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The Pokémon Pikachu 2 GS is a small electronic toy that has an LCD screen. What's unusual is that the screen is not the typical black-and-white LCD found on almost all handheld devices, but instead it has an LCD that displays shades of color. These colors range from transparent to yellow to red to black.

But this coloring doesn't involve RGB subpixels; it seems to use a special trick not seen elsewhere. How does the colorization work?

enter image description here enter image description here

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    \$\begingroup\$ How do you know it doesn't use RGB subpixels? Do you have a teardown or something? \$\endgroup\$ – Funkyguy Nov 20 '15 at 17:34
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    \$\begingroup\$ My guess is that it's either a CSTN (color super-twisted nematic) or CCSTN (color coded super-twisted nematic) display. \$\endgroup\$ – og1L Nov 20 '15 at 19:14
  • \$\begingroup\$ @Funkyguy I can only offer two pieces of evidence, since I don't have a working device at the moment. One is that there are no vertical stripes; all pixels are full square. Two is that the device has a contrast option, and you can see the colors change continuously from yellow to red to black. \$\endgroup\$ – Nayuki Nov 20 '15 at 21:40
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You're right that it's not the same as the technology used on (modern) laptops or flat screen displays. It's almost undoubtedly a color passive matrix display. Nintendo used that kind of display on the Color Gameboy as well. For small displays that don't have to act quickly, a color passive matrix display is a good fit. It's much cheaper to make than an active display which is why this technology, once common on laptop computers, was attractive to manufacturers.

If you look at the screen very closely, you can see the difference: enter image description here

Instead of having individual display elements for discrete red, green, and blue pixels, the matrix works by having three layers of liquid crystals each separated by a color filter -- essentially three stacked monochrome LCD arrays. By turning on one or more of the pixels in each layer, we get color.

How do stacked color displays work?

There are a number of descriptions of these available, but many are 30 or more years old and may not be easy to find. One that can be found online is US patent 4917465 which has good illustrations and shows a particular system using yellow, cyan and magenta filters for a backlit display, but the background section of the patent gives a good description of the different approaches to color LCD before 1990 and the principles are the same as used in reflective displays such as the one discussed here.

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  • \$\begingroup\$ I wonder, couldn't it do with just two colors, yellow on top of black, Both on to make redish \$\endgroup\$ – Passerby Nov 20 '15 at 23:04
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    \$\begingroup\$ I did some research - for reference, a micrograph of the Game Boy Color LCD looks like this: chipmusic.org/forums/topic/13227/a-deep-look-at-gameboy-lcds \$\endgroup\$ – Nayuki Nov 21 '15 at 0:56
  • \$\begingroup\$ @Edward Follow-up question - if there were three layers of color filters, wouldn't the display always be black? \$\endgroup\$ – Nayuki Nov 21 '15 at 0:58
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    \$\begingroup\$ I was able to find a Gameboy Color and verified by looking closely at the display under a magnifying glass that it seems instead to be a design with RGB areas rather than a stacked color passive matrix display. I don't know if the display technology of the GameBoy Color varied by release date. However for the device in question, that seems not to be the case. \$\endgroup\$ – Edward Nov 21 '15 at 4:42
  • \$\begingroup\$ I added a section on how stacked color displays work, which I hope will help answer your questions on that aspect of my answer. \$\endgroup\$ – Edward Nov 21 '15 at 15:58

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