GxEPD2: minimize full refresh time

[StevenBoon]

Hi folks,
I am using an e-ink display - all is working good and well.
GDEH0213Z19 104x212, UC8151D, (HINK-E0213A20-A2 2020-11-19))
But I am using a battery-powered node and would like to minimize its power consumption.
A full refresh takes about 12~13 seconds, but about halfway through, the display is already done with the refresh and it takes quite a few seconds which appear to be useless as nothing is updating.
The logging also doesn't show anything special taking place:

_PowerOn : 71999
_Update_Full : 12693000
_PowerOff : 28000

Do I really need to wait for the full refresh to end, or can I 'cut it' earlier and save a couple of seconds of active time?

[ZinggJM]

@StevenBoon

Do I really need to wait for the full refresh to end, or can I 'cut it' earlier and save a couple of seconds of active time?

Yes and no, with GxEPD2. Do you use b/w only content on the 3-color display?
With b/w only, you don't see anything happening during the waveform phase for color update.
-jz-

[StevenBoon]

So far, I only used b/w. I don't really need the red, so if it can save useful power I can get rid of that!

[ZinggJM]

You are lucky. You have one of the few (2) 3-color e-paper displays that can do that.
See example GxEPD2x_FastBlackWhiteOnColor.ino.

// experimental example GxEPD2x_FastBlackWhiteOnColor.ino: differential refresh on capable 3-color panels

[StevenBoon]

That sounds great! I'm off for the evening, but hope to get around to testing that soon. Sounds like my lucky day

[StevenBoon]

Wow, that is near-instantaneous. The partial refresh completely eliminates the 12+ seconds, cutting it down to tens of milliseconds.
The only problem is that the partial refresh - of course - isn't perfectly drawn: some pixels are not that black.
What is the best solution to this? Draw the same text twice since it's easily fast enough? Or is there another recommendation?

[ZinggJM]

@StevenBoon

Draw the same text twice since it's easily fast enough?

Differential refresh with no differences will (should) have no effect.

Buy a b/w e-paper display for b/w only use.

You could try the example from Good Display for this special use.

[StevenBoon]

I've kept the code running through the night and the display lost close to no contrast. Sometimes a few pixels don't get turned on all the way but I have some space to increase the font.
Funnily enough a tri-color epd is the cheapest one I can get for the dimensions I need.
I think I know enough - thanks for the support!! You're literally singlehandedly carrying the whole hobbyist epd world

[StevenBoon]

Pretty much nailed it - full refresh & draw bitmap on reboot, and BW/partial refresh on deepsleep wake. Thank you!