Why does E-ink displays have to do a full-screen refresh to remove ghosting?

Question

E-ink readers (like a Kindle) has been out for years, but even now after several generations of the products, all of them still does a full-screen refresh to remove the ghosting.

I can't find the answer on the official e-ink website, however this question: Why does "flashing" prevent ghosting on E-Ink displays? does explained how E-ink works but still not anwsering my question:

Why is it necessary to refresh the whole screen?

Wouldn't it be enough to just refresh the part that wasn't empty (white) plus some "padding" (since a answer in the linked question mentioned that "However, each cell might be influenced by its neighbors as well as the applied charge.")

For example, when we read a book, it's very likely that some part of the screen will be always empty (like page margin, line margin, etc...) why can't those reigons be left out (like when you use the "Page Flip" function in Kindle PW2, only the small preview window gets refreshed and the rest part of the screen stays still)

EDIT:

@CL. reminds me in the comment section that no matter how much "padding" is added, the boundary between refreshed and unrefreshed parts is always there... So as long as a cell could be influenced by it's neighbors, this won't work - duh!

• But still, I think it would be nice to add a few padding to keep the ghosting away from interfering with new texts to keep readability without doing a full-screen refresh which I really, really hate.

So what's the technical difficulty to prevent this influencing between the cells? The latest generation of E-ink is "Carta" is introduced in CES 2013, no upgrade for two years already.

Answer 1

The simplest answer is also the most obvious. When issuing new information for the screen, deciding whether each and every pixel needs to change takes longer than issuing a complete refresh, and twice the memory. Both frames would have to be available simultaneously for comparison. This could be done in hardware with a purpose built buffer, but it's still additional hardware. Since it is not capable of real time video, and is so power efficient anyway, we can skip the step of making that decision, pass in the new image inverted, and then invert it again to make sure all the bistable elements were in the correct state.

Edit: I should also point out, that the book file itself stores the sheet as an entire image - the whole image being displayed is different, and thus would require the special handling noted above.

Answer 2

According to the English Wikipedia, a simple change of a cell is not enough to get a 100% black or white cell, it will always be a little grey. The solution is to toggle the cell several times and then set it to the desired color.

(I have to admit that this statement is tagged as "citation needed" on wiki, and that the following are assumptions gained by a closer look on that displays.)

However, this makes sense, as it is exactly what you see on a refresh. And this process needs some time, like one second. This seems to be a limitation of the display, not the electronics, because you can see the transition of the cells. So, they use the literally quick and dirty method for zooming / moving objects, but for a clear view, they do a refresh. As usually many parts of the display have to be refreshed which results in lots of flickering, they simply refresh the entire display. I guess that's also less annoying to the reader.

It is also not a problem of boundaries around cells. Imagine a big, filled, black circle on the display. If it is removed, the entire area remains as grey shadow. If it's a problem of boundaries, the area would turn to perfect white, leaving a grey outline. Also, solving this is as simple as setting all black cells to white, and all white cells next to the circle to white, too.