enabled or not, brightness upscaling is available or not etc. Need to think a bit about how to implement it in a way that makes sense. Tying everything to keyboard OSD steps do not make much sense as that won't work properly with sync.

[waydabber]

So what I should do is to create a syncing mapping section under "Groups" that can be generally used to affect syncing among source and target displays for various controls. There should be some additional specifiers (like HDR enabled/not etc) for a sync mapping. Syncing should then take the mappings into consideration when available for a source->target pair. Multi screen keyboard control should then be linked up with syncing (#2826) so that way the mapping will take effect when using keyboard for control.

Mapping should be bi-directional (probably).

[waydabber]

Thinking more about this, mapping is not really needed for anything besides Brightness. For brightness an universal nits based system (that can act as a common base for matching display brightness levels) is probably better. An optional table could be defined for every display that contains the nits curve, the app then can map brightness differences between any two displays this way. This should work both for syncing and keyboard control.

[jhildenbiddle]

Sounds good. Happy to take the concept for a test drive if/when it's ready. :)