[davidfor]

Originally Posted by ottischwenk

Your post indicates that you don't know Kobo.

And I do know Kobo and have discussed how they process the images for display with the Kobo developer who did the work. This included the issues they have because the use the Adobe RMSDK in some places and other libraries in other places. This included supplying test cases and testing beta version of the firmware to see the results.

Here in this forum the simulation of 256 gray levels was called dithering

It is the only word I have seen used here and elsewhere to describe this. I did search elsewhere, and it seemed to be the correct word. The Wikipedia article on Dithering seems to agree with this. In fact, the first sentence in the section on digital images is:

Dithering is used in computer graphics to create the illusion of color depth in images on systems with a limited color palette.

And that sounds exactly like what you would do for "simulation of 256 gray levels".

If we are using an incorrect term, can you please tell us what the correct one is? You might need to post that in German so that Google does not mess it up in the translation.

and so I used this one

And Kobo does this by assigning a corresponding predefined 4*4 pixel image to the average brightness value of a 4*4 pixel grid and displaying it - at 300 dpi this results in a resolution of 75 (300/4), at 227 it is 57 (227/4).

Which sounds like an example of a dithering algorithm to me. It isn't one that produces a "random" effect, so it probably has some undesirable affects which can depend on what the input. It's probably more like to cause a moiré pattern than other dithering algorithms.

I am curious about your example. Can you tell me which Kobo device and firmware that image was produced with? Even better would be the source of the image so that I can look at it on my devices. If it is somewhere I can legally download it, can you point it out for me to try?