[guesswork] Try to read brightness/color temperature limits from device

[cajus]

Found your github project while trying to understand the USB HID report descriptor of my new Litra Glow. As I wasn't able to find any documentation on the Logitech side, it seems like there's some swarm intelligence which does reverse engineering in various github/whatever-litra repos ;-)

What I found while playing around may get the min/max/step values for brightness/color temperature from the device. I don't know for sure of course:

device.sendReport(17, new Uint8Array([0xFF, 0x04, 0x21, 0x00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00]))
Event for ReportID 17: FF 04 21 05 00 14 00 FA 00 01 07 00 00 00 00 00 00 00 00 
                                   00 14 (min brightness = 20)
                                         00 FA (max brightness = 250)
                                               00 01 (step width = 1)

and

device.sendReport(17, new Uint8Array([0xFF, 0x04, 0x71, 0x00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00]))
Event for ReportID 17: FF 04 71 05 0A 8C 19 64 00 64 07 00 00 00 00 00 00 00 00 
                                   0A 8C (min color temperature = 2700K)
                                         19 64 (max color temperature = 6500K)
                                               00 64 (step width = 100K)

Maybe people with other Litra devices can try/confirm/invalidate that? Just close the issue if you think that's too much guesswork ;-)

[timrogers]

I love this!

Any Beam or Beam LX owners out there who can verify? 👀

[cajus]

Had access to a Beam LX and it works there. For your code, it's 0x06 instead of 0x04 as in the other calls for the LX.

Additionally, I put some minutes in learning how the protocol works: 0x04/0x06 is a feature index and points to the very same thing for both devices. They are not guaranteed to be constant over firmware updates. To do this properly, you've to query the device for the desired feature index before.

I've done a quick WebHID experiment for Chrome based browsers here. You can find a brief documentation about what I found out here.