What the heck is dithering? Wikipedia says, “Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as ‘banding’ in images.” The only reason you probably need to care about that is if you’d like to create your own images to use on your Kindle as a screen-saver (a.k.a. sleep screen), or if you are a content provider who would like to optimize your images for the Kindle.
The reason you may want to dither an image is because the Kindle e-ink screen only displays 16 shades of gray. If it tries to display an image that has gradual changes of darkness/lightness, you will likely end up seeing distinct bands of the different grays that are available, instead of a seamless transition. As an example, let’s suppose we start with an extreme example:
If we display it on the Kindle as is (or even if it is converted to gray-scale first), it will end up looking something like this on the Kindle’s screen:
What I have found to be a good solution when working with the GIMP image editor is to first change it to an “indexed” image (using the Image -> Mode -> Indexed… menu option). In the resulting pop-up, I select the option to “Use custom palette” and select a 16-shade palette I created for this purpose. Then in the “Dithering” section of that pop-up I select “Floyd-Steinberg (normal)”, then click the Convert button. This would convert our original sample image to:
If you were to zoom in that image, you would see that it is not the apparent smooth transition from one shade of gray to another, but that there are a very limited number of actual shades per pixel, but they have been pseudo-randomly blended together to give the smooth effect:
Once you’ve done this dithering by changing the image mode to an indexed palette image, you then want to once again change the mode to grayscale (Image -> Mode -> Grayscale) before saving it as a PNG file. This final mode change is needed because, at least in my experience, the way the Kindle displays palette-based PNG images is unpredictable and less than optimal, but seems to work fine when it is saved as a grayscale PNG, instead.
If you would like to try this and are a GIMP user (it’s a free, open source product, which is why I use it), it comes “out of the box” with a 32-color gray palette which will help smooth things out, but you can use my custom 16-color gray palette by downloading it, unzipping it, and saving it in your custom palettes directory. (Go to Edit -> Preferences from the menu bar, then in the new window find the “Folders” section on the left, and drill down into it to find and select “Palettes”. The main part of that new window should then tell you what folder it is using for custom palettes, which is where you would want to save this new custom palette file — probably something like “C:\Users\YourName\.gimp-2.6\palettes” if you’re on Windows.)