In most cases, DDC is blocked by a monitor setting that tries to take complete control of Brightness, Contrast or Volume.
Usually disabling those functions in the monitor settings will allow Lunar control brightness.
Below are some settings that are known to block DDC, grouped by monitor vendor.
Note: DDC seems to be unresponsive on M7/M5 Smart Monitors even after disabling all the below features. There is no currently known solution for those monitors for now. You'll have to rely on Lunar's fallback controls.
Input Signal Plus
Eye Saver Mode
Eco Saving Plus
Smart ECO Saving
Preset Mode (this option should be set to Custom or Standard to allow DDC)
Picture Mode (this option should be set to Custom or Standard to allow DDC)
Simple hotkeys are what you can configure on Lunar's HOTKEYS page under the Function Keys section. These don't need special permissions.
If you haven't checked the checkbox for Use F1, F2, etc. keys as standard function keys on external keyboards, you will have to hold Fn when pressing hotkeys that contain function keys like F1, F2 etc.
Otherwise, something like Command+F1 will actually send Command+Brightness which is a Media Key.
If you see the Software Controls tag under the monitor name in the Lunar UI, then Lunar can't use DDC to control this monitor.
Lunar can approximate a decrease in brightness by changing the software gamma tables to make the colors look darker.
This doesn't change the hardware brightness as DDC does and can only decrease brightness, not increase it.
You have to manually set the monitor's brightness and contrast (using the monitor physical buttons) to the highest possible values that look good for your monitor.
Yes, it is a known issue.
The MacBook display can increase the brightness on isolated local regions, which is the case when rendering HDR content.
The problem is that the system reports the highest brightness of the built-in display, not an average.
So even if the display contains only a small HDR-extra-bright-white square, Lunar receives that brightness from the system, and sends it to the external monitors.