Feature request: 4 gray support

[Tuom]

Hi @elad661, thank you for a great library. Is it possible to add support for 4 gray colors as in official waveshare driver? It was added there in this commit.

[elad661]

Hi, Does the 4 gray color mode works with the older 2.7" display, or does it require newer hardware?

[Tuom]

Well, I think it's the same hardware. Mine looks exactly like yours on a gif in README.

[elad661]

hmm... looks like (at least in theory) I could add support for a 4 gray mode for partial refresh, but probably not for the fast-updating lookup tables.

However, I don't know when I'll have the time to look into this and I can't promise any results. Waveshare didn't update their documentation explaining how this new functionality works, so implementing it in my library would probably involve copying their new code from here https://github.com/waveshare/e-Paper/blob/master/RaspberryPi_JetsonNano/python/lib/waveshare_epd/epd2in7.py and a lot of trial and error

[Tuom]

Okay, thanks. I will try to do some testing by my own. If I find something, I'll let you know. About documentation, I found that 4 Gray is described in wiki about 3.7inch display.

[Tuom]

Hi @elad661, so I started researching official lib and try to understand how it works. I think I will be able to implement 4 colors into your library. Can I make PR after that? I also managed to get working partial refresh for 4 colors. It seems that its working but there are some pixels which slightly changed color outside the refreshing area. I am attaching video with partial refresh. But you probably cannot see these changed pixels.

https://user-images.githubusercontent.com/17725008/104002826-e1836000-51a1-11eb-9597-bdf65dbb5a3f.mp4

[elad661]

Hi, This looks great! feel free to submit a PR.

I think pixels changing color slightly outside of the refresh area is a problem that also exists without 4-gray support, I don't know if it's a bug in the partial-refresh implementation or a hardware quirk

[Tuom]

I probably found the reason. In official library there is different LUT than you are using. I tried to use that for BW partial refresh and there is the same problem. I search through multiple libraries for this display and also for Good Display GDEW027W3 which seems to be the same. I found that there are different LUT for partial and full refresh. So I found LUT for full BW refresh, partial BW refresh and full 4 gray refresh. I haven't found 4 gray partial LUT yet. I will try to work on PR and also find missing LUT.

[V0r-T3x]

I'll try to study the code too, I tried it on a waveshare 2.7" 3-colors e-paper. The partial refresh work pretty well, but the screen writing look red/dark red on this one. But overall you did a great job ^^.