the e-ink screens have a linear response (see http://kindleworld.blogspot.fr/2011/...91602627321184
), whereas the computer monitors have a gamma-correction curve which darkens the lower values displayed, and brightens the higher values.
You can test this using a gamma-calibration pattern, such as : http://www.povray.org/documentation/...ence/gamma.png
On your monitor, it will probably give you a gamma value of 2.2, and on e-ink 1.0.
For that matter, JPEG and PNG files are coded using gamma compression (usually with the sRGB profile, which has a gamma around 2.2). This means that lower intensity values, corresponding to shadows, are "boosted". This gamma correction also comes from perceptual considerations: you are more sensible to intensity differences in dark areas.
Ideally, an image viewer for the e-ink display should decompress the gamma, i.e apply the formula intensity_displayed=255*(value_from_file/255)^2.2
Since there are only 255 values to compute, this can easily be done using a lookup table.
The gamma value (2.2) could be user-adjustable, because most users prefer brighter images.
You can test this using KCC (KindleComicConverter), which will generate an epub for your pocketboot (chose the DXG profile for the 912) with gamma-decompressed images. Apply it on images with subtle lighting (such as a B&W photograph with not too much contrast), and you'll see the difference: the image produced by KCC is more natural (although darker, of course) than this displayed by the pocketbook image viewer. You can easily test it using the "custom gamma setting" in KCC. https://github.com/ciromattia/kcc