How my e-reader lost its stripes

For a long time, avid reading has been a cherished habit, but in today's world filled with distractions, even books and e-ink readers struggle to compete with the convenience of smartphones. The constant presence of notifications on these devices makes reading both more accessible and more susceptible to being replaced. Recently, I came across a new breed of tiny e-ink readers, which piqued my curiosity.

I purchased an Xteink X3, a remarkably compact device, and quickly installed the CrossPoint firmware on it. This allowed me to download books and customize fonts. I also experimented with converting a photo into a black and white bitmap for the "device sleep" screen, which I found visually appealing. The device's ability to display four shades of grey intrigued me, so I decided to explore the potential of greyscale images.

To my surprise, I encountered a couple of bugs. First, the sleep screen displayed greyscale images with murky dark areas, leading me to believe my eyesight was deteriorating. However, upon creating a test image, I discovered that dark grey was indeed black, while light grey appeared almost white. This issue was easily resolved by adjusting the image.

Another bug emerged when I noticed ghostly remnants of previous screens appearing on the CrossPoint viewer app. But, this problem only occurred on the sleep screen, suggesting that there were two different image renderers at play. Upon closer inspection, I noticed distinctive vertical stripes in the photograph, which were not present in the original bitmap file.

Delving deeper into the technical aspects of e-ink technology, I discovered that images on e-ink screens are created using voltage pulses to move black and white pigment particles, which retain their position even when power is removed. The CrossPoint firmware first draws a black-and-white base, followed by a short voltage-pulse waveform to create grey shades.

However, the image viewer app only performed a fast black-and-white update without executing the grey nudge, resulting in ghosted images. Despite attempting to rectify the issue with different dithering algorithms, the stripes persisted. To better understand the issue, I turned to an AI language model for assistance. Initially, the model suspected that the stripes were due to the Floyd–Steinberg dithering algorithm used to prepare the image.

However, switching to a different algorithm did not solve the problem. To gain further insight, I applied a fast Fourier transform (FFT) to the photograph, which helped identify a repeating pattern approximately two screen pixels wide. This confirmed the presence of the stripes, but also revealed that the issue was caused by the error-diffusion dithering algorithm, which produces fine dither texture that can manifest as the observed bands.

By averaging the brightness of each column in the image, I was able to eliminate the distracting dither texture and reveal the true nature of the stripes. Despite the technical hurdles, the Xteink X3 proved to be a fascinating device, offering a unique reading experience with its ability to display greyscale images.