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Can you get 32 level grayscale out of an E-ink display?

A project log for NekoCal - an E-Ink Calendar

A simple calendar made with e-ink display and stm32 microcontroller.

Wenting Z.Wenting Z. 12/31/2017 at 01:541 Comment

A quick research shows that all current commercial E-ink devices have a maximum grayscale level of 16. Is it a hardware limitation? Or can we get more grayscales just like we have done on CGA, Commodore 64, GameBoy Color and many other vintage hardwares that have a color limitation?

Well, the answer is yes. See my results :p

Just as said on the screen, happy new year 2018! (The upper right is one in 4bpp mode for reference)

Now, how it is done? Basically, to display an image on an Eink display, you need to apply multiple frames to the screen, and the result of superposition would be the image. In order to decide what to apply to display a specific color, a look-up table is used, and it is called “waveform table”. This is usually provided by the driver solution provider and we have no direct access to it since it is confidential. Now all commercial displays can only do 4bpp because there is simply no 5bpp LUT available.

So if we can create one and it would be able to do 5bpp right? Yes, but if we can. The waveform table is actually a 4 dimension LUT, the output depends on the previous grayscale, the target grayscale, the frame number we are at and temperature. Basically it’s just way too hard for us to create such a LUT. Maybe this is also the reason why there is no commerical 5bpp LUT available?

So in order to archive that, I need to first “trim down” the LUT. 4D is way too much. Firstly, I decided to ignore the temperature. Then we can always start at white. Now it is already a 2D LUT. Then I decided to make the target grayscale equals to the frame number: We would use fixed 32 frame sequence for 5bpp mode and one frame correspond to one level grayscale. This requires a big change to the LUT: It is no longer looking up for the data to output since we have made it match, the output have to equal to the grayscale input. So instead it is looking up for the line time. By adjusting the line time we can fine control the grayscale. And here it is.

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Avamander wrote 09/15/2018 at 11:23 point

Cool!

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