How to set min / max range for LCD backlight level (0 to 100, not 0 to 255)

[ShoutingMan]

Have an iMac 2017 27" conversion with the R1811 driver board. I use HDMI, running 5K and 10 bit color. The R1811 firmware supports Backlight levels from 0 to 100. BetterDisplay drives it from 30 to 255. This makes it so the usable range of the macOS brightness software control is the first 10 ticks and the latter 6 ticks just bounce around in brightness as BetterDisplay attempts overdrives the backlight.

I want to tune BetterDisplay so it 1) will command the backlight to below 30 for finer control on the dark side and 2) command to a maximum of 100 for the max software level to have full range control.

I tried a few things based on scanning this forum, but haven't found an obvious solution or setting for this. Particularly, I tried DDC setting for Brightness (Hardware), changing it from 100 to 39 (since 100 is 39% of 255). The result was odd, as the maximum backlight was then 111 instead of 255, which isn't linearly scaled from 255. I dropped the setting to 33, and that results in a maximum backlight setting of 99. So that's good.

But the minimum brightness setting is never set below 30. I can't find a setting that tricks BetterDisplay into using values below 30 for dimmer settings.

Hopefully the graph helps, which shows the absolute brightness measured with a light meter and the backlight setting driven by Better Display against the macOS software brightness control levels. This is for the default DDC control settings.

Any insights into BetterDisplay for configuring would be appreciated!

[waydabber]

Hi there, if the app detected the range as 30-255 then the board might report incorrect values. Anyway, you can change the control range under the display's settings pane, under the DDC Features… sheet. Go to Brightness and change DDC min and max values.

Let me know if this does not work!

[waydabber]

Oh, re-reading your comment, you might have already tried that. If the brightness can't go below minimum using DDC brightness 0 and generally behaves erratically, then it will be difficult to fix as DDC values are integer values represented in 2 bytes (0-65535). So if the 0 value will result in a 30% brightness, then you can't send a DDC command to go below that.

Some boards however use contrast or some other control code for brightness, you can try changing the brightness control code to something else (maybe to the code for contrast) - it might not work but worth a try.

[waydabber]

• also note that by default the 0-50% of the slider range does software dimming, not hardware DDC brightness control. That might cause the flat output for the 0-50% range in your graph?

[ShoutingMan]

Thanks for the followup!

I understand now the flat region is the software simulated brightness. It's that range, where I wish I could set the Backlight to 0 instead of 30 (even at "0" it's still relatively bright).

I've tried as you suggest adjusting the Brightness and Contrast settings. And they are both 0 to 100 by default. Putting the max to 33 to 38 caps the max level to about 100 (from 255). I've played with Contrast a little and it doesn't seem to do anything with this driver board.

Here's some photos and screen shots to illustrate. The photos are of the driver board OSD at 0 and 16 macOS levels. The two screenshots are the DDC settings I last tried.

The display is livable as is. But being able to use its full 0 to 100 backlight would make the overall experience nicer. :)

[ShoutingMan]

But I'mn ot sure I'm using the CLI correctly. Basic info query commands fail, like:

BetterDisplay get -displayName
BetterDisplay get -resolutionList

[waydabber]

The -hardwareBrightness will just show the app's managed hardware brightness. To read the DDC value you'd need to use

betterdisplaycli get -ddc -vcp=luminance

If this returns Failed, the display might not support DDC read. You can start the app from Terminal using -v to see if it complains for an error.

[ShoutingMan]

Thanks. It reads 0 for Backlight settings up to 30 and then maxes out at 31 for Max Backlight of 100.

[waydabber]

Well in this case (if there is no alternative code for luminance) the issue can only corrected via a firmware update. You'll need to contact the makers of the board and ask about why this is and if they can fix this.

[ShoutingMan]

Thanks. And I got confirmation from the MacRumors forum that others see the same behavior with this control board.
https://forums.macrumors.com/threads/diy-5k-monitor-success.2253100/page-90?post=33652979#post-33652979

[ShoutingMan]

And I bought the license, out of appreciation for attempting to solve this and educating me on the finer details of BetterDisplay Pro. :)

[waydabber]

Thank you! :)