Utilizing epaper panels to display artwork is still a niche usage of the technology. Grayscale is great for a lot of subjects and even a commonplace enhancement filter in artistic images and photos, but being able to display colors would certainly be a nice extension of the possibilities. And since I get quite a lot of requests for picture frames with color epaper panels, I started to break new ground with a device using four of these interesting – altough relatively small – color displays. The resolution is 600 x 448 with the number of colors limited to seven, including black and white. Compare that to the 16 million different colors1 that most modern monitors are able to show, this seems like too little.

Available color palette

CIE color space — CC-SA 4.0

In reality, though – since each pixel of a monitor consists of only a red, green and blue subpixel side by side – the only colors that can be displayed directly are shades of those three colors. Every other color to be displayed relies on the fact that the resolution of the human eye is finite and when looked at from afar, the combinations of red, green and blue will blur into the color that is intended to be displayed. Magnify the panel, and the alleged millions of colors will reveal themselves as a combination of the three base colors with different lightnesses.

CC BY-SA 3.0 — Stan Zurek

As a side note, the human eye is not able to directly see arbitrary colors, either: the retina contains only three types of cone cells. Their sensitivities define what we call the visible spectrum.

Light of a specific wavelength will excite the three types of cones to a certain extent and the brain mixes it back to the color matching the original wavelength. So if i.e. light with a wavelength corresponding to orange hits your retina, the cones senstive to green and red will get excited and the brain averages is out to orange.

The careful observer will notice that the pinkish colors of the color space above don’t appear in the visible spectrum. That’s because those colors don’t correspond to a single wavelength. If the brain needs to mix a color based on input from the blue and red cones — which could be presumed to blur to the average wavelength of green — the brain notices that there is no green component and instead invents a new set of colors: the line of purples.

But I digress, so let’s go back to epaper.

Epaper panels usually don’t have subpixels.2 Panels consist of tiny capsules that are filled with differently charged pigments. Applying different voltages forces the pigments either to the surface or submerges them.

CC-BY-SA 3.0 — Senarclens

Multiple capsules make up one pixel. Generating shades of the pigments can be achieved by controlling the pigments so that both end up on the surface in a desired ratio. Again, at a distance, the eye won’t be able to distinguish the base colors anymore and the desired color effect will be perceived.

How can a rich set of colors be achieved with this technique? Just add more pigments with different colors to the capsules. But how to control them independently from each other – after all, charges can only be positive and negative? The solution is to make the pigments different sizes so that they move inside the capsules with different speeds. This also means you need to shuffle them around multiple times until the right amount of every type of pigment ends up on the surface. The result can be seen below:

Wait – this does not look like only seven colors! How is this possible?
The explanation is simple: for every pixel the nearest of the seven directly presentable colors is displayed instead and the difference to the target color value is spread across the adjacent pixels. This process is called dithering. The perceptual mechanism is the same as described earlier with the subpixels of a monitor and the pigments in the capsules of an epaper display.

  1. A monitor is able to show a triangle-shaped subset of the colors in CIE color space. ↩︎

  2. Although some color epaper panels use RGB color filters (which leads to washed-out colors). ↩︎